diff --git a/srfi/.gitignore b/srfi/.gitignore new file mode 100644 index 0000000..1377554 --- /dev/null +++ b/srfi/.gitignore @@ -0,0 +1 @@ +*.swp diff --git a/srfi/2.scm b/srfi/2.scm new file mode 100644 index 0000000..f7e4112 --- /dev/null +++ b/srfi/2.scm @@ -0,0 +1,172 @@ +; Checking of a LAND* special form +; +; LAND* is a generalized AND: it evaluates a sequence of forms one after another +; till the first one that yields #f; the non-#f result of a form can be bound +; to a fresh variable and used in the subsequent forms. +; +; When an ordinary AND is formed of _proper_ boolean expressions: +; (AND E1 E2 ...) +; expression E2, if it gets to be evaluated, knows that E1 has returned non-#f. +; Moreover, E2 knows exactly what the result of E1 was - #t - so E2 can use +; this knowledge to its advantage. If E1 however is an _extended_ +; boolean expression, E2 can no longer tell which particular non-#f +; value E1 has returned. Chances are it took a lot of work to evaluate E1, +; and the produced result (a number, a vector, a string, etc) may be of +; value to E2. Alas, the AND form merely checks that the result is not an #f, +; and throws it away. If E2 needs it, it has to recompute the value again. +; This proposed LAND* special form lets constituent expressions get +; hold of the results of already evaluated expressions, without re-doing +; their work. +; +; Syntax: +; LAND* (CLAWS) BODY +; +; where CLAWS is a list of expressions or bindings: +; CLAWS ::= '() | (cons CLAW CLAWS) +; Every element of the CLAWS list, a CLAW, must be one of the following: +; (VARIABLE EXPRESSION) +; or +; (EXPRESSION) +; or +; BOUND-VARIABLE +; These CLAWS are evaluated in the strict left-to-right order. For each +; CLAW, the EXPRESSION part is evaluated first (or BOUND-VARIABLE is looked up). +; If the result is #f, LAND* immediately returns #f, thus disregarding the rest +; of the CLAWS and the BODY. If the EXPRESSION evaluates to not-#f, and +; the CLAW is of the form +; (VARIABLE EXPRESSION) +; the EXPRESSION's value is bound to a freshly made VARIABLE. The VARIABLE is +; available for _the rest_ of the CLAWS, and the BODY. As usual, all +; VARIABLEs must be unique (like in let*). +; +; Thus LAND* is a sort of cross-breed between LET* and AND. +; +; Denotation semantics: +; +; Eval[ (LAND* (CLAW1 ...) BODY), Env] = +; EvalClaw[ CLAW1, Env ] andalso +; Eval[ (LAND* ( ...) BODY), ExtClawEnv[ CLAW1, Env]] +; +; Eval[ (LAND* (CLAW) ), Env] = EvalClaw[ CLAW, Env ] +; Eval[ (LAND* () FORM1 ...), Env] = Eval[ (BEGIN FORM1 ...), Env ] +; Eval[ (LAND* () ), Env] = #t +; +; EvalClaw[ BOUND-VARIABLE, Env ] = Eval[ BOUND-VARIABLE, Env ] +; EvalClaw[ (EXPRESSION), Env ] = Eval[ EXPRESSION, Env ] +; EvalClaw[ (VARIABLE EXPRESSION), Env ] = Eval[ EXPRESSION, Env ] +; +; ExtClawEnv[ BOUND-VARIABLE, Env ] = Env +; ExtClawEnv[ (EXPRESSION), Env ] = EnvAfterEval[ EXPRESSION, Env ] +; ExtClawEnv[ (VARIABLE EXPRESSION), Env ] = +; ExtendEnv[ EnvAfterEval[ EXPRESSION, Env ], +; VARIABLE boundto Eval[ EXPRESSION, Env ]] +; +; +; If one has a Scheme interpreter written in Prolog/ML/Haskell, he can +; implement the above semantics right away. Within Scheme, it is trivial to +; code LAND* with R4RS "define-syntax". Alas, Gambit does not have this +; facility. So this implementation uses macros instead. +; +; The following LAND* macro will convert a LAND* expression into a "tree" of +; AND and LET expressions. For example, +; (LAND* ((my-list (compute-list)) ((not (null? my-list)))) +; (do-something my-list)) +; is transformed into +; (and (let ((my-list (compute-list))) +; (and my-list (not (null? my-list)) (begin (do-something my-list))))) +; +; I must admit the LAND* macro is written in a pathetic anti-functional style. +; To my excuse, the macro's goal is a syntactic transformation of source +; code, that is, performing a re-writing. IMHO, rewriting kind of suggests +; mutating. +; +; Sample applications: +; +; The following piece of code (from my treap package) +; (let ((new-root (node:dispatch-on-key root key ...))) +; (if new-root (set! root new-root))) +; could be elegantly re-written as +; (land* ((new-root (node:dispatch-on-key root key ...))) +; (set! root new-root)) +; +; A very common application of land* is looking up a value +; associated with a given key in an assoc list, returning #f in case of a +; look-up failure: +; +; ; Standard implementation +; (define (look-up key alist) +; (let ((found-assoc (assq key alist))) +; (and found-assoc (cdr found-assoc)))) +; +; ; A more elegant solution +; (define (look-up key alist) +; (cdr (or (assq key alist) '(#f . #f)))) +; +; ; An implementation which is just as graceful as the latter +; ; and just as efficient as the former: +; (define (look-up key alist) +; (land* ((x (assq key alist))) (cdr x))) +; +; Generalized cond: +; +; (or +; (land* (bindings-cond1) body1) +; (land* (bindings-cond2) body2) +; (begin else-clause)) +; +; Unlike => (cond's send), LAND* applies beyond cond. LAND* can also be used +; to generalize cond, as => is limited to sending of only a single value; +; LAND* allows as many bindings as necessary (which are performed in sequence) +; +; (or +; (land* ((c (read-char)) ((not (eof-object? c)))) +; (string-set! some-str i c) (++! i)) +; (begin (do-process-eof))) +; +; Another concept LAND* is reminiscent of is programming with guards: +; a LAND* form can be considered a sequence of _guarded_ expressions. +; In a regular program, forms may produce results, bind them to variables +; and let other forms use these results. LAND* differs in that it checks +; to make sure that every produced result "makes sense" (that is, not an #f). +; The first "failure" triggers the guard and aborts the rest of the +; sequence (which presumably would not make any sense to execute anyway). +; +; $Id: vland-gambit.scm,v 1.1 1998/12/28 23:54:29 srfimgr Exp $ + +;; +;; Adapted to Blackhole for Gambit by Álvaro Castro-Castilla + +(define-macro (and-let* claws . body) + (let* ((new-vars '()) (result (cons 'and '())) (growth-point result)) + + (define (andjoin! clause) + (let ((prev-point growth-point) (clause-cell (cons clause '()))) + (set-cdr! growth-point clause-cell) + (set! growth-point clause-cell))) + + (if (not (list? claws)) + (error "Syntax error: bindings must be a list")) + (for-each + (lambda (claw) + (cond + ((symbol? claw) ; BOUND-VARIABLE form + (andjoin! claw)) + ((and (pair? claw) (null? (cdr claw))) ; (EXPRESSION) form + (andjoin! (car claw))) + ; (VARIABLE EXPRESSION) form + ((and (pair? claw) (symbol? (car claw)) + (pair? (cdr claw)) (null? (cddr claw))) + (let* ((var (car claw)) (var-cell (cons var '()))) + (if (memq var new-vars) + (error "Syntax error: duplicate variable in the bindings:" var)) + (set! new-vars (cons var new-vars)) + (set-cdr! growth-point `((let (,claw) (and . ,var-cell)))) + (set! growth-point var-cell))) + (else + (error "Syntax error: an ill-formed binding in a syntactic form let-and*" + claw)) + )) + claws) + (if (not (null? body)) + (andjoin! `(begin ,@body))) + result)) diff --git a/srfi/test-2.scm b/srfi/test-2.scm new file mode 100644 index 0000000..58db05d --- /dev/null +++ b/srfi/test-2.scm @@ -0,0 +1,45 @@ +(import (std srfi/64)) + +(import ./2) + +(test-begin "srfi-2" 28) + +(test-equal (and-let* () 1) 1) +(test-equal (and-let* () 1 2) 2) +(test-equal (and-let* () ) #t) + +(test-equal (let ((x #f)) (and-let* (x))) #f) +(test-equal (let ((x 1)) (and-let* (x))) 1) +(test-equal (and-let* ((x #f)) ) #f) +(test-equal (and-let* ((x 1)) ) 1) +;(test-error (and-let* ( #f (x 1))) ) +(test-equal (and-let* ( (#f) (x 1)) ) #f) +;(test-error (and-let* (2 (x 1))) ) +(test-equal (and-let* ( (2) (x 1)) ) 1) +(test-equal (and-let* ( (x 1) (2)) ) 2) +(test-equal (let ((x #f)) (and-let* (x) x)) #f) +(test-equal (let ((x "")) (and-let* (x) x)) "") +(test-equal (let ((x "")) (and-let* (x) )) "") +(test-equal (let ((x 1)) (and-let* (x) (+ x 1))) 2) +(test-equal (let ((x #f)) (and-let* (x) (+ x 1))) #f) +(test-equal (let ((x 1)) (and-let* (((positive? x))) (+ x 1))) 2) +(test-equal (let ((x 1)) (and-let* (((positive? x))) )) #t) +(test-equal (let ((x 0)) (and-let* (((positive? x))) (+ x 1))) #f) +(test-equal (let ((x 1)) (and-let* (((positive? x)) (x (+ x 1))) (+ x 1))) 3) +#;(test-error + #t + (let ((x 1)) (and-let* (((positive? x)) (x (+ x 1)) (x (+ x 1))) (+ x 1))) + ) + +(test-equal (let ((x 1)) (and-let* (x ((positive? x))) (+ x 1))) 2) +(test-equal (let ((x 1)) (and-let* ( ((begin x)) ((positive? x))) (+ x 1))) 2) +(test-equal (let ((x 0)) (and-let* (x ((positive? x))) (+ x 1))) #f) +(test-equal (let ((x #f)) (and-let* (x ((positive? x))) (+ x 1))) #f) +(test-equal (let ((x #f)) (and-let* ( ((begin x)) ((positive? x))) (+ x 1))) #f) + +(test-equal (let ((x 1)) (and-let* (x (y (- x 1)) ((positive? y))) (/ x y))) #f) +(test-equal (let ((x 0)) (and-let* (x (y (- x 1)) ((positive? y))) (/ x y))) #f) +(test-equal (let ((x #f)) (and-let* (x (y (- x 1)) ((positive? y))) (/ x y))) #f) +(test-equal (let ((x 3)) (and-let* (x (y (- x 1)) ((positive? y))) (/ x y))) 3/2) + +(test-end "srfi-2")