Allow underscore lambda parameters

The parser treats _ as a keyword for ordinary identifiers, but lambda
parameters can use it as a throwaway name. After the Text cleanup, that
path rejected call arguments containing lambda c, _, err.

Accept _ through parameter parsing only, where it is valid. Keep normal
identifier diagnostics unchanged and add a parser regression covering
the lambda call-argument case.

Author: Kristian Larsson

compiler/lib/src/Acton/Parser.hs

@@ -1115,8 +1115,9 @@ singleStar = (lexeme . try) (char '*' <* notFollowedBy (char '*'))
 identifier :: Parser Text
 identifier = (lexeme . try) $ do
     off <- getOffset
-    void $ lookAhead (satisfy (\c -> isAlpha c || c == '_') <?> "identifier")
-    x <- takeWhile1P (Just "identifier") (\c -> isAlphaNum c || c=='_')
+    c <- satisfy (\c -> isAlpha c || c == '_') <?> "identifier"
+    cs <- hidden (takeWhileP Nothing (\c -> isAlphaNum c || c == '_'))
+    let x = T.cons c cs
     if S.isKeywordText x
       then parseError (TrivialError off (Just (Tokens (N.fromList (T.unpack x)))) (Set.fromList [Label (N.fromList "identifier")]))
       else return x
@@ -1130,6 +1131,12 @@ name = do off <- getOffset
 
 escname = name <|> addLoc (S.name . head <$> plainstrLiteral)  -- Assumes an escname cannot contain hex escape sequences
 
+paramName :: Parser S.Name
+paramName = name <|> do
+    off <- getOffset
+    rword "_"
+    return $ S.Name (Loc off (off + 1)) (T.singleton '_')
+
 tvarname = do off <- getOffset
               x <- identifier
               if isTypeVarName x
@@ -2448,13 +2455,13 @@ yield_expr = addLoc $ do
 --- Params ---------------------------------------------------------------------
 
 parm :: Bool -> Parser (S.Name, Maybe S.Type, Maybe S.Expr)
-parm ann = do n <- name
+parm ann = do n <- paramName
               mbt <- if ann then optional (colon *> ttype) else return Nothing
               mbe <- optional (equals *> expr)
               return (n, mbt, mbe)
 
 pstar :: Bool -> Parser S.Type -> Parser (S.Name, Maybe S.Type)
-pstar ann startype = do n <- name
+pstar ann startype = do n <- paramName
                         mbt <- if ann then optional (colon *> startype) else return Nothing
                         return (n, mbt)
 

compiler/lib/test/ActonSpec.hs

@@ -113,6 +113,12 @@ main = do
         completed `shouldSatisfy` monotonic
         last progress `shouldBe` (6, 6)
 
+      it "allows underscore parameters in lambdas" $ do
+        let input = "def f():\n    rpc(a, lambda c, _, err: cb(c, err))\n"
+        case parseModuleTest input of
+          Left err -> expectationFailure $ "Parse failed: " ++ err
+          Right _ -> return ()
+
       describe "Basic Syntax" $ do
         testParse env0 ["syntax1"]