whole-function comptime folding for zero-runtime value calls

std/enums/enums.hon

@@ -0,0 +1,45 @@
+import "@std/meta"
+
+EnumMap func($E, $V type) type {
+    match typeinfo!(E) {
+        .enum |info|: return struct {
+	        present [info.fields.len]mut bool # fixme: replace with bitset
+	       	values [info.fields.len]mut V
+        }
+        else: compile_error!("EnumMap key must be an enum")
+    }
+}
+
+init func(
+	$E, $V type,
+	values meta.EnumFieldStruct(E, ?V, some!(null)),
+) EnumMap(E, V) {
+	map EnumMap(E, V) = undefined
+
+	match typeinfo!(E) {
+		.enum |info|: expand for info.fields |field, i| {
+			map.present[i] = false
+
+			if field!(values, field.name) |value| {
+				map.present[i] = true
+				map.values[i] = value
+			}
+		}
+		else: compile_error!("EnumMap key must be an enum")
+	}
+
+	return map
+}
+
+get func($E, $V type, map @EnumMap(E, V), key E) ?V {
+	# fixme: linear lookup; implement an enum index/discriminant map for O(1) lookup
+	match typeinfo!(E) {
+		.enum |info|: expand for info.fields |field, i| {
+			if key == field!(E, field.name) {
+				if (map.present[i]) return map.values[i]
+				return null
+			}
+		}
+		else: compile_error!("EnumMap key must be an enum")
+	}
+}