diff options
| -rw-r--r-- | txr.1 | 111 |
1 files changed, 82 insertions, 29 deletions
@@ -12227,40 +12227,66 @@ forms, the symbol .code it is implicitly bound to 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. If +a subexpression which is thereby identified as the +.IR it-form . +This +.code it +alias provides a convenient reference to that place or value, similar to the +word "it" in the English language, and similar anaphoric pronouns in other +languages. + +If .code it is bound to a place form, the binding is established as if using the .code placelet -operator. Otherwise, +operator: the form is evaluated only once, even if the +.code it +alias is used multiple times in the +.meta then +or +.meta else +expressions. Otherwise, if the form is not a syntactic place .code it is bound as an ordinary lexical variable to the form's value. +An +.I it-candidate +is an an expression viable for having its value or storage location +bound to the +.code it +symbol. An it-candidate is any expression which is not a constant expression +according to the +.code constantp +function, and not a symbol. + The .code ifa -macro imposes several restrictions on the +macro imposes applies several rules to the .meta cond -expression. Firstly, the +expression: +.RS +.IP 1. +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 +expression is ill-formed, and throws an exception at +macro-expansion time. +.IP 2. +If the .meta cond expression is a function call with two or more arguments, -at most one of them may be an it-candidate: an expression -viable for having its value or storage location bound to the -.code it -symbol. If there are two or more it-candidates, the +at most one of them may be an it-candidate. +If two or more arguments are it-candidates, the situation +is ambiguous. The .code ifa -expression is ill-formed. - +expression is ill-formed and throws an exception at macro-expansion +time. +.IP 3. If .meta cond is an atom, or a function call expression with no arguments, @@ -12271,50 +12297,60 @@ symbol is not bound. Effectively, macro behaves like the ordinary .code if operator. - +.IP 4. 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 +variable is bound to the argument expression, except when the function being called is .codn not , .codn null , or .codn false . -That special situation is rewritten according to the following pattern: +This binding occurs regardless of whether the expression is +an it-candidate. +.IP 5. +If +.meta cond +is a function call with exactly one argument to the Boolean negation +function which goes by one of the three names +.codn not , +.codn null , +or +.codn false , +then that situation is handled by a rewrite according to the following pattern: .cblk .mets (ifa (not << expr ) < then << else ) -> (ifa < expr < else << then ) .cble -and likewise for +which applies likewise for .code null or .code false substituted for .codn not . - -Note the reversal of +The Boolean inverse function is removed, and the .meta then and -.metn else . - +.meta else +expressions are exchanged. +.IP 6. If .meta cond is a function call with two or more arguments, then it is only well-formed if at most one of those arguments is an it-candidate. If there is one such argument, then the .code it -variable is bound to it. Otherwise the variable is bound +variable is bound to it. +.IP 7. +Otherwise the variable is bound to the leftmost argument expression, regardless of whether that argument expression is an it-candidate. +.RE -An it-candidate is any expression which is not a constant expression -according to the -.code constantp -function, and not a symbol. - +.IP In all other regards, the .code ifa macro behaves similarly to @@ -12349,18 +12385,35 @@ form. .cblk (ifa t 1 0) -> 1 + ;; Rule 7: it binds to (* x x), which is + ;; the only it-candidate. (let ((x 6) (y 49)) (ifa (> y (* x x)) ;; it binds to (* x x) (list it))) -> (36) + ;; Rule 4: it binds to argument of evenp, + ;; even though 4 isn't an it-candidate. (ifa (evenp 4) (list it)) -> (4) + ;; Rule 5: (ifa (not (oddp 4)) (list it)) -> (4) + + ;; Violation of Rule 1: + ;; while is not a function + (ifa (while t (print 42)) + (list it)) + --> exception! + + ;; Violation of Rule 2: + (let ((x 6) (y 49)) + (ifa (> (* y y y) (* x x))) + (list it)) + --> exception! .cble .coNP Macro @ conda |