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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 145 |
1 files changed, 145 insertions, 0 deletions
@@ -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 *) |