Destructuring sequence expressions with bindings (historical revision 33030)

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Destructuring sequence expressions with bindings

Automatic destructuring of expressions is a handy feature, which can be successfully used in writing procedural macros, for example. Some programming languages use it extensively, in particular ML and its descendents Haskell and Miranda. And Chicken offers an egg called matchable, which does it as well.

This library provides an alternative to matchable, a bunch of macros, all starting with the bind prefix, and all being derived from bind and related procedures. They all destructure arbitrary mixtures of (pseudo-) lists, vectors and strings, which match a pattern, and can be easily enhanced, to accept other sequence types as well, arrays, for example.

The syntax of the fundamental bind macro is as follows

[syntax] (bind pat (where fender ...) .. seq xpr ....)

Here, a pattern, pat, is a nested psudolist of (mostly) symbols, seq a sequencce expression, i.e. a mixture of pseudolists, vectors and strings, fender a check expression on pattern variables and xpr .... constitute the body of the macro. Note the special use of dots here and below: Three dots repeat the expression to the left zero or many times, two dots zero or one times and four dots one or many times.

This macro binds pattern variables, i.e. symbols of pat, to corresponding sequenceds of seq, checks, if the fenders succeed and exectutes the body in this context.

The bind macro is patterned after Paul Graham's dbind, cf. his classic "On Lisp", p. 232. Here is a Chicken version for lists:

(define-syntax dbind
  (ir-macro-transformer
    (lambda (form inject compare?)
      (letrec (
        (mappend
          (lambda (fn lists)
            (apply append (map fn lists))))
        (destruc
          (lambda (pat seq)
            (let loop ((pat pat) (seq seq) (n 0))
              (if (pair? pat)
                (let ((p (car pat)) (recu (loop (cdr pat) seq (+ n 1))))
                  (if (symbol? p)
                    (cons `(,p (list-ref ,seq ,n)) recu)
                    (let ((g (gensym)))
                      (cons (cons `(,g (list-ref ,seq ,n))
                                  (loop p g 0))
                            recu))))
                (if (null? pat)
                  '()
                  `((,pat (list-tail ,seq ,n))))))))
        (dbind-ex
          (lambda (binds body)
            (if (null? binds)
              `(begin ,@body)
              `(let ,(map (lambda (b) (if (pair? (car b)) (car b) b))
                          binds)
                 ,(dbind-ex
                    (mappend (lambda (b) (if (pair? (car b)) (cdr b) '()))
                             binds)
                    body)))))
        )         
        (let ((pat (cadr form))
              (seq (caddr form))
              (body (cdddr form))
              (gseq 'seq))
          `(let ((,gseq ,seq))
             ,(dbind-ex (destruc pat gseq) body)))))))

This code works as follows: First, destruc traverses the pattern and groups each symbol with some list accessing code, using gensyms to step down the pattern while grouping the gensym bound object with all pairs depending on this gensym. So, for example,

(destruc '(a (b . c) . d) 'seq)

will result in

((a (list-ref seq 0))
 ((#:g (list-ref seq 1)) (b (list-ref #:g 0)) (c (list-tail #:g 1)))
 (d (list-tail seq 2)))

This tree is then transformed via dbind-ex into a nested let

(let ((a (list-ref seq 0))
      (#:g (list-ref seq 1))
      (d (list-tail seq 2)))
  (let ((b (list-ref #:g 0))
        (c (list-tail #:g 1)))
    body))

Note, that the destructuring procedures are local to this macro. This is necessary in Chicken for the macro to work, in particular in compiled code, unless you define them for-syntax, but that would implicitly export them. But since they are of no interest outside of the macro, local procedures are preferable.

Note further, that ir-macro-transformer does all the necessary renaming transparently behind the scene, even if the helpers where defined in another module. In particular, gseq needn't be a gensym.

And note, that Graham's code didn't check for seq's length, i.e. (dbind (a b) '(1 2 3) (list a b) would happily return '(1 2).

Graham's original code works on the sequence datatype, so vectors and strings are destructured as well. Sequences don't exist in Scheme, unless you import-for-syntax Felix' sequences egg. To make this module self-contained, I prefer to supply access-routines closed over a table, which provides sequence versions of list-ref and list-tail, the only sequence routines used by destruc above, as well as a sequence version of length, which is needed to do the length checks.

There are some features, which I would like to have and which are implemented as well. First wildcards, represented by the underscore symbol. It matches everything, but binds nothing. So it can appear multiple times in the same macro. Wildcard symbols are simply not collected in the destruc routine.

Second, non-symbol literals, which don't bind anything, of course, but match only expressions evaluating to themselves. This and the length checks are treated simply by pairing them as well with check-routines in destruc but separating the pairs with leading symbol from those with leading nil or literal in dbind-ex. The former are bound with lets as in Graham's code, the latter's cadrs being evaluated before the recursive call to dbind-ex.

The last feature missing is fenders, which is important in particular for bind-case and can easily be implemented with a where clause: A pattern matches successfully if only each pattern variable can be bound and the checks as well as the where clause are satisfied. If the where clause doesn't pass, the next pattern is tried in bind-case or a bind-exception is signalled in bind.

(use bindings)

Examples


(use bindings)

(let ((stack #f) (push! #f) (pop! #f))
  (bind-set! (stack (push! pop!))
    (list
      '()
      (vector
        (lambda (xpr) (set! stack (cons xpr stack)))
        (lambda () (set! stack (cdr stack))))))
  (push! 1)
  (push! 0)
  stack)
; -> '(0 1)

(begin
  (bind-define (top push! pop!)
    (let ((lst '()))
      (vector
        (lambda () (car lst))
        (lambda (xpr) (set! lst (cons xpr lst)))
        (lambda () (set! lst (cdr lst))))))
  (push! 0)
  (push! 1)
  (pop!)
  (top))
; -> 0

(bind a 1 a)
; -> 1

(bind (x y z w) '(1 2 3 4) (list x y z w))
; -> '(1 2 3 4)

(bind (x . y) '#(1 2 3 4) (list x y))
; -> '(1 #(2 3 4))

(bind (x (y (z u . v)) w) '(1 #(2 "foo") 4)
  (list x y z u v w))
; -> '(1 2 #\f #\o "o" 4)

(bind (x (y (z . u)) v . w) (vector 1 (list 2 (cons 3 4)) 5 6)
  (list x y z u v w))
; -> '(1 2 3 4 5 #(6))

((bind-lambda (a (b . C) . d)
   (list a b C d))
 '(1 #(20 30 40) 2 3))
; -> '(1 20 #(30 40) (2 3))

((bind-lambda* ((a (b . C) . d) (e . f))
   (list a b C d e f))
 '(1 #(20 30 40) 2 3) '#(4 5 6))
; -> '(1 20 #(30 40) (2 3) 4 #(5 6))

(bind* loop (x (a . b) y) '(5 #(1) 0)
  (if (zero? x)
    (list x a b y)
    (loop (list (- x 1) (cons a (cons a b)) (+ y 1)))))
; -> '(0 1 (1 1 1 1 1 . #()) 5)

(bind* loop (x y) '(5 0)
  (if (zero? x)
    (vector x y)
    (loop (vector (- x 1) (+ y 1)))))
; -> '#(0 5)

(bind-let (((x y (z . w)) '(1 2 #(3 4 5))))
  (list x y z w))
; -> '(1 2 3 #(4 5))

(bind-let (
  (((x y) z) '(#(1 2) 3))
  (u (+ 2 2))
  ((v w) '#(5 6))
  )
  (list x y z u v w))
; -> '(1 2 3 4 5 6)

(bind-let loop (((a b) '(5 0)))
  (if (zero? a)
    (list a b)
    (loop (list (list (- a 1) (+ b 1))))))
; -> '(0 5)

(bind-let loop (
  ((x . y) '(1 2 3))
  ((z) '#(10))
  )
  (if (zero? z)
    (list x y z)
    (loop (list (cons (+ x 1) (map add1 y)) (list (- z 1))))))
; -> '(11 (12 13) 0)

(bind-let* (
  (((x y) z) '(#(1 2) 3))
  (u (+ 1 2 x))
  ((v w) (list (+ z 2) 6))
  )
  (list x y z u v w))
; -> '(1 2 3 4 5 6)

(bindrec ((o?) e?)
  (vector (list (lambda (m) (if (zero? m) #f (e? (- m 1)))))
          (lambda (n) (if (zero? n) #t (o? (- n 1)))))
  (list (o? 95) (e? 95)))
; -> '(#t #f)

(bind-letrec (
  ((o? (e?))
   (list (lambda (m) (if (zero? m) #f (e? (- m 1))))
         (vector (lambda (n) (if (zero? n) #t (o? (- n 1)))))))
  )
  (list (o? 95) (e? 95)))
; -> '(#t #f)

((bindable? ()) '())
; -> #t

((bindable? (a (b C) . d)) '(1 (2 3) . 4))
; -> #t

((bindable? (a (b C) . d)) '(1 #(2 3) 4 5))
; -> #t

((bindable? (a (b . C) . d)) '(1 (2 3) 4))
; -> #t

((bindable? (a (b . C) . d)) '#(1 2 3 4 5))
; -> #f

((bindable? (a (b C) d)) '(1 (2 3) 4 5))
; -> #f

((bindable? (a b) (even? a)) '#(1 2))
; -> #f 

(bind-case '#(1 2)
  (() '())
  ((a) (list a))
  ((a b) (list a b))
  ((a b C) (list a b C)))
; -> '(1 2))

(define (my-map fn lst)
  (bind-case lst
    (() '())
    ((x . xs) (cons (fn x) (my-map fn xs)))))
(my-map add1 '(1 2 3)))
; -> '(2 3 4)

(define (vector-reverse vec)
  (let ((result (make-vector (vector-length vec) #f)))
    (let loop ((vec vec))
      (bind-case vec
        (() result)
        ((x . xs)
         (vector-set! result
                      (vector-length xs)
                      x)
         (loop (subvector vec 1)))))))
(vector-reverse #(0 1 2 3))
; -> #(3 2 1 0)

((bind-case-lambda
   ((a (b . C) . d) (list a b C d))
   ((e . f) (where (zero? e)) e)
   ((e . f) (list e f)))
 '(1 2 3 4 5))
; -> '(1 (2 3 4 5)))

((bind-case-lambda
   ((e . f) (where (zero? e)) f)
   ((e . f) (list e f)))
 '#(0 2 3 4 5))
;-> '#(2 3 4 5))

((bind-case-lambda
   ((a (b . C) . d) (list a b C d))
   ((e . f) (list e f)))
 '(1 #(2 3 4) 5 6))
; -> '(1 2 #(3 4) (5 6))

((bind-case-lambda*
   (((a b C . d) (e . f))
    (list a b C d e f)))
 '(1 2 3) '#(4 5 6))
; -> '(1 2 3 () 4 #(5 6))

((bind-case-lambda*
   (((a (b . C) . d) (e . f))
    (list a b C d e f)))
 '(1 #(20 30 40) 2 3) '(4 5 6))
; -> '(1 20 #(30 40) (2 3) 4 (5 6))

;;adding arrays to generic sequence table
(bind-table-add!  array?
                  array-length
                  (lambda (seq k)
                    (array-item k seq))
                  (lambda (seq k)
                    (array-drop k seq)))

(bind (x y z) (array 1 2 3) (list x y z))
;-> '(1 2 3)

(bind (x (y z)) (vector 0 (array 1 2)) (list x y z))
;-> '(0 1 2)

(bind (x (y . z)) (vector 0 (array 1 2 3 4)) (list x y z))
;-> '(0 1 @(2 3 4))

Copyright (c) 2011-2015, Juergen Lorenz
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Version History

4.1: macro-rules and friends moved to procedural-macros, where clauses now follow patterns
4.0: code completely rewritten,simplified and reorganized, some small syntax changes
3.5.2: code reorganized again to fix compile problems
3.5.1: code reorganized
3.5: let-macro and letrec-macro added, list-of? replaced by list-of to accept zero to many predicates
3.4: where and key clauses now work together in macro-rules, moreover, macro-rules can be used in define-macro, so it needn't be imported for-syntax
3.3: bindings library split in two; fixed a compile-time bug, thanks to John Foerch and Moritz Heidkamp
3.2: wild-card processing added
3.1: support for once formal parameters, rename-prefix clause moved to parameter, let-macro and letrec-macro changed to er-macro and renamed
3.0.1: bugfix in define-er-macro, bind/cc added, three macros moved to macro-helpers
3.0: library now includes low-level-macros
2.5: all macros are low-level. Module depends on low-level-macros
2.4.1: documentation procedure beautified
2.4: generic functions rewritten, records removed from tests
2.3.4: internal generic-pair? added again bugfixed
2.3.3: exception-handler added, internal generic-pair? removed
2.3.2: bind-case improved, format replaced by print
2.3: code partially rewritten, syntax of bindable? changed, matching records outsourced to the tests
2.2: renamed bind? to bindable?, bind/cc moved to list-bindings
2.1: generics (and hence bind and friends) now accept records, bind/cc added
2.0: bind-matches? and bind-loop changed to bindable? and bind*, where clauses and generic functions added, syms->vars removed
1.0: all binding macros can now destructure arbitrary nested sequences, i.e mixtures of lists, pseudo-lists, vectors and strings; dependency on contracts removed.
0.1: initial import