I'm trying to use Happy for the first time, and I'm having trouble with this very simple example: ===== BEGIN: PolyParser.y ===== { module Main where import Char ( isAlpha, isDigit, isSpace ) } %name calc %tokentype { Token } %error { parseError } %token id { TokenId $$ } ',' { TokenCOMMA } wombat { TokenWombat $$ } %% Poly : id ',' wombat { parseError :: [Token] -> a parseError _ = error "Parse error" data Poly = Poly Int String deriving Show data Token = TokenId Int | TokenCOMMA | TokenWombat String deriving Show lexer :: String -> [Token] lexer [] = [] lexer (c:cs) | isSpace c = lexer cs | isAlpha c = lexVar (c:cs) | isDigit c = lexNum (c:cs) lexer (',':cs) = TokenCOMMA : lexer cs lexNum cs = TokenInt (read num) : lexer rest where (num,rest) = span isDigit cs lexVar cs = case span isAlpha cs of (wombat,rest) -> TokenWombat wombat : lexer rest (id,rest) -> TokenId id : lexer rest main = getContents >>= print . calc . lexer } ===== END: PolyParser.y ===== The resulting PolyParser.hs has an indentation problem (second-to-last line, below), so it won't compile. happyReduce_1 = happySpecReduce_3 4 happyReduction_1 happyReduction_1 _ _ _ = HappyAbsSyn4 (parseError :: [Token] -> a parseError _ = error "Parse error" Thank you in advance to anyone who can tell me where I've gone wrong.