From 0f958749e189e4dacd7a1f70cfc33460e1228d3b Mon Sep 17 00:00:00 2001
From: Peter Mikkelsen <peter@pmikkelsen.com>
Date: Wed, 7 Jul 2021 16:32:02 +0000
Subject: Make '=..'/2 work according to spec. Introduce types.c for functions
 which tells us something about term types. Should be used a lot more instead
 of explicitly looking into terms->tag everywhere

---
 types.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 58 insertions(+)
 create mode 100644 types.c

(limited to 'types.c')

diff --git a/types.c b/types.c
new file mode 100644
index 0000000..6f2b33d
--- /dev/null
+++ b/types.c
@@ -0,0 +1,58 @@
+#include <u.h>
+#include <libc.h>
+#include <bio.h>
+
+#include "dat.h"
+#include "fns.h"
+
+/* Type tests */
+int
+islist(Term *t)
+{
+	return (isemptylist(t) || isnonemptylist(t));
+}
+
+int
+ispartiallist(Term *t)
+{
+	if(t->tag == VariableTerm)
+		return 1;
+	else if(t->tag == CompoundTerm && runestrcmp(t->text, L".") == 0 && t->arity == 2)
+		return ispartiallist(listtail(t));
+	else
+		return 0;
+}
+
+int
+isemptylist(Term *t)
+{
+	return (t->tag == AtomTerm && runestrcmp(t->text, L"[]") == 0);
+}
+
+int
+isnonemptylist(Term *t)
+{
+	if(t->tag == CompoundTerm && runestrcmp(t->text, L".") == 0 && t->arity == 2)
+		return islist(listtail(t));
+	else
+		return 0;	
+}
+
+/* Other functions */
+Term *
+listhead(Term *t)
+{
+	if(t->tag == CompoundTerm && runestrcmp(t->text, L".") == 0 && t->arity == 2)
+		return t->children;
+	else
+		return nil;
+}
+
+Term *
+listtail(Term *t)
+{
+	if(t->tag == CompoundTerm && runestrcmp(t->text, L".") == 0 && t->arity == 2)
+		return t->children->next;
+	else
+		return nil;
+}
\ No newline at end of file
-- 
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