diff options
| -rw-r--r-- | ChangeLog | 9 | ||||
| -rw-r--r-- | share/txr/stdlib/ifa.tl | 16 | ||||
| -rw-r--r-- | txr.1 | 145 |
3 files changed, 166 insertions, 4 deletions
@@ -1,5 +1,14 @@ 2015-06-19 Kaz Kylheku <kaz@kylheku.com> + * txr.1: Documented ifa. + + * share/txr/stdlib/ifa.tl: Tightened up the tests for + situations when the macro is ill-formed, following + the improved specification. Also, eval-error is thrown + instead of just error. + +2015-06-19 Kaz Kylheku <kaz@kylheku.com> + Test ifa macro. * Makefile (TEST_OUT): Include .tl files. diff --git a/share/txr/stdlib/ifa.tl b/share/txr/stdlib/ifa.tl index f7c4fcb0..180e5231 100644 --- a/share/txr/stdlib/ifa.tl +++ b/share/txr/stdlib/ifa.tl @@ -29,15 +29,23 @@ (cond ((or (atom test) (null (cdr test))) ^(let ((it ,test)) (if it ,then ,else))) - ((member (first test) '(not null)) ^(ifa ,(second test) ,else ,then)) + ((member (first test) '(not null false)) + (unless (eql (length test) 2) + (throwf 'eval-error "ifa: wrong number of arguments to ~s" + (first test))) + ^(ifa ,(second test) ,else ,then)) (t (let* ((sym (first test)) (args (rest test)) (n-candidate-args [count-if candidate-p args]) (pos-candidate (or [pos-if candidate-p args] 0))) - (unless (fboundp sym) - (error "ifa: only works with global functions.")) + (unless (or (lexical-fun-p e sym) + (and (or (functionp (symbol-function sym)) + (null (symbol-function sym))))) + (throwf 'eval-error "ifa: test expression must be \ + \ a simple function call")) (when (> n-candidate-args 1) - (error "ifa: ambiguous situation: not clear what can be \"it\".")) + (throwf 'eval-error "ifa: ambiguous situation: \ + \ not clear what can be \"it\"")) (let* ((temps (mapcar (ret (gensym)) args)) (it-temp [temps pos-candidate])) ^(let* (,*(zip temps args) (it ,it-temp)) @@ -10796,6 +10796,151 @@ is returned. (format t "frobosity value ~a exceeds 150\en" fv)) .cble +.coNP Macro @ ifa +.synb +.mets (ifa < cond < then <> [ else ]) +.syne +.desc +The +.code ifa +macro provides a anaphoric conditional operator resembling the +.code if +operator. Around the evaluation of the +.meta then +and +.meta else +forms, the symbol +.code it +is implicitly bound to the value of a subexpression of +.metn cond , +providing a reference to that value, similar to the word +"it" in the English language, and similar anaphoric pronouns +in other languages. + +The +.code ifa +macro imposes several restrictions on the +.meta cond +expression. Firstly, the +.meta cond +expression must be either an atom, or a function call form: +a compound expression with a symbol in the leftmost position +which resolves to a function. Otherwise the +.code ifa +macro invocation is ill-formed. + +Secondly, if the +.meta cond +expression is a function call with more than two arguments, +at most one of them may be an it-candidate: an expression +viable for having its value bound to the +.code it +symbol. If there are two or more it-candidates, the +.code ifa +expression is ill-formed. + +If +.meta cond +is an atom, or a function call expression with no arguments, +then the +.code it +symbol is not bound. Effectively, +.code ifa +macro behaves like the ordinary +.code if +operator. + +If +.meta cond +is an invocation of the functions +.code not +or +.codn null , + +If +.meta cond +is a function call with exactly one argument, then the +.code it +variable is bound to the value of that argument, except when +the function being called is +.codn not , +.code null , +or +.codn false . +That special situation is rewritten according to the following pattern: + +.cblk +.mets (ifa (not << expr ) < then << else ) -> (ifa < expr < else << then ) +.cble + +Note the reversal of +.meta then +and +.metn else . + +If +.meta cond +is a function call with two or more arguments, then it is only +well-formed if zero or one of those arguments are it-candidates. +If there is an argument which is an it-candidate, then the +.code it +variable is bound to its value. Otherwise the variable is bound +to the value of the leftmost argument expression, regardless of whether that +argument expression is an it-candidate. + +An it-candidate is any expression which is not a constant expression +according to the +.code constantp +function, and not a symbol. + +In all other regards, the +.code ifa +macro behaves similarly to +.codn if . + +The +.meta cond +expression is evaluated, and, if applicable, +the value of the appropriate argument is captured and bound to +the variable +.code it +whose scope extends over the +.meta then +form, as well as over +.metn else , +if present. + +If +.meta cond +yields a true value, then +.meta then +is evaluated and the resulting value is returned, otherwise +.meta else +is evaluated if present and its value is returned. +A missing +.meta else +is treated as if it were the +.code nil +form. + +.TP* Examples: +.cblk + (ifa t 1 0) -> 1 + + (let ((x 6) (y 49)) + (ifa (> y (* x x)) ;; it binds to (* x x) + (list it))) + -> (36) + + (ifa (evenp 4) + (list it)) + -> (4) + + (ifa (not (oddp 4)) + (list it)) + -> (4) +.cble + .coNP Macro @ dotimes .synb .mets (dotimes >> ( var < count-form <> [ result-form ]) << body-form *) |