foof-loop: A Simple, Extensible Scheme Looping Facility

This is the reference manual for foof-loop version 9 (BETA). This file was written by Taylor R. Campbell and is placed in the Public Domain.

The most recently released version of this file is permanently stored at This document is not finished: to be written is a section on augmented error reporting for the authors of iterators.

  1. foof-loop: A Simple, Extensible Scheme Looping Facility
  2. Requirements
  3. Repository
  4. Introduction
    1. Basic Looping: DO, Named LET, and LOOP
    2. Controlling Loops
    3. Updating Loop Variables
    4. Iterator-Supplied Loop Variables
    5. Iterating over Numbers
    6. Accumulating Results
    7. Other LOOP Clauses: WHILE, UNTIL, LET, and LET-VALUES
  5. Loop Structure
    1. Lazy Loops
    2. Loop Expansion
      1. Efficiency Concerns
        1. Use of Assignment
        2. Local Closure for FINISH
        3. Multiple Return Values
        4. Index Variables and Generic Arithmetic
  6. Iterators
    1. Iterating across Lists
    2. Iterating across Vectors and Strings
    3. Iterating through Input
    4. Iterating over Numbers
    5. Accumulating
      1. Accumulating Lists
      2. Accumulating Numerical Results
  7. Writing Custom Iterators
    1. Reporting Syntax Errors
      1. Pitfalls of Reporting Syntax Errors
  8. Design Notes and Rationale
    1. Upper and Lower Bounds on Intervals




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Every useful program has loops. There are too many flavours of loops to classify all, but patterns worth naming recur. Named LET and DO capture two flavours, but Scheme lacks any name for the pattern of looping through a list, CARing and CDRing to the end; or the pattern of looping through a vector, stepping indices from one bound to another. Procedures such as MAP and FOLD use these patterns, but compose poorly; one cannot easily FOLD a list and MAP a vector in parallel to yield a folded value and a mapped vector. Foof-loop gives names to these patterns by which they can be composed together in a unified LOOP macro.

Basic Looping: DO, Named LET, and LOOP

Consider a loop that counts the number of items in a list that satisfy a predicate. In standard Scheme, this might be written with named LET or DO; let us choose DO, which is slightly the conciser for this form of loop:

 (define (count-matching-items list predicate)
   (do ((list list (cdr list))
        (count 0
               (if (predicate (car list))
                   (+ count 1)
       ((null-list? list) count)))

This is straightforward, but if we wish to substitute a vector for a list, we must restructure of the loop so that it steps through the indices of the vector, rather than through the list's pairs:

 (define (count-matching-items vector predicate)
   (do ((index 0 (+ index 1))
        (count 0
               (if (predicate (vector-ref vector index))
                   (+ count 1)
       ((= index (vector-length vector))

Even worse, to count the matching items read from an input port, we must rewrite the loop using named LET, because we need an idiom which DO cannot express. We must read from the port when the loop is entered and before we decide whether to stop the loop, because the decision depends on the object we read:

 (define (count-matching-items reader input-port predicate)
   (let continue ((count 0))
     (let ((item (reader input-port)))
       (if (eof-object? item)
           (continue (if (predicate item)
                         (+ count 1)

Named LET captures a very general loop structure, and DO captures a much more specific loop structure, but neither captures any mechanism of iteration. When looping through a list, one must CAR and CDR, and check for PAIR?; when looping through a vector, one must start at 0, increment indices, and check against the length; when looping through input, one must first read an object and check for the end of input. None of these patterns, of looping through a list, of looping through a vector, or of looping through input, is given any name in Scheme.

Foof-loop provides just these names. To loop for each item in a list, we say just that:

 (define (count-matching-items list predicate)
   (loop ((for item (in-list list))
          (with count 0
            (if (predicate item)
                (+ count 1)
     => count))

How do we read this mountain of syntactic sugar? LOOP is the macro that invokes foof-loop. Its first operand is a list of loop clauses. The loop clause (FOR ITEM (IN-LIST LIST)) invokes the IN-LIST iterator, which steps through LIST, letting ITEM be successive elements of the list for each iteration of the loop. The loop clause (WITH COUNT 0 <update>) introduces COUNT as a variable whose value is 0 in the first iteration of the loop, and whose value at each next iteration is that of the expression <update>. The loop repeats until any one of the loop clauses terminates it; in this case, loop will terminate when the list is exhausted of elements. Finally, when the loop has terminated, it returns the value of the expression after =>, here the number of matching items we counted.

To count elements of a vector, we need only substitute IN-VECTOR for IN-LIST. Likewise, to count objects read from an input port, we need only substitute IN-PORT, and specify the reader in (FOR ITEM (IN-PORT INPUT-PORT READER)).

(Foof-loop gives names to mechanisms of iteration, but provides no generic iterators of collections, sequences, or associations, whose mechanism is chosen from whether the input is a list, a vector, &c. The author has experimented inconclusively with a generic protocol, based on Dylan's collections, to implement such iterators.)

Controlling Loops

An astute reader might wonder why we need => before the final expression whose value the loop will return; another astute reader might wonder how LOOP can be as general as DO or named LET without any program body inside the loop. Both will be relieved to learn that the LOOP form has a program body; the => sets the final expression apart from the body. Not only can we write a program body as in DO, but we can even give a name to the loop and specify where in the body to continue to the next iteration; then the loop does not implicitly continue after the body, which is evaluated in a tail position. For example, to find a matching item, rather than to count all of them, we can write:

 (define (find-matching-item list if-absent predicate)
   (loop continue ((for item (in-list list)))
     => (if-absent)
     (if (predicate item)

Here the loop continues only if ITEM fails to satisfy PREDICATE. Then only if we have run through every element of LIST, and found none satisfying PREDICATE, shall we evaluate the final expression and call IF-ABSENT. As before, we can substitute IN-VECTOR, IN-PORT, or any other iterator we please, for IN-LIST here.

We are not limited to continuing the loop in tail positions in the loop body. We could, for example, write a (single-list) MAP procedure thus:

 (define (map procedure list)
   (loop recur ((for element (in-list list)))
     => '()
     (cons (procedure element) (recur))))

If the list is exhausted of elements, then the loop yields nil; otherwise, it recursively maps the remaining elements of the list, and makes a pair of the mapped ELEMENT and all the mapped remaining elements.

Finally, we need not supply a final expression at all, in which case what the loop returns is unspecified. For example, this program writes each element of a list LIST followed by a newline:

 (loop ((for element (in-list list)))
   (write element)

Updating Loop Variables

A third astute reader -- not necessarily distinct from the first two -- may wonder still how LOOP can be as expressive as named LET, with which one can update loop variables (which we introduced in LOOP using WITH clauses) when explicitly continuing the loop. One can do this too with LOOP by passing arguments when continuing the loop; LOOP is a drop-in replacement for named LET, if we use the loop's name only in operator positions, the reason for which will become clear shortly. This definition of REVERSE-MAP is unchanged by substituting LOOP for LET:

 (define (reverse-map procedure list)
   (let continue ((list list) (result '()))
     (if (null-list? list)
         (continue (cdr list)
                   (cons (procedure (car list) result))))))

The loop clauses (<variable> <initializer>) and (WITH <variable> <initializer>) are both equivalent to named LET bindings. Passing arguments to CONTINUE updates the corresponding loop variables by position. As with named LET, the update binds new variables, and does not assign existing ones.

But the gripe with named LET that one hears most often is the distance between the list of loop variables and the expressions to update them. Worse, the only association between the variables and expressions is position. Foof-loop gives the option of named update: the loop's name is bound to a macro, not a procedure, so that when continuing the loop, after any positional arguments, which are optional, an operand of the form (=> <variable> <expression>) updates the value of loop variable <variable> to <expression>. For example, to partition a list into two lists, of the elements that satisfy a predicate and the elements that do not, we can update one of the two lists:

 (define (partition predicate list)
   (loop continue ((for element (in-list list))
                   (with satisfied '())
                   (with unsatisfied '()))
     => (values (reverse satisfied)
                (reverse unsatisfied))
     (if (predicate element)
         (continue (=> satisfied (cons element satisfied)))
         (continue (=> unsatisfied (cons element unsatisfied))))))

Note well that the arrow => has two different meanings in this code.

Iterator-Supplied Loop Variables

Sometimes it is not enough just to step through each element of a list or a vector; we may need to know at what index in the sequence we are, or we may need to move to another position. Some iterators can expose their underlying loop variables. IN-LIST can expose the pair whose car is the current element of the list. It also ensures that modifying the cdr of that pair does not affect the iteration, so that we can safely reverse a list in place with it:

 (define (reverse! list)
   (loop ((for element pair (in-list list))
          (with tail '() pair))
     => tail
     (set-cdr! pair tail)))

This procedure steps through the list, letting TAIL be () initially and then the last pair we encountered; for each pair we encounter, we set its cdr to be the last one. We happen not to use ELEMENT here, although we could use it in a REVERSE-MAP! procedure:

 (define (reverse-map! procedure list)
   (loop ((for element pair (in-list list))
          (with tail '() pair))
     => tail
     (set-car! pair (procedure element))
     (set-cdr! pair tail)))

Often we find nested sublists which we want to coalesce into the containing list. For example, we may want to flatten nested BEGINs in Scheme code. We can flatten a BEGIN as we go by updating the loop variable of IN-LIST whenever we encounter a BEGIN, rather than simply letting IN-LIST implicitly update it to be its cdr:

 (define (flatten-begins list)
   (loop continue ((for element pair (in-list list))
                   (with subforms '()))
     => (reverse subforms)
     (if (and (pair? element)
              (eq? 'BEGIN (car element)))
         (continue (=> pair (append (cdr element) (cdr pair))))
         (continue (=> subforms (cons element subforms))))))

If ELEMENT is a nested BEGIN, then at the next iteration, PAIR will be the first pair of the body to which the remaining elements of the enclosing body have been appended, and ELEMENT will be the first element of the concatenation.

Vector iterators expose as a loop variable the index into the vector. This procedure shuffles a vector by the Fisher-Yates method, given a procedure (RANDOM-INTEGER <n>) yielding an integer between 0 (inclusive) and <n> (exclusive):

 (define (shuffle-vector! vector)
   (loop ((for element i (in-vector vector)))
     (let ((j (random-integer (+ i 1))))
       (vector-set! vector i (vector-ref vector j))
       (vector-set! vector j element))))

Iterating over Numbers

There are two iterators for iterating over numbers: UP-FROM and DOWN-FROM. (These names violate the convention of the prefix of IN- on iterator names, but after long and arduous discussion, the author and his minions concluded them better than any alternatives.) For example, to make a list of a given length by applying a procedure to each index for the element at that index, we can write:

 (define (list-tabulate length procedure)
   (loop ((for index (up-from 0 (to length)))
          (with list '() (cons (procedure index) list)))
     => (reverse list)))

The keyword TO is part of the syntax of UP-FROM which specifies an upper bound for the iteration. It is not a separate procedure or special operator. We can step by a specified increment by adding (BY <increment>); for instance, this loop makes a list of the even integers less than N:

 (loop ((for integer (up-from 0 (to N) (by 2)))
        (with evens '() (cons integer evens)))
   => (reverse evens))

We can also omit TO in order to iterate without bound; presumably, then, we intend to terminate on some other condition. For example, if we assume lists to be finite and non-circular, this procedure computes their lengths:

 (define (unsafe-length list)
   (loop ((for element (in-list list))
          (for length (up-from 0)))
     => length))

The loop clause (FOR <variable> (UP-FROM <start> [(TO <end>)] [(BY <step>)])) iterates with <variable> initially bound to <start> and then updated at each iteration to its previous value plus <step>, until its value has reached or exceeded <end>.

DOWN-FROM shares syntax with UP-FROM but subtly differs in semantics, and will illustrate a convention in the design of foof-loop which I shall give its own paragraph to emphasize its importance:

Lower bounds are inclusive. Upper bounds are exclusive.

Thus UP-FROM includes the start and excludes the end, but DOWN-FROM excludes the start and includes the end, if it is reached. If <start> - <end> is not an even multiple of <step>, DOWN-FROM too will exclude the end. The same consequences apply to IN-VECTOR and the similar IN-VECTOR-REVERSE. This bears repeating.

Lower bounds are inclusive. Upper bounds are exclusive.

Whether we iterate forward or we iterate backward, whether over ranges of numbers or over ranges of elements in vectors, the lower bound of the iteration is inclusive and the upper bound of the iteration is exclusive.

The number in DOWN-FROM, the index in IN-VECTOR-REVERSE, and the loop variable in any other iterator of descending numbers, is decremented by the step, not when loop variables are updated, but on entry to the loop. This means that if the loop variable is initialized to 0, its first value in the loop body will be -1, not 0; and if it is updated to N, the next value that it may have in the loop body will be N - 1.

Let me say it once more, to be sure that the reader understand.

Lower bounds are inclusive. Upper bounds are exclusive.

Reverse iteration is extremely counter-intuitive. One might be tempted to say that starting bounds should be inclusive and that ending bounds should be exclusive. An earlier version of foof-loop followed this convention, which led to off-by-one adjustments galore:

 (loop ((for element
             (in-vector-reverse vector
                                (- (vector-length vector) 1)

This fragment is ugly and error-prone. It may lead to negative indices into a vector. (As an aside, C does not permit pointers to locations preceding the starts of array objects, but does permit pointers to locations one element past the ends of array objects.) The same fragment reads much better if lower bounds are inclusive and upper bounds are exclusive:

 (loop ((for element
             (in-vector-reverse vector (vector-length vector) 0)))

If one must use any other bounding convention, one should draw attention to it as an exception to the rule. Use inclusive lower bounds and exclusive upper bounds if possible; if not, write the rules explicitly using WITH, LET, and UNTIL clauses. For example, to loop with both bounds inclusive, write:

 (loop (...
        (with i start (+ i step))
        (until (> i stop))

Accumulating Results

The only iterators we have seen are those that step through sequences. Every time we have accumulated a list of values, we have always written a manual loop variable, a manual CONS, and a manual REVERSE at the end. We can simplify with accumulating iterators such as LISTING. With LISTING, (single-list) MAP becomes:

 (define (map procedure list)
   (loop ((for element (in-list list))
          (for result (listing (procedure element))))
     => result))

Accumulators, as the reader has surely noticed, are named not by prefixing IN- but by suffixing ING. Most accumulators have the same general syntax:

 (FOR <accumulated-result> 
      (<accumulating> [(INITIAL <initial-value>)]
                      <datum and conditions>))

<Initial-value> is a value to seed the accumulation with. In LISTING, for example, it is the tail of the list after all the accumulated elements, and by default is nil. <Datum and conditions> has several forms, of which we have already seen the form for accumulating a single value unconditionally. To accumulate the value only if some condition is true, write (IF <condition>) after the datum:

 (define (filter predicate list)
   (loop ((for element (in-list list))
          (for result
               (listing element
                        (if (predicate element)))))
     => result))

Sometimes we derive the value we want to accumulate from a non-false condition, as in COND's => clause. For example, to filter and map a list's elements, we accumulate all non-false values:

 (define (filter-map procedure list)
   (loop ((for element (in-list list))
          (for result
               (listing (procedure list) => (lambda (x) x))))
     => result))

Finally, the => clause is extended as in SRFI 61. An example of this is left to the imagination of the reader.

Along with LISTING, there are three other similar list-related accumulators: LISTING-REVERSE, APPENDING, and APPENDING-REVERSE. For elements elt_0, elt_1, ..., elt_n-1, elt_n, LISTING yields

 (LIST elt_0 elt_1 ... elt_n-1 elt_n);


 (LIST elt_n elt_n-1 ... elt_1 elt_0);


 (APPEND elt_0 elt_1 ... elt_n-1 elt_n);


         (REVERSE elt_n-1)
         (REVERSE elt_1)
         (REVERSE elt_0)),

although it can be implemented considerably more efficiently than (APPENDING (REVERSE ...)).

There are two accumulators that make lists in place: LISTING! and LISTING-INTO!. LISTING must make a list in reverse and then reverse it at the end, a process which requires twice as many pairs as the result uses; LISTING! makes a list forward by setting the cdr of each previous pair, and requires a total of one intermediate pair, the initial one. (LISTING-INTO! <initial-pair> <datum and conditions>) uses no intermediate pairs because it is seeded with an initial pair:

 (let ((x (cons 'initial '())))
   (loop ((for i (up-from 0 (to 10)))
          (for result (listing-into! x (* i i) (if (even? i))))))
 ;Value: (INITIAL 0 4 16 36 64)

Beware, though, that LISTING! and LISTING-INTO! are dangerous because they are non-reentrant. If the loop is continued more than once from the same iteration, or if control may transfer non-locally into the loop body and repeat iterations, LISTING! and LISTING-INTO! may overwrite old results.


We have seen FOR and WITH clauses in LOOP expressions: FOR to dispatch to an iterator, and WITH to write the user's loop variables. There remain four others: WHILE, UNTIL, LET, and LET-VALUES.

To read the leading non-empty lines of a file, one might wish to iterate through the successive lines from an input port, making a list of them. The iteration should terminate either when there are no more lines to be read or when the line just read is empty. We can terminate the loop by naming it and only conditionally continuing; thus our initial attempt may resemble this procedure:

 (define (read-initial-non-empty-lines input-port)
   (loop continue ((for line (in-port input-port read-line))
                   (for lines (listing line)))
     => lines
     (if (string-empty? line)

The final expression, LINES, is simple here, so we may not bat an eye at its duplication, although in the backs of our minds we should note that to avoid copying an elaborate expression would require more work. But this program fails altogether, because the variable LINES is not available inside the body of the loop; only once all the lines have been listed -- in reverse -- by IN-LIST will it reverse the list and bind the variable LINES, in the final expression. Fortunately, we can force the loop to terminate and evaluate the final expression with all the bindings we expect, using WHILE or UNTIL:

 (define (read-initial-non-empty-lines input-port)
   (loop ((for line (in-port input-port read-line))
          (until (string-empty? line))
          (for lines (listing line)))
     => lines))

Iteration will terminate either when READ-LINE returns an EOF object, tested implicitly by IN-PORT, or when the line read is an empty string, tested explicitly by the user. In either case, LISTING will bind LINES to the list of lines, which we return in the final expression. By the way, the order of the clauses is irrelevant; we could just as well have written:

 (define (read-initial-non-empty-lines input-port)
   (loop ((until (string-empty? line))
          (for lines (listing line))
          (for line (in-port input-port read-line)))
     => lines))

(LET <variable> <expression>) binds <variable> to <expression> in the body of the loop. (LET-VALUES <bvl> <producer>) is similar, but binds multiple values, rather than a single value; (LET <variable> <expression>) is equivalent to (LET-VALUES (<variable>) <expression>).

Variables from iterators are bound in the right-hand sides of LET and LET-VALUES clauses; these, and variables from LET and LET-VALUES clauses, are bound in the conditions of WHILE and UNTIL clauses. Thus variables from LET and LET-VALUES are bound, and WHILE and UNTIL conditions tested, after all iterators have agreed to let the loop continue.

Thus concludes the introduction to foof-loop. The rest of this document is a reference manual for foof-loop.

Loop Structure

[syntax] (LOOP [<loop-name>] (<loop-clause> ...) [=> <final-expression>] [<body>])

The LOOP form follows the general syntax:

 (LOOP [<loop-name>] (<loop-clause> ...)
   [=> <final-expression>]

<Loop-clause> may have one of the following forms:

 (<loop-variable> <initializer> [<update>])
 (WITH <loop-variable> <initializer> [<update>])
 (FOR <variable> ... (<iterator> <argument> ...))
 (LET <variable> <expression>)
 (WHILE <condition>)
 (UNTIL <condition>)

A loop steps through each iterator in parallel, repeatedly evaluating the body, until the termination condition of any loop clause is true. Then if the <final-expression> is supplied, it is evaluated in an environment with the final values of all the loop variables bound, as well as any final bindings supplied by the iterators; if there is no <final-expression>, the value of the invocation of the loop is unspecified.

If <loop-name> is not supplied, the loop continues at the end of the loop body, which may be empty. If <loop-name> is supplied, it is bound to a macro that must be called in the loop body to continue the loop. This macro may be called arbitrarily many times, in tail or non-tail positions; it is appropriate for recursive loops. (Note, though, that some iterators are non-reentrant, which may make multiple calls unsafe. These iterators are discouraged, however, and by convention are marked with a suffix of !; e.g., LISTING! and LISTING-INTO!.)

The macro <loop-name> accepts arguments by which to update the loop variables. Arguments may be passed by position, or by name, or both. Positional arguments come first, corresponding with the loop variables in leading WITH clauses. After positional arguments are updates to named variables, written (=> <variable> <update>). Loop variables from iterators may be updated only by name.

All arguments to <loop-name> are optional: all loop variables have default update expressions. A loop variable from (WITH <variable> <initializer> [<update>]) by default either is updated to the value of <update>, or remains unchanged if no <update> was supplied. Loop variables from iterators by default are updated according to the iterator.

 (loop continue ((with a 0)
                 (with b '()
                   ;; Default update for B:
                   (cons a b))
                 (for c d (in-list '(i j k p q r))))
   (write (list a b c d))
   (continue (+ a 1)    ;First position, for the first (WITH A 0).
             (=> d (cddr d))))   ;D is from (FOR C D (IN-LIST ...)).
 ;; Output:
 ;(0 () i (i j k p q r))
 ;(1 (0) k (k p q r))
 ;(2 (1 0) q (q r))

Many variables are involved in a loop, some written by the user and some which arise only from iterators. Each variable falls into one of several classes:

The loop is controlled by the loop clauses:

 (WITH <variable> <initializer> [<update>])

Introduces a loop variable named <variable>, initialized to the value of <initializer>. On each iteration of the loop, the variable may be explicitly updated; if it is not explicitly updated, but <update> was supplied, then the variable is updated to the value of that expression in an environment with all loop variables, entry variables, body variables, and user variables bound; otherwise, <variable> remains unchanged at the next iteration of the loop.

 (<variable> <initializer> [<update>])

Identical to (WITH <variable> <initializer> [<update>]). This form is provided to make the LOOP syntax compatible with named LET, and very similar to DO:

     (let next ((x 0))
       (if (< x 10)
           (begin (write x) (next (+ x 1))))),
     (loop next ((x 0))
       (if (< x 10)
           (begin (write x) (next (+ x 1))))),
     (do ((x 0 (+ x 1)))
         ((>= x 10))
       (write x)), and
     (loop ((x 0 (+ x 1))
            (until (>= x 10)))
       (write x))

are all equivalent.

Prefer the WITH syntax; it is visually distinct and easier for pretty-printers to indent better with less effort.

 (FOR <variable> ... (<iterator> <argument> ...))

General iteration form. <Iterator> can do as it pleases with the <variable>s and <argument>s, but by convention the <variable>s are variables to be bound in the loop body or the final expression, and the <argument>s are expressions or directives controlling the iteration.

The FOR clauses have arbitrary order; relying on it is an error.

 (LET <variable> <expression>)
 (LET-VALUES <bvl> <expression>)

LET binds <variable> to the value of <expression> inside the body bindings, and before the user termination conditions and loop body. LET-VALUES binds all the variables in the bound variable list <bvl> to the respective values returned by <expression>. The clause (LET <variable> <expression>) is equivalent to the clause (LET-VALUES (<variable>) <expression>).

 (WHILE <condition>)
 (UNTIL <condition>)

After no iterator has terminated the loop, <condition> is evaluated in an environment with all of loop variables, entry variables, body variables, and user variables bound. If true, the iteration continues for WHILE and terminates for UNTIL, and vice versa.

Lazy Loops

[syntax] (LAZY-LOOP <loop-name> (<loop-clause> ...) => <final-expression> <body>)

Lazily evaluated variant of LOOP which depends on SRFI 45 (Primitives for Expressing Iterative Lazy Algoritms). Equivalent to a LOOP form wrapped in LAZY with all calls to the <loop-name> also wrapped in LAZY. (See the SRFI 45 for details on LAZY.) The loop's body and final expression both must yield promises.

   ;;; Assume SRFI-40-style streams (see SRFI 40, A Library of
   ;;; Streams, for details, particularly on even versus odd streams).
   ;;; Also assume an IN-STREAM iterator.
   (define (stream-filter predicate stream)
     (lazy-loop filter ((for element (in-stream stream)))
       => stream-nil
       (if (predicate element)
           (stream-cons element (filter))
   ;;; An equivalent definition of STREAM-FILTER.
   (define (stream-filter predicate stream)
      (loop filter ((for element (in-stream stream)))
        => stream-nil
        (if (predicate element)
            (stream-cons element (lazy (filter)))
            (lazy (filter))))))

The laziness annotations are added around the whole loop and around calls to the loop, rather than around the loop body, in order to delay any outer bindings of the loop (for example, many expressions supplied as arguments to loop iterators) and any loop variable updates when recursively invoking the loop.

Loop Expansion

When a LOOP form is fully expanded, the result has the form of:

 (LET-VALUES ((<outer-bvl> <outer-producer>)
   (DEFINE (LOOP-PROCEDURE <loop-variable> ...)
     (LET-VALUES ((<entry-bvl> <entry-producer>)
       (LET ((FINISH (LAMBDA ()
                       (LET-VALUES ((<final-bvl> <final-producer>))
         (IF (OR <termination-condition> ...)
             (LET-VALUES ((<body-bvl> <body-producer>)
               (LET-VALUES ((<user-bvl> <user-producer>)
                 (IF (OR <user-termination-condition> ...)
   (LOOP-PRODEDURE <loop-initializer> ...))

Each <*-bvl> is a bound variable list supplied by an iterator or by the user. Each <*-producer> is an expression yielding multiple values to be distributed to the corresponding bound variable list. The <user-bvl>s are from user-supplied LET and LET-VALUES clauses; the <user-termination-conditions> are from WHILE and UNTIL clauses.

LOOP-PROCEDURE is a name invisible to all of the forms involved in the loop except for the macro to which the loop's name is bound in the loop body. This macro processes its positional and named arguments to form a call to LOOP-PROCEDURE updating all the loop variables appropriately.

Efficiency Concerns

Efficient code is a primary goal of LOOP. It should be possible to write floating-point matrix multiplication routines as fast with LOOP as by hand, and perhaps faster because the details of the matrix operations can be swept under the rug of a matrix iterator. There may be some questions, however, of the efficiency of the code that LOOP generates.

Use of Assignment

Many other loop macros, such as Common Lisp LOOP and SRFI 42 Eager Comprehensions, update loop variables by assignment, rather than by rebinding. Even hand-written loops using named LET or DO are sometimes written with assignment, because it is more convenient with large numbers of loop variables which would otherwise be repeated mostly unchanged for every recursive call to the loop procedure.

This is a death-knell for efficient code, especially in many highly optimizing Scheme compilers, because most Scheme compilers generate heap-allocated cells to store mutable variables. Compilers' lives are simplified by assuming that all variables are immutable; they are free to move references to the variables as they please. Transforming a program with SET! into a program without SET! but with heap-allocated cells is trivial, and lets the compilers assume that variables are immutable; eliding those cells anywhere but in closure references is considerably harder, and not worth the effort for idiomatic Scheme code.

It may be the case that a pure tree interpreter can execute assignment- ridden code more efficiently than procedure calls with large numbers of arguments, because of the overhead of recursively processing each of those arguments. It is also the case that tree interpreters are slow anyway.

Using assignment to update loop variables is a losing proposition in the end. LOOP never introduces assignments to mutable variables; all loop variables are updated by recursive procedure call, i.e. rebinding. Custom iterators may, but should not, use assignment.

Local Closure for FINISH

Because the construction of a closure for FINISH is costly in many Scheme implementations which have extremely naive compilers, the current implementation of LOOP creates the closure only if necessary to preserve scope; it will integrate the body of FINISH if and only if there are no user termination conditions, or there are no iterator- supplied termination conditions, iterator-supplied body bindings, and user-supplied bindings around the body.

However, programmers who are worried about the closure for FINISH would probably be better served by a Scheme implementation that goes to the slighest effort necessary to recognize the local closure and fold the unconditional jumps into the code for the conditional.

Multiple Return Values

Opponents of multiple return values may object to the pervasion of LET-VALUES throughout the expansion and the inefficiency thereof in Scheme implementations that do not subscribe earnestly to the doctrine of multiple return values. But LET-VALUES is the only way to allow iterators to bind multiple variables that may have complex inter- dependencies without the indirection and cost of intermediate data structures.

There is no cost in Scheme systems with efficient support for multiple return values. There will be a cost in Scheme systems with costly multiple return values, whether LOOP uses multiple return values, and incurs the cost implicitly, or whether LOOP generates code to construct intermediate data structures. But on Scheme systems with efficient multiple return value support, LOOP incurs no cost. By using multiple return values, LOOP defers the question of the cost to the Scheme implementation.

Furthermore, for most cases, there is only a single return value anyway. LET-VALUES itself can avoid whatever overhead there may be of multiple return values, simply by recognizing single-value cases. Unfortunately, the reference implementation of SRFI 11 does not do this, and will generate CALL-WITH-VALUES invocations for every binding clause in the LET-VALUES. Here is a better implementation:

Index Variables and Generic Arithmetic

Many Scheme implementations provide specialized arithmetic operators on limited integers fixed to an interval whose values fit in machine registers. These limited integers, often called fixnums, are typically guaranteed to contain the domain of vector indices. Fixnum arithmetic is usually much faster than generic arithmetic -- when it does not overflow and yield completely bogus answers.

LOOP generates generic arithmetic for index variables, in iterators such as UP-FROM and IN-VECTOR. There are a number of reasons for this:

1. There is no portable specialized arithmetic for LOOP to use.

2. LOOP is meant not only for code inside inner loops that must be as fast as possible; programs written with LOOP should not fail obscurely because they passed some iterator a non-fixnum value where a fixnum was expected.

3. The iterators provided by foof-loop would be complicated by support for fixnum arithmetic. Do we add, for example, UP-FROM-FIXNUM, UP-FROM-UNSIGNED-FIXNUM, and so on? Do we have IN-VECTOR, IN-VECTOR/UNSAFE, and so on?

4. Programs that use fixnum arithmetic are harder to understand, because with fixnum arithmetic comes a plethora of subtle implications. For example, the usual formula for the average of two integers -- (QUOTIENT (+ A B) 2) -- is no longer valid under fixnum arithmetic, because the sum may overflow the bounds of fixnums. A program that once used Scheme vectors but has been converted to use sparse vectors, whose indices are arbitrary integers, may no longer be able to use fixnum arithmetic.

It is up to the compiler to specialize the generic arithmetic where appropriate, such as the indices inside IN-VECTOR, and to insert pre- loop type and range checks where appropriate. Programmers who use compilers incapable of this should consider finding better compilers, or improving the ones they use.

Common Lisp, by comparison, has no specialized arithmetic operators; rather, it lets programmers declare the types of variables' values. This is even more specific than specialized arithmetic, and does not require N arithmetic routines for every kind of specialized arithmetic, so that it composes with macros well. This is a better way to improve the efficiency of arithmetic, and requires minimal changes to LOOP -- at most, support for iterators to supply declarations in the loop.


Iterating across Lists

[syntax] (IN-LIST <list> [<successor>])


   (FOR <element> [<pair>] (IN-LIST <list> [<successor>]))

Iterates for each successive pair in <list>, binding the variable <pair>, if supplied, to that pair, and the variable <element> to the car of that pair. The successor of the pair is taken by applying <successor> to the current pair. The iteration runs out when the successor is no longer a pair.

<Successor>, if supplied, must be a unary procedure. The default value of <successor> is CDR. <List> and <successor> are evaluated once before the loop begins.

<Pair> is a loop variable; its value in the final expression is unspecified.

<Element> is a body variable.

The next pair is taken before the loop body, so the loop body may reliably apply SET-CDR!, or its equivalent for the given successor procedure, to the pair, without altering the iteration. Note, though, that <successor> will be called irrespective of whether <pair> is updated explicitly.

   (loop ((for a (in-list '(a b c)))
          (for b (in-list '(p q r))))
     (write (list a b))
   ;; Output:
   ;(a p)
   ;(b q)
   ;(c r)
   (loop ((for x (in-list '(1 2 3)))
          (with y 0 (+ y (* x 2))))
     => y)
   ;Value: 12
   ;;; PAIR-FOLD from SRFI 1.
   (define (pair-fold kons knil list)
     (loop ((for elt pair (in-list list))
            (with knil knil (kons pair knil)))
       => knil))
[syntax] (IN-LISTS <lists> [<tail>])


 (FOR <elements> [<pairs>] (IN-LISTS <lists> [<tail>]))

Iterates for each successive pair in each parallel element of <lists>, binding the variable <pairs>, if supplied, to be a list of those pairs, and the variable <elements> to a list of the cars of those pairs, prepended to the value of <tail> evaluated at each iteration.

no <tail> is supplied, its default value is the empty list.  The

successor of each pair is taken by CDR. The iteration runs out when any successor is no longer a pair.

<Lists> must be a non-empty list; it is an error if otherwise. It is evaluated once before the loop begins.

<Pairs> is a loop variable; its value in the final expression is unspecified.

<Elements> is a body variable.

The cdrs are taken before the loop body, so the loop body may reliably apply SET-CDR! to any of the pairs without altering the iteration.

   ;;; Transpose a matrix represented as a nested list.
   (loop ((for columns (in-lists '((a b c) (d e f))))
          (with rows '() (cons columns rows)))
     => rows)
   ;Value: ((c f) (b e) (a d))
   (define (every? predicate list . lists)
     (loop proceed ((for elts (in-lists (cons list lists))))
       (and (apply predicate elts)
   (define (any predicate list . lists)
     (loop proceed ((for elts (in-lists (cons list lists))))
       (or (apply predicate elts)
   ;;; SRFI 1's signature for FOLD.  Notice that KNIL is the *last*
   ;;; argument to the FOLD procedure, so we have to tack it on to the
   ;;; end of the argument list.
   (define (fold kons knil list . lists)
     (loop ((with knil knil (apply kons arguments))
            (for arguments 
                 (in-lists (cons list lists)
                           (cons knil '()))))
       => knil))

Iterating across Vectors and Strings

[syntax] (IN-VECTOR <vector> [<low> [<high>]])
[syntax] (IN-STRING <string> [<low> [<high>]])
[syntax] (IN-VECTOR-REVERSE <vector> [<high> [<low>]])
[syntax] (IN-STRING-REVERSE <string> [<high> [<low>]])


   (FOR <element> [<index>] (IN-VECTOR         <vector> [<low> [<high>]]))
   (FOR <element> [<index>] (IN-STRING         <string> [<low> [<high>]]))
   (FOR <element> [<index>] (IN-VECTOR-REVERSE <vector> [<high> [<low>]]))
   (FOR <element> [<index>] (IN-STRING-REVERSE <string> [<high> [<low>]]))

These iterate for each successive index in the vector or string, respectively, binding the variable <index>, if supplied, to that index, and the variable <element> to the element at that index.

IN-VECTOR and IN-STRING run from <low>, inclusive, to <high>, exclusive; IN-VECTOR-REVERSE and IN-STRING-REVERSE run from <high>, exclusive, to <low>, inclusive.

<Vector> or <string> must be a vector or a string, respectively. <Low> and <high>, if supplied, must be exact, non-negative integers, and valid bounds for <string> or <vector>. The default values of <low> and <high> are 0 and the length of the vector or string, respectively. <Vector>, <string>, <low>, and <high> are all evaluated once before the loop begins.

<Index> is a loop variable; its value in the final expression is the value of the last bound of iteration (the value of <high> for IN-VECTOR and IN-STRING; the value of <low> for IN-VECTOR-REVERSE and IN-STRING-REVERSE). With the reverse iterators, IN-VECTOR-REVERSE and IN-STRING-REVERSE, explicitly updating <index> when continuing the loop will cause the next value seen by the loop body to be one *less* than what the index was updated to. That is, on loop entry, <index> is assumed to be an exclusive upper bound, and so it is tested for termination and then decremented to find the actual index.

<Element> is a body variable.

Because the lower bound is inclusive and the upper bound is exclusive, for IN-VECTOR and IN-STRING, the first value of <index> is <low>, and the value of <index> in the final expression is <high>; while for IN-VECTOR-REVERSE and IN-STRING-REVERSE, the first value of <index> in the loop body is one less than <high>, and the value of <index> in the final expression is <low>.

   (loop ((for a i (in-vector '#(foo bar baz)))
          (for b j (in-string-reverse "abcdefghi" 6 3)))
     => (list i j)
     (write (list a i b j))
   ;Value: (3 3)
   ;; Output:
   ;(foo 0 f 5)
   ;(bar 1 e 4)
   ;(baz 2 d 3)
   ;;; Find the first index of an element satisfying a predicate.
   (define (vector-index vector predicate)
     (loop proceed ((for elt index (in-vector vector)))
       (if (predicate elt)
   ;;; Copy characters from one string to another.
   (define (string-copy! target tstart source sstart send)
     (loop ((for char (in-string source sstart send))
            (with index tstart (+ index 1)))
       (string-set! target index char)))
   (loop proceed 
       ((for v i
           '#(a b c d e f g h i j k l m n o p q r s t u v w x y z))))
     (write (list v i))
     (proceed (=> i (+ 1 (* i 2)))))
   ;; Output:
   ;(a 0)
   ;(b 1)
   ;(d 3)
   ;(h 7)
   ;(p 15)

Iterating through Input

[syntax] (IN-PORT <input-port> [<reader> [<eof?>]])


 (FOR <datum> (IN-PORT <input-port> [<reader> [<eof?>]]))

Iterates for successive data read from <input-port>, binding the variable <datum> to the datum read by applying the value of <reader>. Iteration terminates when the datum read satisfies the predicate <eof?>.

<Input-port> is usually an input port. <Reader> and <eof?>, if supplied, must be unary procedures. The default value of <reader> is READ-CHAR, and the default value of <eof?> is EOF-OBJECT?. <Input-port>, <reader>, and <eof?> are all evaluated once before the loop begins.

<Datum> is an entry variable.

   (define (read-line input-port)
     (let ((initial (peek-char input-port)))
       (if (eof-object? initial)
           (loop ((for char (in-port input-port))
                  (until (char=? char #\newline))
                  (with chars '() (cons char chars)))
             => (list->string (reverse chars))))))
   (define (read-all input-port)
     (loop ((for datum (in-port input-port read))
            (with data '() (cons datum data)))
       => (reverse data)))
[syntax] (IN-FILE <pathname> [<reader> [<eof?>]])


 (FOR <datum> (IN-FILE <pathname> [<reader> [<eof?>]]))

Opens an input port from the file named by <pathname>, iterates as with IN-PORT from the newly opened input port, and closes the input port after iteration terminates.

<Datum> is an entry variable.

   (define (read-lines-from-file pathname)
     (loop ((for line (in-file pathname read-line))
            (with lines '() (cons line lines)))
       => (reverse lines)))
   (define (file-length pathname)
     (loop ((for char (in-file pathname))
            (with count 0 (+ count 1)))
       => count))

Iterating over Numbers

[syntax] (UP-FROM <low> [(TO <high>)] [(BY <step>)])
[syntax] (DOWN-FROM <high> [(TO <low>)] [(BY <step>)])


   (FOR <number> (UP-FROM   <low>  [(TO <high>)] [(BY <step>)]))
   (FOR <number> (DOWN-FROM <high> [(TO <low>)]  [(BY <step>)]))

Iterates for each successive number starting at <low> and incrementing by <step> at every iteration, for UP-FROM; or starting at <high> and decrementing by <step> at every iteration, for DOWN-FROM. <Number> is bound to the current number. If the TO parameter is supplied, iteration terminates when the number exceeds the given bound.

<Low>, <high>, and <step> must all be exact numbers. If a TO parameter is supplied, they must furthermore all be real numbers. They are all evaluated once before the loop begins.

<Number> is a loop variable in UP-FROM. In DOWN-FROM, the treatment of <number> is somewhat subtle in order to do the right thing as much as possible, illustrated in the examples (to which the baffled reader may wish to skip immediately). <Number> is a loop variable, and alternately an entry variable or a body variable, depending on the use of DOWN-FROM. If DOWN-FROM is given a lower bound, then <number> is a body variable, and its value will never descend below <low> as a body variable. The final value of the variable will be not <low>, but the least value it attains before descending below <low>, which may or may not be <low>. If DOWN-FROM is not given a lower bound, then <number> is an entry variable, initialized to <step> less than the value of the loop variable <number>. There are two consequences to this, and two reasons motivating it:

(C1) After updating <number> as a loop variable to some number N, <number> as a body variable in the next iteration will be <step> *less* than N.

(C2) <Number> will be initialized to the value of <high>, but unless the loop terminates with zero iterations, neither the loop variable <number> nor the body variable <number> will ever be observed with the value <high>. In the rare case that the loop terminates with zero iterations, <number> may be observed in the final expression with the value of <high>.

(R1) As repeated in the introduction, lower bounds are inclusive, and upper bounds are exclusive. However, <high> - <low> may not be an integral multiple of <step>, so that <number> may never be <low>. With the semantics chosen, <number> will at least always be in the interval [<low>, <high>], and in fact as a body variable always in the interval [<low>, <high>); only in the rare case of a loop terminated after zero iterations will <number> be observable with the value <high>. (Of course, with explicit updates, it may take on any value.)

(R2) When there is no lower bound, it is often useful (a) to write user-supplied termination conditions in terms of <number>, and (b) to use, in the final expression, the value of <number> that induced termination of the loop. The example of a vector quick-sorting routine below illustrates such a use of <number> in scanning backward from the end of a subvector past elements greater than the pivot.

The more persevering reader, no doubt now exhausted by the above tortuous contortions of explanation, will be relieved to escape into a collection of illustrative examples.

   (define (iota count start step)
     (loop ((for n (up-from 0 (to count)))
            (for result (listing (+ start (* n step)))))
       => result))
   ;;; Note that the following is incorrect, if the arguments to IOTA
   ;;; may be inexact numbers:
   (define (iota count start step)
     (loop ((for n (up-from start
                            (to (+ start (* count step)))
                            (by step)))
            (for result (listing n)))
       => result))
   (define (sieve n)
     (let ((table (make-bit-string (- n 2) #t)))
       (define (prime? k) (bit-string-ref table (- k 2)))
       (define (not-prime! k) (bit-string-clear! table (- k 2)))
       (define (purge-multiples i)
         (loop ((for j (up-from (* i i)
                                (to n)
                                (by i))))
           (not-prime! j)))
       (loop proceed ((for i (up-from 2 (to n)))
                      (with primes '()))
         => (reverse primes)
         (if (prime? i)
             (begin (purge-multiples i)
                    (proceed (=> primes (cons i primes))))
   (define (vector-quick-sort! elt< vector start end)
     (loop sort ((start start) (end end))
       (if (< 1 (- end start))
           (let ((pivot (select-pivot vector start end)))
             (loop continue ((i start) (j end))
               (let ((i (loop ((for i (up-from i))
                               (while (elt< (vector-ref vector i) pivot)))
                          => i))
                     (j (loop ((for j (down-from j))
                               (while (elt< pivot (vector-ref vector j))))
                          => j)))
                 (if (< i j)
                     (begin (vector-exchange! vector i j)
                            (continue (+ i 1) j))
                     (begin (sort (=> end i))
                            (sort (=> start (+ j 1)))))))))))


Accumulating iterators all follow the same basic forms: accumulate each value of an expression into a result, with an optional condition for the accumulation. Accumulators differ according to the way that they accumulate, whether the intermediate result of accumulation is available in the body of the loop or whether only the fully accumulated result is available in the final expression, and whether the accumulator accepts an optional initial value argument.

Accumulators generally have the following forms:

   (FOR <result> (<accumulating> <datum>))

<Datum> is evaluated and its value accumulated.

   (FOR <result> (<accumulating> <datum> (IF <condition>)))

<Condition> is evaluated; if it yields true, <datum> is evaluated and accumulated.

   (FOR <result> (<accumulating> <condition> => <mapper>))

<Condition> is evaluated; if it yields true, <mapper> is evaluated and its value applied to the value of <condition>. What <mapper> returns is accumulated.

   (FOR <result> (<accumulating> <generator> <tester> => <mapper>))

<Generator> and <tester> are evaluated, and the value of <tester> applied to arguments of the values yielded by <generator>. If <tester> returns true, <mapper> is evaluated and its value applied also to arguments of the values yielded by <generator>. What <mapper> returns is accumulated.

Subexpressions such as <datum>, <condition>, &c., are evaluated in an environment with all loop, entry, and body variables bound from the current iteration. Some accumulators also have initial values, supplied with an optional (INITIAL <initial-value>) form before any of the other arguments to the accumulator.

Accumulators that are allowed to operate by destructively modifying internal state are marked with ! suffixes. These accumulators are /non-reentrant/, which means that it is unsafe to write loops that may be recursively run more than once, and that non-local re-entry may destroy values returned from the loop.

Accumulating Lists

[syntax] (LISTING [(INITIAL <tail>)] ...)
[syntax] (LISTING-REVERSE [(INITIAL <tail>)] ...)


    (FOR <result> (LISTING [(INITIAL <tail>)] ...))
    (FOR <result> (LISTING-REVERSE [(INITIAL <tail>)] ...))

These each produce lists of the accumulated data with a final cdr of <tail>. LISTING yields a list in the order that the data were produced; LISTING-REVERSE yields a list in the reverse order of that.

<Tail>, if supplied, is evaluated once before the beginning of the loop. The default value of <tail> is the empty list.

For LISTING, <result> is a final variable.

For LISTING-REVERSE, <result> is a loop variable; its value in the final expression is the accumulated list in reverse.

   (define (list-tabulate length procedure)
     (loop ((for i (up-from 0 (to length)))
            (for list (listing (procedure i))))
       => list))
   (define (take list count)
     (loop ((for i (up-from 0 (to count)))
            (for elt (in-list list))
            (for prefix (listing elt)))
       => prefix))
   (define (append-reverse list tail)
     (loop ((for elt (in-list list))
            (for result (listing-reverse (initial tail) list)))
       => result))
   (define (unzip5 list)
     (loop ((for component (in-list list))
            (for result1 (listing (first component)))
            (for result2 (listing (second component)))
            (for result3 (listing (third component)))
            (for result4 (listing (fourth component)))
            (for result5 (listing (fifth component))))
       => (values result1 result2 result3 result4 result5)))
[syntax] (APPENDING [(INITIAL <tail>)] ...)
[syntax] (APPENDING-REVERSE [(INITIAL <tail>)] ...)


   (FOR <result> (APPENDING [(INITIAL <tail>)] ...))
   (FOR <result> (APPENDING-REVERSE [(INITIAL <tail>)] ...))

These append the accumulated data into a list with a final cdr of <tail>. APPENDING maintains the order of the lists and their elements; APPENDING-REVERSE reverses the elements of the lists. The accumulated data must all be proper lists.

<Tail>, if supplied, is evaluated once in the environment outside the loop. The default value of <tail> is the empty list.

For APPENDING, <result> is a final variable.

For APPENDING-REVERSE, <result> is a loop variable; its value in the final expression is the reverse-concatenated list.

   (define (concatenate lists)
     (loop ((for list (in-list lists))
            (for result (appending list)))
       => list))
[syntax] (LISTING! [(INITIAL <tail>)] ...)
[syntax] (LISTING-INTO! <pair> [(INITIAL <tail>)] ...)


   (FOR <result> (LISTING! [(INITIAL <tail>)] ...))
   (FOR <result> (LISTING-INTO! <pair> [(INITIAL <tail>)] ...))

LISTING! builds up a list with a final cdr of <tail>. LISTING-INTO! builds up a list with a final cdr of <tail> and destructively stores it in the cdr of <pair>. Both use as little intermediate storage as possible, which may involve using destructive operations, and as a consequence are non-reentrant.

<Tail>, if supplied, is evaluated once in the environment outside the loop. The default value of <tail> is the empty list.

<Result> is a final variable.

Accumulating Numerical Results

[syntax] (SUMMING [(INITIAL <initial-sum>)] ...)
[syntax] (MULTIPLYING [(INITIAL <initial-sum>)] ...)


   (FOR <result> (SUMMING [(INITIAL <initial-sum>)] ...))
   (FOR <result> (MULTIPLYING [(INITIAL <initial-product>)] ...))

Accumulates sums or products by adding the accumulated data to <initial-sum>, or multiplying the accumulated data by <initial-product>.

<Initial-sum> and <initial-product>, if supplied, must be numbers, and are evaluated once before the loop begins. The default value of <initial-sum> is 0, and the default value of <initial-product> is 1.

<Result> is a loop variable.

   (define (count predicate list)
     (loop ((for elt (in-list list))
            (for count (summing 1 (if (predicate elt)))))
       => count))
[syntax] (MINIMIZING [(INITIAL <initial-minimum>)] ...)
[syntax] (MAXIMIZING [(INITIAL <initial-maximum>)] ...)


   (FOR <result> (MINIMIZING [(INITIAL <initial-minimum>)] ...))
   (FOR <result> (MAXIMIZING [(INITIAL <initial-maximum>)] ...))

Finds the minimum or maximum datum accumulated. If an initial value expression is provided, it is evaluated once before the beginning of the loop, all of the data involved must be real numbers, and <result> will be a real number. If no initial value is provided, the data must be either real numbers or false, and <result> will be either a real number or false.

<Result> is a loop variable.

Writing Custom Iterators

(NOTE: This section documents the implementation of iterators for Taylor R. Campbell's current version of foof-loop. It does *not* document the version originally written by Alex Shinn (after whose nickname, foof, the system is named), which has a somewhat less flexible interface for custom iterators.)

Extensibility is a primary goal of foof-loop. Writing custom iterators is a matter of specifying the syntax of the iterator, all of the loop variables and other variables bound by the loop, and the loop's termination conditions. In order to implement foof-loop in portable SYNTAX-RULES, iterators are macros in continuation-passing style.

When LOOP encounters a clause of the form

 (FOR <variable> ... (<iterator> . <arguments>)),

it invokes <iterator> as (<iterator> (<variable> ...) <arguments> <continuation> . <environment>), where <environment> is information that LOOP needs in order to continue processing the other loop clauses, and where <continuation> is a macro that returns control back to LOOP given a description of all the variables bound by and termination conditions tested for the iterator at hand.

The continuation should be invoked as follows:

  ((<outer-bvl> <outer-producer>) ...)
  ;; Outer variables to be bound once outside the whole loop.
  ;; Each <outer-producer> is evaluated once in the enclosing
  ;; environment of the LOOP form.
  ((<loop-variable> <initializer> <update>) ...)
  ;; Loop variables.  Each <initializer> is evaluated in an
  ;; environment where the outer variables are bound.  Each <update>
  ;; is evaluated in an environment where the outer variables, loop
  ;; variables, entry variables, and body variables are all bound.
  ((<entry-bvl> <entry-producer>) ...)
  ;; Entry variables, to be bound each time the loop is entered.
  ;; Each <entry-producer> is evaluated in an environment where the
  ;; outer variables and loop variables are bound.
  (<termination-condition> ...)
  ;; Conditions that will terminate the loop.  These are all evaluated
  ;; in an environment where the outer variables, loop variables, and
  ;; entry variables are bound.  The first condition, of any loop
  ;; clause, that holds true will terminate the iteration, short-
  ;; circuiting the rest of the termination conditions.
  ((<body-bvl> <body-producer>) ...)
  ;; Variables to be bound around the body of the loop, if the loop
  ;; does not terminate.
  ((<final-bvl> <final-producer>) ...)
  ;; Variables to be bound around the final expression of the loop, if
  ;; the loop does terminate.
  . environment).

The <*-bvl>s are bound variable lists, and the variables in them are bound to the respective values of the corresponding <*-producer> expressions, as with LET-VALUES. The bindings are made in parallel, also as with LET-VALUES. To make interdependent bindings, use multiple return values in a single binding clause. See the section titled Loop Expansion for the placement of these parts in the code generated by LOOP.

The order of the arguments to the continuation is intended to reflect the order of control through the loop: first the outer variables are bound, then the loop variables are initialized, then the entry variables are bound, then the termination conditions are tested, and finally either the body variables bindings or the final variables are bound, depending on the termination conditions.

For example, here is a simplified definition of IN-LIST, omitting support for a user-supplied loop variable, a user-supplied list successor procedure, and the guarantee that SET-CDR! may be safely applied to the pair without altering the iteration. For the sake of concision, we use the name next for the continuation and rest for the environment. In this simplified IN-LIST, we expect one variable in FOR and one argument after IN-LIST -- i.e., (FOR <element-variable> (IN-LIST <list-expression>)) --; hence, the initial two arguments to the actual macro are (element-variable) and (list-expression).

 (define-syntax in-list
   (syntax-rules ()
     ((IN-LIST (element-variable) (list-expression) next . rest)
      (next (((LIST) list-expression))      ;Outer bindings
            ((PAIR LIST (CDR PAIR)))        ;Loop variables
            ()                              ;Entry bindings
            ((NOT (PAIR? PAIR)))            ;Termination conditions
            (((element-variable)            ;Body bindings
              (CAR PAIR)))
            ()                              ;Final bindings
            . rest))))

Reporting Syntax Errors

The above definition of IN-LIST will work, but, if used incorrectly, it would yield a strange syntax error incomprehensible to the casual user of foof-loop, involving a great deal of state internal to the LOOP macro. This is not what users like to see; users like to see useful, comprehensible error messages that are directly related to what they wrote. foof-loop provides two macros for reporting syntax errors: SYNTACTIC-ERROR and LOOP-CLAUSE-ERROR. SYNTACTIC-ERROR is a general way to report errors in macros:

 (SYNTACTIC-ERROR <message> <form> ...)

will cause an error to be reported when this form is expanded, ideally involving the given message and subforms. Many loop iterators are implemented in terms of internal utilities handling the general patterns of several specialized cases, such as IN-VECTOR and IN-STRING, which generate very similar code. In order to report specific syntax errors that correspond with what was actually the input to the LOOP macro, these iterators pass error context descriptors to one another. Error context is of the form

 (<name> (<variable> ...) <arguments> <message>),

where <name> is the name of the iterator, the <variable>s are the forms passed in the FOR clause before the iterator, <arguments> is the tail of the iterator subform inside the FOR clause, and <message> is a message explaining the error. Because of limitations in SYNTAX-RULES, the error reporting macros cannot construct a message containing <name>; thus <message> should identify the name of the iterator. The macro LOOP-CLAUSE-ERROR invokes SYNTACTIC-ERROR with a FOR form that corresponds with the original FOR form seen by LOOP:

 (LOOP-CLAUSE-ERROR (<name> (<variable> ...) <arguments> <message>))

expands to

                  (FOR <variable> ...
                       (<name> . <arguments>))).

Using LOOP-CLAUSE-ERROR for more robust syntax error notification, we can write the simplified IN-LIST by adding a fall-through syntax rule that matches any possible invocation of the iterator from LOOP:

 (define-syntax in-list
   (syntax-rules ()
     ((IN-LIST (element-variable) (list-expression) next . rest)
      (next (((LIST) list-expression))      ;Outer bindings
            ((PAIR LIST (CDR PAIR)))        ;Loop variables
            ()                              ;Entry bindings
            ((NOT (PAIR? PAIR)))            ;Termination conditions
            (((element-variable)            ;Body bindings
              (CAR PAIR)))
            ()                              ;Final bindings
            . rest))
     ((IN-LIST variables arguments next . rest)
      ;; Ignore NEXT and REST; we only report the syntax error.
       (IN-LIST variables arguments
                "Malformed IN-LIST form in LOOP:")))))

With the appropriate definition of SYNTACTIC-ERROR shown below, if we misuse IN-LIST, we should get a helpful error message in most Schemes:

 (loop ((for x y z (in-list)))
   (body x y z))
 Syntax error: Malformed IN-LIST clause in LOOP:
               (for x y z (in-list))

Pitfalls of Reporting Syntax Errors

The principle behind passing error context to any macros in terms of which iterators are implemented is to present coherent syntax errors reporting the original iterator form. In an ideal world, this would always be easy, but because we are limited to the tools of SYNTAX-RULES and SYNTAX-RULES alone, this is not as easy as we should like. There are cases where it is convenient, for example, to supply default values for optional parameters by recursively calling the iterator macro. This means, though, that if there is a syntax error for some other reason -- for example, if some of the arguments are named, or there are optional variables *and* optional arguments --, which goes undetected until some default values have been supplied, the wrong form will be reported to the user.

To avoid this, it is better to rewrite iterators into invocations of internal macros that take error context as an extra parameter. This precludes writing one iterator in terms of another one, however, which is an unfortunate consequence of the way that foof-loop is designed, which is itself an unfortunate consequence of the tools available. We should like to be able to have a macro debugger that can examine the expansion history of the forms leading up to a syntax error, but this is not a reliably portable concept; few Schemes even record the expansion history. (I can name only one: MIT Scheme.)

Foof-loop provides no facilities for checking syntax errors beyond what can be detected by SYNTAX-RULES patterns. For example, there is no way to verify that a variable to be bound is, in fact, a variable; if the user writes anything other than a variable, there will probably be a very strange error much later on involving invalid definitions with strange lambda lists. It is not difficult to imagine the interface to such checks; however, they are considerably more trouble to implement than they would be worth in program development at the moment.

Design Notes and Rationale

For a comparison of (an older rendition of) foof-loop with other contemporary looping macros, see the Usenet article with message-id <> posted on comp.lang.scheme in September of 2006. Also, for extended comparative examples, see These design notes are specifically about Taylor R. Campbell's variant.

Upper and Lower Bounds on Intervals

In the iterators provided, lower bounds are always included and upper bounds are always excluded, whether one starts from the upper bound and descends to the lower bound, or whether one starts from the lower bound and ascends to the upper bound.

Inconsistent bounds make programs confusing. Strict adherence to the inclusive-lower/exclusive-upper convention is the only sensible option. Between the R5RS, foof-loop, and many libraries that already adhere to it, this convention may not eliminate all off-by-one errors, but it makes programs much easier to write correctly from the start. Reverse iteration, from more positive to more negative numbers, is an especally sordid source of off-by-one errors.

If you write any of your own custom iterators that involve upper or lower bounds, please adhere to these rules. If you have an extremely good reason not to adhere to these rules, I'd like to hear it.