cannot find symbol »class β« in generated java code

[mabre]

Frege 3.24.100 does not generate valid java code for this frege code, unless one of the type signatures is provided or new_acc seq`` is removed. Frege 3.23.451 and ghc can compile run the code.

module Lexer 
where

main :: IO ()
main = case alexScan "lorem" 42 of
            Nothing -> print 1

-- alexScan :: String -> Integer -> Maybe String
alexScan "foo" sc = Nothing
alexScan input sc = alexScanUser undefined input sc

-- alexMonadScan :: String
alexMonadScan = do
  case alexScan "foo" 1 of 
    Nothing -> "eof"

-- alexScanUser :: a -> String -> Integer -> Maybe b
alexScanUser user input sc
  = case alex_scan_tkn input "" of
      ("", input') -> Nothing

-- alex_scan_tkn :: String -> String -> (String, String)
alex_scan_tkn input last_acc =
  let new_acc = alexMonadScan in
    new_acc `seq` (last_acc, input)

calling: javac -cp /home/markus/Downloads/frege/frege3.24.100.jar:. -d . -sourcepath . -encoding UTF-8 ./Lexer.java 
./Lexer.java:108: error: cannot find symbol
                      PreludeBase.<β>undefined(), Thunk.<String/*<Character>*/>lazy(arg$1), arg$2
                                   ^
  symbol:   class β
  location: class Lexer
./Lexer.java:107: error: cannot find symbol
            (Lazy<PreludeBase.TMaybe<α>>)(() -> Lexer.<β, α>alexScanUser(
                                                       ^
  symbol:   class β
  location: class Lexer
2 errors
E CannotFindSymbol.hs:27: java compiler errors are most
    likely caused by erroneous native definitions

[Ingo60]

Java doesn't allow unknown types coming out of nowhere, instead all such types must be declared in advance.

Since the type of undefined in the application of alexScanUser doesn't influence the type of the function alexScan there is no such type.

Workaround would be to just pass something that does have a known type. Since the argument is not used, any type would do. Even

(undefined::Bool)

does work.