* lisplib.c (place_set_entries): Add *place-macro* and

define-place-macro to list of names.

* share/txr/stdlib/place.tl (*place-macro*): New global hash.
(sys:tigger-load, sys:pl-expand): New functions.
(call-update-expander, call-clobber-expander, call-delete-expander):
Recognize and expand place macros.
(define-place-macro): New macro.
(first, rest): Places redefined using define-place-macro, replacing
the old hack of copying the expanders from one table entry to another.

* txr.1: Documented place macros.

1 files changed, 44 insertions, 3 deletions

diff --git a/txr.1 b/txr.1
index 2cebedce..b6b952a8 100644
--- a/txr.1
+++ b/txr.1
@@ -9924,10 +9924,14 @@ are both open-ended. Code can be written quite easily in \*(TL to introduce
 new kinds of places, as well as new place-mutating operators.
 New places can be introduced with the help of the
 .code defplace
-macro.  New place update macros (place operators) are written using the
+macro, or possibly the
+.code define-place-macro
+macro in simple cases when a new syntactic place can be expressed as a
+transformation to the syntax of an existing place.
+New place update macros (place operators) are written using the
 ordinary macro definer
-.code defmacro
-but with the help of special utility macros called
+.codn defmacro ,
+with the help of special utility macros called
 .codn with-update-expander ,
 .codn with-clobber-expander ,
 and
@@ -30251,6 +30255,43 @@ cells:
          ,body)))
 .cble
 
+.coNP Macro @ define-place-macro
+.synb
+.mets (define-place-macro < name < macro-style-params
+.mets \ \  << body-form *)
+.syne
+.desc
+In some situations, an equivalence exists between two forms, only one
+of which is recognized as a place. The
+.code define-place-macro
+macro can be used to establish a form as a place in terms of a translation to
+an equivalent form which is already a place.
+
+The
+.code define-place-macro
+has the same syntax as
+.codn defmacro .
+It specifies a macro transformation for a compound form which has the
+.meta name
+symbol in its leftmost position.
+This macro expansion is applied when such a form is used as a place.
+It is applied after all other expansions, and no other macro-expansions
+are applied afterward.
+
+.TP* "Example:"
+
+Implementation of
+.code first
+in terms of
+.codn car :
+
+.cblk
+  (define-place-macro first (obj)
+    ^(car ,obj))
+.cble
+
+.coNP Operator @ defmacro
+
 .coNP Macro @ rlet
 .synb
 .mets (rlet >> ({( sym << init-form )}*) << body-form *)