diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 80 |
1 files changed, 45 insertions, 35 deletions
@@ -2955,11 +2955,8 @@ directive with optional .code elif and .code else -clauses is a syntactic sugar -which translates to a combination of -.code @(cases) -and -.codn @(require) . +clauses allows one of multiple bodies of pattern matching directives to be +conditionally selected by testing the values of Lisp expressions. .coIP @(choose) Multiple clauses are applied to the same input. The one whose effect persists @@ -4348,6 +4345,11 @@ Example: .dir if +The +.code if +directive allows for conditional selection of pattern matching clauses, +based on the Boolean results Lisp expressions. + The syntax of the .code if directive can be exemplified as follows: @@ -4411,37 +4413,33 @@ then matching continues with otherwise it proceeds with .codn {@c} . -The +More precisely, how the .code if -directive is actually a syntactic sugar which is translated to -.code @(cases) -and -.codn @(require) . -That is to say, the following pattern: - -.cblk - @(cases) -.mets @(require << lisp-expr-1 ) - A - @(or) -.mets @(require << lisp-expr-2 ) - B - @(or) - C - @(end) -.cble - -corresponds to the somewhat shorter and clearer: - -.cblk -.mets @(if << lisp-expr-1 ) - A -.mets @(elif << lisp-expr-2 ) - B - @(else) - C - @(end) -.cble +directive works is as follows. The Lisp expressions are evaluated in order, +starting with the +.code if +expression, then the +.code elif +expressions if any are present. If any Lisp expression yields a true result +(any value other than +.codn nil) +then evaluation of Lisp expressions stops. The corresponding clause of that +Lisp expression is selected and pattern matching continues +with that clauses. The result of that clause (its success or failure, +and any newly bound variables) is then taken as the result of the +.code if +directive. If none of the Lisp expressions yield true, and an +.code else +clause is present, then that clause is processed and its result determines +the result of the +.code if +directive. If none of the Lisp expressions +yield true, and there is no +.code else +clause, then the +.code if +directive is deemed to have trivially succeeded, allowing matching to continue +with whatever directive follows it. .dir gather @@ -40636,6 +40634,18 @@ of these version values, the described behaviors are provided if is given an argument which is equal or lower. For instance .code -C 103 selects the behaviors described below for version 105, but not those for 102. +.IP 136 +A request for compatibility with \*(TX 136 or earlier restores the old behavior +of the +.code if +directive, which in used to be a syntactic sugar for a +.code cases +directive with +.code require +at the top of each block. Though semantically well-defined and working as +documented, the behavior was confusing, since failed matching caused potential +evaluation of multiple clauses, whereas programmers expect an if/elif/else +ladder to select exactly one clause. .IP 128 Compatibility with \*(TX 128 or earlier brings back the behavior that expressions in quasiliterals are evaluated according to \*(TX evaluation |