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Inspired by Paul Graham's classic "On Lisp" this module introduces anaphoric macros, which are unhygienic by design. Hence they can not be implemented with syntax-rules! In fact, they introduce new identifiers behind the scene, mostly named it, which can be referenced in the body without being declared. With the exception of define-anaphor, which is hygienic, and define-properties, which introdues as many suffixed names as there are name arguments given, all macros in this module start with an a prefix to remind the user, that they pollute the namespace on purpose. Corresponding operators without this prefix are well known.
Remember, that in a natural language an anaphor is an expression, which refers back in the conversation: "Buy this book and read it."
anaphora[syntax] (anaphora sym)
where sym is optional. Called without argument it returns the list of exported symbols, with one of those symbols as argument it returns its call-structure.
aif[syntax] (aif test consequent [alternative])
Anaphoric version of if. Binds the result of test to the symbol it, which can than be used in the consequent or the mandatory anternative.
(aif (memv 3 '(1 2 3 4 5)) it #f)
awhen[syntax] (awhen test xpr . xprs)
Anaphoric version of when, i.e. the one-armed if, which allows multiple expressions to be evaluated, if test succeeds. As with aif, the result of test is stored in the variable it, which can be refered in xpr ...
(awhen (memv 3 '(1 2 3 4 5)) (print it) (reverse it))
acond[syntax] (acond ((test xpr ...) ... [(else xpr1 ...)]))
Anaphoric version of cond. The result of each test is stored in the variable it, which can be used in the corresponding expressions xpr ...
(acond ((memv 6 '(1 2 3 4 5)) it) ((memv 3 '(1 2 3 4 5)) it) (else it))
Note, that the ordinary cond macro does something similar with the literal symbol =>.
awhile[syntax] (awhile test xpr . xprs)
Anaphoric version of while.
The body xpr . xprs is evaluated as often, as the test is true. As usual, the result of this test, which often is the result of a poll operation, is named it and can be referenced in the body.
(let ((lst '(1 2 3 4 5 #f)) (res '())) (awhile (car lst) (set! res (cons (car lst) res)) (set! lst (cdr lst))) res)
Of course, this is not the preferred programming style in Scheme ...
aand[syntax] (aand arg ...)
Anaphoric version of and. When sequentially evaluating the arguments arg ..., the anaphor it will be bound to the value of the previous argument.
(let ((lst '(1 2 3))) (aand lst (cdr it) (cdr it)))
alambda[syntax] (alambda args xpr . xprs)
Anaphoric version of lambda.
The resulting procedure is bound to the anaphor self. This way, anonymous functions can be recursive as well.
(map (alambda (n) (if (zero? n) 1 (* n (self (- n 1))))) '(1 2 3 4 5))
define-anaphor[syntax] (define-anaphor name from rule)
Hygienic macro which writes an anaphoric macro with implicit it, name, from another procedure or macro, from, with rule either #:cascade or #:first Note, that most of the macros above could have been created by means of define-anaphor.
(define-anaphor alist list #:cascade) (alist 1 (+ it 2) (* it 3)) ; -> '(1 3 9) (define-anaphor awhen when #:first) (awhen (* 1 2 3 4 5) (* 2 it)) ; -> 240
define-properties[syntax] (define-properties name ...)
Abstracting away get and put!. Defines for each name two macros, name and name!, the first being an accessor to the property name, the second the corresponding mutator.
(define-properties color weight) (color! 'foo 'red) (color 'foo) ; -> 'red (weight! 'foo 5) (weight 'foo) ;-> 5 (color! 'foo 'blue) (color 'foo) ; -> 'blue (weight! 'foo 50) (weight 'foo) ; -> 50
list-recurser[procedure] (list-recurser recurser base)
generates a procedure which traverses on the cdrs of its only list argument.
(define (lsome? ok?) (list-recurser (lambda (lst th) (or (ok? (car lst)) (th))) #f)) ((lsome? odd?) '(2 4 5 6)) ; -> #t
alist-recurser[syntax] (alist-recurser recur-xpr base-xpr)
anaphoric macro with internal symbols it, representing the current list, and go-on, representing traversal along cdrs. The result is a procedure, which recurs on its only list argument.
(define (alsome? ok?) (alist-recurser (or (ok? (car it)) (go-on)) #f)) ((alsome? odd?) '(2 4 6)) ; -> #f
tree-recurser[procedure] (tree-recurser recurser base)
generates a procedure which traverses on the cars and the cdrs of its only tree argument.
(define tcopy (tree-recurser (lambda (tree left right) (cons (left) (or (right) '()))) identity)) (tcopy '(1 (2 (3 4) 5) 6)) ; -> '(1 (2 (3 4) 5) 6)
atree-recurser[syntax] (atree-recurser recur-xpr base-xpr)
anaphoric macro with internal symbols it, representing the current tree, go-left and go-right, representing traversal along cars and cdrs respectively. The result is a procedure, which recurs on its only list argument.
(define atcopy (atree-recurser (cons (go-left) (or (go-right) '())) it)) (atcopy '(1 (2 3 (4)) 5)) ; -> '(1 (2 3 (4)) 5)
Nov 02, 2013
Copyright (c) 2011-2013, Juergen Lorenz All rights reserved.
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- added define-anaphor, define-properties, tree-recurser, atree-recurser, list-recurser and alist-recurser
- helper-module anaphora-helper removed
- documentation dispatcher added
- initial import