diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 54 |
1 files changed, 48 insertions, 6 deletions
@@ -42169,19 +42169,35 @@ The pattern macro bindings are stored in a hash table held by the variable whose keys are symbols, and whose values are expander functions. There are no lexically scoped pattern macros. -If a +Pattern macros defined with .code defmatch -macro uses the +may specify the special macro parameters +.code :form +and +.code :env +in their parameter lists. The values of these parameters are determined +in a manner particular to +.codn defmatch . + +The .code :form -parameter to gain access to the form, what the parameter retrieves -is the pattern matching form, or a constituent thereof, in which the -the macro is being invoked. For instance, if the operator is being used -inside a pattern given to a +parameter captures the pattern matching form, or a constituent thereof, in +which the the macro is being invoked. For instance, if the operator is being +used inside a pattern given to a .code when-match macro invocation, then the form will be that entire .code when-match form. +The +.code :env +parameter captures a specially constructed macro-time environment object in +which all of the variables to the left of the pattern appear as lexical +variables. The parent of this environment is the surrounding macro environment. +If the pattern macro needs to treat a variable which already has a binding +differently from an unbound variable, it can look up the variable in this +environment. + .TP* Example: .verb @@ -42202,6 +42218,24 @@ form. "~s: bindable symbol expected, not ~s" 'foo sym)) ...) + + ;; Pattern macro which uses = equality to backreference + ;; an existing lexical binding, or else binds the variable + ;; if it has no existing lexical binding. + (defmatch var= (sym :env e) + (if (lexical-var-p e sym) + (with-gensyms (obj) + ^@(require (sys:var ,obj) + (= ,sym ,obj))) + ^(sys:var ,sym))) + + ;; example use: + (when-match (@(var= a) @(var= a)) '(1 1.0) a) + -> 1 + + ;; no match: (equal 1 1.0) is false + (when-match (@a @a) '(1 1.0) a) + -> nil .brev .coNP Special variable @ *match-macro* @@ -42222,6 +42256,14 @@ then there is no such binding: pattern operator forms based on .code sym do not undergo place macro expansion. +The macro expanders in +.code *match-macro* +are two-parameter functions. The first argument passes the operator +syntax to be expanded. The second argument is used for passing the +environment object which the expander can capture using +.code :env +in its macro parameter list. + .SS* Quasiquote Operator Syntax .coNP Macro @ qquote .synb |