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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 90 |
1 files changed, 90 insertions, 0 deletions
@@ -2972,6 +2972,12 @@ clauses, in conjunction with .meIP @(define < name >> ( args ...)) Introduces a function. Functions are described in the Functions section below. +.meIP @(call < expr << args *) +Performs function indirection. Evaluates +.metn expr , +which must produce a symbol that names a pattern function. Then that +pattern function is invoked. + .coIP @(gather) Searches text for matches for multiple clauses which may occur in arbitrary order. For convenience, lines of the first clause are treated as separate @@ -7131,6 +7137,90 @@ is called directly from the top level, its .code fun call goes to the toplevel definition. +.NP* Indirect Calls + +Function indirection may be performed using the +.code call +directive. If +.meta fun-expr +is an expression which evaluates to a symbol, and +that symbol names a function which takes no arguments, then +.cblk + @(call fun-expr) +.cble +may be used to invoke the function. Of course, additional +expressions may be supplied which specify arguments. + +Example 1: + +.cblk +\ @(define foo (arg)) + @(bind arg "abc") + @(end) + @(call @'foo b) +.cble + +In this example, the effect is that +.code foo +is invoked, and +.code b +ends up bound to +.strn abc . + +The +.code call +directive here uses the +.code @'foo +expression to calculate the name of the function to be invoked. +The +.code @ +symbol indicates that the expression which follows is \*(TL , +and +.code 'foo +is the \*(TL syntax for quoting a symbol. (See the +.code quote +operator). + +Of course, this particular +.code call +expression can just be replaced by the direct invocation +syntax +.codn "@(foo b)" . + +The power of +.code call +lies in being able to specify the function as a value which +comes from elsewhere in the program, as in the following example. + +.cblk +\ @(define foo (arg)) + @(bind arg "abc") + @(end) + @(bind f @'foo) + @(call f b) +.cble + +Here the +.code call +directive obtains the name of the function from the +.code f +variable. + +Note that function names are resolved to functions in the environment +that is apparent at the point in execution where the +.code call +takes place. Very simply, the directive +.code "@(call f args ...)" +is precisely equivalent to +.code "@(s args ...)" +if, at the point of the call, +.code f +is a variable which holds the symbol +.code s +and symbol +.code s +is defined as a function. Otherwise it is erroneous. + .SS* Modularization .dirs load include |