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A generic macro-expansion and code-walking and code-transformation facility.
[procedure] (expand EXPRESSION [HOOK [ARGUMENT ...]])
Recursively expands macros in EXPRESSION, calling the procedure HOOK (if provided). The hook is called with 5 or more arguments:
(HOOK EXPR CLASS WALK ENV MARK ARGUMENT ...)
EXPR is the currently traversed subexpression, CLASS is a symbol identifying the expression class, WALK is a procedure of one or more arguments that should be called to walk sub-expressions of EXPR recursively. The additional arguments ARGUMENT ... will be passed to the hook as additional parameters, which in turn should pass the same number of extra arguments to recursive invocations of WALK (plus the sub-expression to walk). ENV is a list of lists containing the lexical environment, where each list is a list of lexically bound identifiers (symbols). The frontmost list holds the current environment. MARK is a procedure of one argument that can be used to mark subexpressions which should not be traversed by HOOK.
Invoking WALK with the same (eq?) EXPR that has been given as the first argument to HOOK performs default processing and walking of subexpressions of EXPR.
Note that the default processing returns S-expressions. If you want to use expand to create some other type (for example an abstract syntax tree of dedicated node records), you have to provide treatment and translation for all expression classes.
[procedure] (expand/context EXPRESSION CONTEXT [HOOK [ARGUMENT ...]])
Similar to expand, but allows passing additional expansion-parameters, which should be created by calling expansion-context.
[procedure] (expansion-context #!key evaluate compilation unsafe cooked classes prehook environment)
Creates an expansion-context that is used to pass additional expansion parameters to expand/context. The keyword arguments have the following meaning:
- should macro-expansion time evaluation be performed (for example, in an eval-when clause) (boolean, default: #f).
- does the expansion happen during compilation (as opposed to interpretation) (boolean, default: #t when called during compilation or #f otherwise).
- does the expansion happen in an unsafe compilation context (boolean, default: #f).
- list of symbols identifying the classes of expressions for which the hook should be called during expansion (list or #t, default: #t).
- whether internal forms that perform expansion-time evaluation or other special processing (usually compiler-specific) be processed as they appear (#t) or whether they should be passed through to the hook function - passing #t for evaluate implies cooked mode.
- a procedure accepting at least 3 arguments, an expression, an environment and a continuation. This procedure will be called before any macroexpansion with the to-be-expanded expression in the first argument position. The continuation argument should be called with the actual expression to be expanded (or the original one) and any additional arguments. Returning from the prehook procedure without calling the continuation is undefined.
- the initial environment used in the expansion.
The following expression classes exist:
- variable reference
- literal constant
- quoted literal constant
- an undefined value - note that this means really undefined, not (void)
- if conditional
- begin sequence
- let binding form
- lambda expression
- procedure application
- compiler special form
- require-extension or require-for-syntax form (or internal equivalent)
- special internal form - should normally be treated as a call to an identity function
Note that external macro packages like syntax-case expand their input completely, so the prehook will already get macroexpanded forms.
[procedure] (expansion-context? X)
Returns #t if X is an expansion context, or #f otherwise.
[procedure] (install-toplevel-expander EXPANDER [OVERRIDE-MACROEXPAND])
Installs EXPANDER as the toplevel macro expander for subsequent interpretation or compilation. If OVERRIDE-MACROEXPAND is given and true, then macroexpand will also invoke the given expander and no further internal macroexpansion takes place. Use this when you want to install a custom macroexpansion package that does complete processing of syntax transformation. If you merely want to do some custom pre-processing, pass #f for this argument (or omit it completely).
(define exp '(begin (define (count-bits2 n result) (if (zero? n) result (count-bits2 (bitwise-and n (sub1 n)) (add1 result)))) (time (do ((i 0 (add1 i))) ((>= i 10)) (print (count-bits2 i 0))))) )
(pp (expand exp)) ; expand all macros
(define exp2 ; trace global variable references (expand exp (lambda (x c w e m) (if (and (symbol? x) (not (ormap (cut memq x <>) e))) (w (m `(let ((x ,x)) (print "ref: " ',x " = " x) x) ) ) (w x) ) ) ) )
(pp exp2) (pp (eval exp2)) ; the code can be evaluated as usual
(define exp3 ; trace all calls (expand/context exp (expansion-context classes: '(app)) (lambda (x c w e m) (let ((fn (gensym))) (w (m `((let ((,fn ,(car x))) (print "calling: " ,fn) ,fn) ,@(map w (cdr x) ) ) ) ) ) ) ) )
(pp exp3) (pp (eval exp3))
(define (instrument-calls exp pred? mod) ;; instrument calls that satisfy a given predicate (expand/context exp (expansion-context classes: '(app)) (lambda (x c w e m) (if (pred? x) (w (m (mod (map w x)))) (w x) ) ) ) )
(define exp4 ; trace calls (instrument-calls exp (constantly #t) (lambda (call) (let ((v (gensym))) `(begin (print "calling: " ',call) ,call) ) ) ) )
(pp exp4) (eval exp4)
;;;; count direct calls to "car"
(define n 0)
(define (count exp) (expand/context exp (expansion-context classes: '(app)) (lambda (x c w e m) (match x (('car arg) (inc! n) `(car ,(w arg)) ) (_ (w x)) ) ) ) )
(for-each count (read-file "codewalk.scm")) (pp n) ; should be "7"
Copyright (c) 2006, Felix L. Winkelmann All rights reserved.
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Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. Neither the name of the author nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.
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- certain internal lambda-forms didn't get properly expanded [Thanks to Benedikt Rosenau for reporting this]
- doesn't use interpreter-specific expansion hook anymore (doesn't work with syntax-case, otherwise)
- minor bugfixes, added support for "prehook" and "environment" context args
- initial release