FEATURE: The begnnings of some schema definition utilities.

Also allow failure messages to be generated from any expression.

src/parse/mod.rs

@@ -663,7 +663,7 @@ make_fn!(
     do_each!(
         pos => pos,
         _ => word!("fail"),
-        msg => must!(wrap_err!(either!(format_expression, string_expression), "Expected failure message")),
+        msg => must!(wrap_err!(expression, "Expected failure message")),
         (Expression::Fail(FailDef{
             pos: pos,
             message: Box::new(msg),

std/schema.ucg

@@ -0,0 +1,15 @@
+let one_of = module {
+    val=NULL,
+    types=[],
+} => {
+    let reducer = macro(acc, t) => acc{
+        ok = acc.ok || (acc.val is t),
+    };
+    let any = macro(val, types) => reduce reducer {ok=false, val=val}, types;
+
+    let result = any(mod.val, mod.types).ok;
+};
+
+let must = macro(m, msg) => select m.result, fail msg, {
+    true = m.result,
+};

std/tests/schema_test.ucg

@@ -0,0 +1,12 @@
+let t = import "std/testing.ucg".asserts{};
+let schema = import "std/schema.ucg";
+
+assert t.ok{
+    test = schema.must(schema.one_of{val=1, types=["float", "int"]}, "Must be a float or an int"),
+    desc = "1 is an int",
+};
+
+assert t.not_ok{
+    test = schema.one_of{val=1, types=["float", "str"]}.result,
+    desc = "1 is not a float or string",
+};