From 378546295be6b3a72ee3792d9175501bc77c016d Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Thu, 13 Oct 2022 20:23:35 -0700
Subject: New function: macroexpand-struct-clause.

* stdlib/struct.tl (macroexpand-struct-clause): New function.

* autoload.c (struct_set_entries): Autoload struct module
on macroexpand-struct-clause.

* txr.1: Documented.

* stdlib/doc-syms.tl: Updated.
---
 autoload.c         |  2 +-
 stdlib/doc-syms.tl |  1 +
 stdlib/struct.tl   |  5 +++++
 txr.1              | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 57 insertions(+), 1 deletion(-)

diff --git a/autoload.c b/autoload.c
index e266ed43..7a6cd79d 100644
--- a/autoload.c
+++ b/autoload.c
@@ -220,7 +220,7 @@ static val struct_set_entries(val fun)
     lit("defstruct"), lit("qref"), lit("uref"), lit("new"), lit("lnew"),
     lit("new*"), lit("lnew*"),
     lit("meth"), lit("umeth"), lit("usl"), lit("defmeth"), lit("rslot"),
-    lit("define-struct-clause"), nil
+    lit("define-struct-clause"), lit("macroexpand-struct-clause"), nil
   };
   val vname[] = {
     lit("*struct-clause-expander*"), nil
diff --git a/stdlib/doc-syms.tl b/stdlib/doc-syms.tl
index 2b0edcaf..08dcc8e2 100644
--- a/stdlib/doc-syms.tl
+++ b/stdlib/doc-syms.tl
@@ -1190,6 +1190,7 @@
       ("macroexpand-match" "N-02CCCB67")
       ("macroexpand-params" "N-037EB49A")
       ("macroexpand-place" "N-00684FF9")
+      ("macroexpand-struct-clause" "N-01046265")
       ("macrolet" "N-00AC12C0")
       ("madv-dontneed" "N-027D1E84")
       ("madv-normal" "N-027D1E84")
diff --git a/stdlib/struct.tl b/stdlib/struct.tl
index 10b07710..855ac6e1 100644
--- a/stdlib/struct.tl
+++ b/stdlib/struct.tl
@@ -415,6 +415,11 @@
             (mac-param-bind ,form ,params (cdr ,slot) ,*body)))
        ,keyword)))
 
+(defun macroexpand-struct-clause (clause : form)
+  (iflet ((xfun (and (consp clause) [*struct-clause-expander* (car clause)])))
+    [xfun clause form]
+    (cons clause nil)))
+
 (compile-only
   (load-for (struct sys:param-parser-base "param")))
 
diff --git a/txr.1 b/txr.1
index 4c2ee369..cfd70018 100644
--- a/txr.1
+++ b/txr.1
@@ -32313,6 +32313,56 @@ or a subtype thereof.
   fa.(write 4) -> ;; error: fa has no such method
 .brev
 
+.coNP Function @ macroexpand-struct-clause
+.synb
+.mets (macroexpand-struct-clause < clause <> [ form ])
+.syne
+.desc
+If
+.code clause
+is a compound expression whose operator symbol was defined by
+.code define-struct-clause
+then
+.code macroexpand-struct-clause
+expands the clause and returns the expansion, which is
+a list of zero or more clauses.
+Otherwise, the function returns a one-element list containing the
+.meta clause
+argument, as if by the
+.mono
+.meti (list << clause )
+.onom
+expression.
+
+The
+.meta form
+parameter, if present, is used for reporting errors.
+Note: clauses are usually expanded during the processing of a
+.code defstruct
+macro; in that situation, the entire unexpanded
+.code defstruct
+form serves the role,
+
+.TP* Examples:
+
+.verb
+  ;; try to expand :delegate, using incorrect syntax.
+  (macroexpand-struct-clause '(:delegate x (a b)))
+  --> error "** source location n/a: nil: too few elements ..."
+
+  ;; same, but with error reporting form.
+  (macroexpand-struct-clause '(:delegate x (a b)) '(abc xyz))
+  --> error: "** expr-1:1: abc: too few elements ..."
+
+  ;; correct :delegate syntax
+  (macroexpand-struct-clause '(:delegate x (a b) a.foo))
+  --> ((:method x (a b) (qref a.foo (x b))))
+
+  ;; not a defstruct macro clause
+  (macroexpand-struct-clause '(1 2 3))
+  -> ((1 2 3))
+.brev
+
 .coNP Special variable @ *struct-clause-expander*
 .desc
 The
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