matcher: allow hash pattern to omit values.

The @(hash ...) operator now allows key-only patterns
like (42) or (@x), where x could be bound or unbound.
This has separate semantics from when a value is present.

* share/txr/stdlib/match.tl (compile-hash-match): Implement.

* tests/011/patmatch.tl: Test.

* txr.1: Document.

1 files changed, 23 insertions, 1 deletions

diff --git a/txr.1 b/txr.1
index f0ff1866..85374e50 100644
--- a/txr.1
+++ b/txr.1
@@ -41159,7 +41159,7 @@ object's structure type: the type itself, rather than its symbolic name.
 
 .coNP Pattern operator @ hash
 .synb
-.mets @(hash >> {( key-pattern << value-pattern )}*)
+.mets @(hash >> {( key-pattern <> [ value-pattern ])}*)
 .syne
 .desc
 The
@@ -41188,6 +41188,9 @@ pair against that object as described below. Each of
 the pairs must successfully match, otherwise the overall
 match fails.
 
+The following requirements apply to key-value pattern pairs in which
+the value pattern is specified.
+
 If
 .meta key-pattern
 is a trivial pattern, then the semantics of the match is that
@@ -41267,6 +41270,25 @@ such that the semantics can then be understood in terms of the
 .code coll
 operator matching against an association list.
 
+The following requirements apply when the
+.meta value-pattern
+is omitted.
+
+If
+.meta key-pattern
+is a nontrivial pattern other than a variable pattern
+for a variable which has an existing binding, then the pattern
+is applied against the list of keys from the hash table, which
+are retrieved as if using the
+.code hash-keys
+function.
+
+If
+.meta key-pattern
+is a variable pattern referring to an existing binding, then that pattern is
+taken as a literal object.  The match is successful if that object occurs as a
+key in the hash table.
+
 .TP* Example:
 
 .verb