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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 82 |
1 files changed, 82 insertions, 0 deletions
@@ -11957,6 +11957,34 @@ except that the symbol args is to be understood as a generated symbol (gensym). The ap macro nests properly with op and do, in any combination, in regard to the @@n notation. +.SS Macro ret + +.TP +Syntax: + + (ret <form>) + +.TP +Description: + +The ret macro's argument is treated similarly to the arguments of the op +operator, except that the first form of the op operator denotes +a function to be called where as the one and only argument form of the +ret operator denotes a value. + +The ret macro produces a function which takes any number of arguments, +and returns the value specified by <form>. + +The following equivalence holds: + + (ret x) <--> (op identity x) + +The expression (ret @2) returns a function similar to +(lambda (x y . z) y). + +The expression (ret 42) returns a function similar to +(lambda (. rest) 42). + .SS Function chain .TP @@ -12075,6 +12103,60 @@ The iffi function defaults to the identity function when <else-func> is omitted or nil, and therefore is useful in situations when one value is to be replaced with another one when the condition is true, otherwise left alone. +.SH Functions tf and nilf + +.TP +Syntax: + + (tf <arg>*) + (nilf <arg>*) + +.TP +Description: + +The tf and nilf functions take zero or more arguments, and ignore them. +The tf function returns t, and the nilf function returns nil. + +.TP +Example: + + ;; tf and nilf are useful when functions are chained together. + ;; test whether (trunc n 2) is odd. + + (defun trunc-n-2-odd (n) + [[chain (op trunc @1 2) [iff oddp tf nilf]] n) + +In this example, two functions are chained together, and n is passed +through the chain such that it is first divided by two via the +function denoted by (op trunc @1 2) and then the result is passed into the +function denoted by [iff oddp tf nilf]. The iff function passes its +argument into oddp, and if oddp yields true, it passes the same argument to tf. +Here tf proves its utility by ignoring that value and returning t. +If the argument (the divided value) passed into iff is even, then +iff passes it into the nilf function, which ignores the value and +returns nil. + +.SH Function retf + +.TP +Syntax: + + (retf <value>) + +.TP +Description: + +The retf function returns a function. That function can take zero or +more arguments. When called, it ignores its arguments and returns <value>. + +See also: the ret macro. + +.TP +Example: + + ;; the function returned by (retf 42) ignores 1 2 3 and returns 42. + (call (retf 42) 1 2 3) -> 42 + .SH INPUT AND OUTPUT (STREAMS) TXR Lisp supports input and output streams of various kinds, with |