diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 40 |
1 files changed, 37 insertions, 3 deletions
@@ -1297,15 +1297,20 @@ constituent strings, with an optional separator string between all of the values. .IP @(bind) -Binds one or more variables against another variable using a structural -pattern. A limited form of unification takes place which can cause a match to -fail. +Binds one or more variables against a value using a structural +pattern match. A limited form of unification takes place which can cause a +match to fail. .IP @(set) Destructively assigns one or more existing variables using a structural pattern, using syntax similar to bind. Assignment to unbound variables triggers an error. +.IP @(rebind) +Evaluates an expression in the current binding environment, and +then creates new bindings for the variables in the structural pattern. +Useful for temporarily overriding variable values in a scope. + .IP @(output) A directive which encloses an output clause in the query. An output section does not match text, but produces text. The directives above are not @@ -2718,6 +2723,35 @@ Destructuring assignment. D assumed to contain the list A ends up with "A", B ends up with ("B1" "B2") and C gets ("C1" and "C2"). +.SS The Rebind Directive + +The @(rebind) directive resembles @(set) but it is not an assignment. +It combines the semanticss of @(local), @(bind) and @(set). +The expression on the right hand side is evaluated in the current +environment. Then the variables in the pattern on the left are introduced +as new bindings, whose values come from the pattern. + +Rebind makes it easy to create temporary bindings based on existing +bindings. + + @(define pattern-function (arg)) + @;; inside a pattern function: + @(rebind recursion-level @(+ recursion-level 1)) + @;; ... + @(end) + +When the function terminates, the previous value of recursion-level +is restored. The effect is like the following, but much easier +to write and faster to execute: + + @(define pattern-function (arg)) + @;; inside a pattern function: + @(local temp) + @(set temp recursion-level) + @(local recursion-level) + @(set recursion-level @(+ temp 1)) + @;; ... + @(end) .SH BLOCKS |