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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 46 |
1 files changed, 29 insertions, 17 deletions
@@ -35291,34 +35291,46 @@ It specifies a macro transformation for a compound form which has the .meta name symbol in its leftmost position. -Forms which are used as syntactic places are subject to a modified -macro-expansion algorithm. If a place macro applies to a place, then it is -expanded, otherwise it is taken as-is. Then the place is expanded as a ordinary -macro (possibly a symbol macro), if possible, but only through at most -one round of macro-expansion, as if by -.codn macroexpand-1 . -If the macro-expansion succeeds, its its output might be, again, a place -macro form, and so the process is tried again. Place macros can expand to other place -macros or ordinary macros and vice versa. - Place macro expansion doesn't use an environment; place macros are in a single global namespace, special to place macros. There are no lexically scoped place macros. Such an effect can be achieved by having a place macro expand to an a form which is the target of a global or local macro, as necessary. -That said, it needs to be made clear that the ordinary macro-expansion -that may be interleaved within place-macro-expansion does take place -in the surrounding macro environment. A place macro can expand to a form -which matches a lexical macro visible at that point. +To support place macros, forms which are used as syntactic places are subject +to a modified macro-expansion algorithm: +.RS +.IP 1. +If a place macro exists for a form that is being used as a place, then the +that place macro is invoked to expand the +form, and the expansion is taken in place of the original form. This process +repeats until the form can no longer be expanded as a place macro, or +the place macro declines to expand the form by returning the unexpanded +input. +.IP 2. +A form that has been fully expanded as a place macro is then subject +to a single-round of macro-expansion, as if by +.codn macroexpand-1 , +which takes place in the original form's lexical environment. +If the form doesn't expand, or the result of expansion is +.code nil +or a +non-symbolic atom, then the process terminates. Otherwise, the +process is repeated from step 1. +.RE + +.IP The .code define-place-macro -form does not cause +macro does not cause .meta name to become .codn mboundp . -There can exist both an ordinary macro and a place macro of the same name; -moreover, this is potentially useful. +There can exist both an ordinary macro and a place macro of the same name. +In this situation, when the macro call appears as a place form, it is +expanded as a place macro, according to the above steps. When the macro call +appears as an evaluated form, not being used as a place, the form is +expanded using the ordinary macro. .TP* "Example:" |