From 0f66ac2b5412eb432f165b680fb495f972f33917 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Wed, 2 Jun 2021 22:47:07 -0700
Subject: matcher: better error handling for backquotes.

* share/txr/stdlib/match.tl (transform-qquote): Handle hash
error case with separate pattern. Use compile-error and *match
form instead of error. Diagnose splicing unquote and nested
quasiquote.
---
 share/txr/stdlib/match.tl | 17 ++++++++++++-----
 txr.1                     |  9 ++++++++-
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/share/txr/stdlib/match.tl b/share/txr/stdlib/match.tl
index df143dac..dca0ebf9 100644
--- a/share/txr/stdlib/match.tl
+++ b/share/txr/stdlib/match.tl
@@ -989,11 +989,9 @@
 
 (defun transform-qquote (syn)
   (match-case syn
-    ((sys:hash-lit @props . @(coll (@key @val)))
-     (if props
-       (error "~s: only equal hash tables supported" syn)
-       ^@(hash ,*(zip [mapcar transform-qquote key]
-                      [mapcar transform-qquote val]))))
+    ((sys:hash-lit nil . @(coll (@key @val)))
+     ^@(hash ,*(zip [mapcar transform-qquote key]
+                    [mapcar transform-qquote val])))
     ((sys:struct-lit @type . @args)
      ^@(struct ,(transform-qquote type)
                ,*[mapcar transform-qquote args]))
@@ -1003,6 +1001,15 @@
     ((sys:unquote @pat) (if (symbolp pat)
                           ^(sys:var ,pat)
                           ^(sys:expr ,pat)))
+    ((sys:hash-lit @(have) . @nil)
+     (compile-error *match-form*
+                    "only equal hash tables supported"))
+    ((@(or sys:qquote) . @nil)
+     (compile-error *match-form*
+                    "pattern-matching quasiquote doesn't support nesting"))
+    ((sys:splice . @nil)
+     (compile-error *match-form*
+                    "pattern-matching quasiquote doesn't support splicing"))
     ((@ca . @cd) (cons (transform-qquote ca)
                        (transform-qquote cd)))
     (@else else)))
diff --git a/txr.1 b/txr.1
index 7b1446dd..5aaf35e2 100644
--- a/txr.1
+++ b/txr.1
@@ -41361,11 +41361,18 @@ A JSON hash quasiquote is translated into
 .onom
 exactly like a hash.
 .meIP >> ( car . << cdr )
-Tree structure is translated according ot the
+Tree structure is translated according to the
 .mono
 .meti <> ([ car ] . <> [ cdr ])
 .onom
 pattern: it is recursively examined for translations.
+.meIP >> ^ nested-qq-syntax
+A nested quasiquote pattern is diagnosed as an error.
+.meIP >> ,* expr
+Splicing syntax is diagnosed as an error.
+.meIP >> ~* expr
+Splicing JSON syntax is diagnosed as an error inside a JSON quasiliteral.
+.meIP >> ~* expr
 .meIP < obj
 Any other quasiquoted object is left untranslated.
 .RE
-- 
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