From 4d2ab06904fd69c0fc7ba3c5237127726ab5c98b Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Sun, 15 Nov 2015 08:13:42 -0800
Subject: Functors: structs callable as functions.

* lib.c (generic_funcall): If an object is used as a function,
get its lambda method and call that.

* txr.1: Document functors.
---
 lib.c |  3 +++
 txr.1 | 30 ++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/lib.c b/lib.c
index 2771c18d..09e85693 100644
--- a/lib.c
+++ b/lib.c
@@ -4762,6 +4762,9 @@ val generic_funcall(val fun, struct args *args_in)
       default:
         callerror(fun, lit("too many arguments"));
       }
+    } else if (structp(fun)) {
+      fun = method(fun, lambda_s);
+      break;
     }
     /* fallthrough */
   default:
diff --git a/txr.1 b/txr.1
index 65748eb3..281bc167 100644
--- a/txr.1
+++ b/txr.1
@@ -18150,6 +18150,28 @@ A structure type is associated with a static initialization function
 which may be used to store initial values into static slots. It is
 invoked when the type is created.
 
+.NP* Functors
+
+A structure object can be invoked as a function, if it supports an "anonymous"
+method. An anonymous method is one whose name is the
+.code lambda
+symbol. When arguments are applied to an object
+.codn s ,
+the object and those arguments are passed to the
+.code lambda
+method. If there is no such method, the call is erroneous.
+
+That is to say, the following equivalences apply, except that
+.code s
+is evaluated only once:
+
+.cblk
+  (call s args ...)  <-->  s.(lambda args ...)
+
+  [s args ...]  <-->  [s.lambda s args ...]
+
+  (mapcar s list)  <-->  (mapcar (meth s lambda) list)
+.cble
 
 .coNP Macro @ defstruct
 .synb
@@ -18541,6 +18563,14 @@ are placed into a
 named
 .codn name .
 
+A method named
+.code lambda
+allows a structure to be used as if it were a function. When arguments
+are applied to the structure as if it were a function, the
+.code lambda
+method is invoked with those arguments, with the object itself inserted
+into the leftmost argument position.
+
 .coNP Macro @ new
 .synb
 .mets (new >> { name | >> ( name << arg *)} >> { slot << init-form }*)
-- 
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