Tweak variant/result mangling to produce valid wit

Resolves #86

Author: lukewagner

design/mvp/canonical-abi/definitions.py

@@ -1154,7 +1154,10 @@ def mangle_flags(labels):
   return 'flags { ' + ', '.join(labels) + ' }'
 
 def mangle_varianttype(cases):
-  mangled_cases = (c.label + '(' + mangle_maybevaltype(c.t) + ')' for c in cases)
+  mangled_cases = ('{label}{payload}'.format(
+                     label = c.label,
+                     payload = '' if c.t is None else '(' + mangle_valtype(c.t) + ')')
+                   for c in cases)
   return 'variant { ' + ', '.join(mangled_cases) + ' }'
 
 def mangle_enumtype(labels):
@@ -1167,12 +1170,12 @@ def mangle_optiontype(t):
   return 'option<' + mangle_valtype(t) + '>'
 
 def mangle_resulttype(ok, error):
-  return 'result<' + mangle_maybevaltype(ok) + ', ' + mangle_maybevaltype(error) + '>'
+  match (ok, error):
+    case (None, None) : return 'result'
+    case (None, _)    : return 'result<_, ' + mangle_valtype(error) + '>'
+    case (_, None)    : return 'result<' + mangle_valtype(ok) + '>'
+    case (_, _)       : return 'result<' + mangle_valtype(ok) + ', ' + mangle_valtype(error) + '>'
 
-def mangle_maybevaltype(t):
-  if t is None:
-    return '_'
-  return mangle_valtype(t)
 
 ## Lifting Canonical Modules
 

design/mvp/canonical-abi/run_tests.py

@@ -397,11 +397,11 @@ def test_mangle_functype(params, results, expect):
 test_mangle_functype([Flags(['a','b'])], [Enum(['a','b'])],
                      'func flags { a, b } -> enum { a, b }')
 test_mangle_functype([Variant([Case('a',None),Case('b',U8())])], [Union([U8(),List(String())])],
-                     'func variant { a(_), b(u8) } -> union { u8, list<string> }')
+                     'func variant { a, b(u8) } -> union { u8, list<string> }')
 test_mangle_functype([Option(Bool())],[Option(List(U8()))],
                      'func option<bool> -> option<list<u8>>')
 test_mangle_functype([], [('a',Result(None,None)),('b',Result(U8(),None)),('c',Result(None,U8()))],
-                     'func() -> (a: result<_, _>, b: result<u8, _>, c: result<_, u8>)')
+                     'func() -> (a: result, b: result<u8>, c: result<_, u8>)')
 
 def test_cabi(ct, expect):
   got = canonical_module_type(ct)