diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 96 |
1 files changed, 91 insertions, 5 deletions
@@ -1302,6 +1302,15 @@ to the later clauses. Multiple clauses are applied to the same input. Evaluation stops on the first successful clause. +.IP @(require) +The require directive is similar to the do directive: it evaluates one or more +TXR Lisp expressions. If the result of the rightmost expression is nil, +then require triggers a match failure. See the TXR LISP section far below. + +.IP "@(if), @(elif), @(else)" +The if directive with optional elif and else clauses is a syntactic sugar +which translates to a combination of @(cases) and @(require) + .IP @(choose) Multiple clauses are applied to the same input. The one whose effect persists is the one which maximizes or minimizes the length of a particular variable. @@ -1424,11 +1433,6 @@ The load directive loads another TXR file and interprets its contents. The do directive is used to evaluate TXR Lisp expressions, discarding their result values. See the TXR LISP section far below. -.IP @(require) -The require directive is similar to the do directive: it evaluates one or more -TXR Lisp expressions. If the result of the rightmost expression is nil, -then require triggers a match failure. See the TXR LISP section far below. - .PP .SH INPUT SCANNING AND DATA MANIPULATION @@ -2046,6 +2050,88 @@ but the other one matches five lines, then the overall clause is considered to have made a five line match at its position. If more directives follow, they begin matching five lines down from that position. +.SS The Require Directive + +The syntax of @(require) is: + + @(require <lisp-expression>) + +The require directive evaluates a TXR Lisp expression. (See TXR LISP far +below.) If the expression yields a true value, then it succeeds, and matching +continues with the directives which follow. Otherwise the directive fails. + +In the context of the @(require) directive, should not be delimited by an @. + +Example: + + @# require that 4 is greater than 3 + @# This succeeds; therefore, @a is processed + @(require (> (+ 2 2) 3)) + @a + + +.SS The If Directive + +The syntax of the directive can be exemplified as follows + + @(if <lisp-expr>) + . + . + . + @(elif <lisp-expr>) + . + . + . + @(elif <lisp-expr>) + . + . + . + @(else) + . + . + . + @(end) + +The @(elif) and @(else) clauses are all optional. If @(else) is present, it must be +last, before @(end), after any @(elif) clauses. Any of the clauses may be empty. + +See the TXR Lisp section about TXR Lisp expressions. In this directive, TXR Lisp +expressions are not introduced by the @ symbol. + +For example: + + @(if (> (length str) 42)) + foo: @a @b + @(else) + {@c} + @(end) + +In this example, if the length of the variable str is greater than 42, then +matching continues with "foo: @a b", otherwise it proceeds with {@c}. + +The if directive is actually a syntactic sugar which is translated to @(cases) +and @(require). That is to say, the following pattern: + + @(cases) + @(require <lisp-expr-1>) + A + @(or) + @(require <lisp-expr-2>) + B + @(or) + C + @(end) + +corresponds to the somewhat shorter and clearer: + + @(if <lisp-expr-1>) + A + @(elsif <lisp-expr-2>) + B + @(else + C + @(end) + .SS The Gather Directive Sometimes text is structured as items that can appear in an arbitrary order. |