diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 38 |
1 files changed, 37 insertions, 1 deletions
@@ -30165,13 +30165,34 @@ The return value of the macro is the macro expansion. It is substituted in place of the entire macro call form. That form is then expanded again; it may itself be another macro call, or contain more macro calls. -.TP* "Dialect Note:" +A global macro defined using +.code defmacro +may decline to expand a macro form. Declining to expand is achieved by +returning the original unexpanded form, which may be captured using the +.code :form +parameter. When a global macro declines to expand a form, the form is +taken as-is. At evaluation time, it will be treated as a function call. +Note: when a local macro defined by +.code macrolet +declines, more complicated requirements apply; see the description of +.codn macrolet . + +.TP* "Dialect Notes:" A macro in the global namespace introduced by .code defmacro may co-exist with a function of the same name introduced by .codn defun . This is not permitted in ANSI Common Lisp. +ANSI Common Lisp doesn't describe the concept of declining to expand, except in +the area of compiler macros. Since TXR Lisp allows global macros and functions +of the same name to co-exist, ordinary macros can be used to optimize functions +in a manner similar to Common Lisp compiler macros. A macro can be written +of the same name as a function, and can optimize certain cases of the function +call by expanding them to some alternative syntax. Cases which it doesn't +optimize are handled by declining to expand, in which case the form remains +as the original function call. + .TP* Example: .cblk @@ -30248,6 +30269,21 @@ macros to have any visibility to any surrounding lexical variable bindings, which are only instantiated in the evaluation phase, after expansion is done and macros no longer exist. +A local macro defined using +.code defmacro +may decline to expand a macro form. Declining to expand is achieved by returning the original +unexpanded form, which may be captured using the +.code :form +parameter. When a local macro declines to expand a form, the macro definition +is temporarily hidden, as if it didn't exist in the lexical scope. If another +macro of the same name is thereby revealed (a global macro or another local macro +at a shallower nesting level), then an expansion is tried with that macro. If +no such macro is revealed, or if a lexical function binding of that name is +revealed, then no expansion takes place; the original form is taken as-is. +When another macro is tried, the process repeats, resulting in a search which +proceeds as far as possible through outer lexical scopes and finally the +global scope. + .coNP Function @ macro-form-p .synb .mets (macro-form-p < obj <> [ env ]) |