support for polymorphic token types

[trofi]

One of my co-workers attempted to make a polymorphic token type
for generated parsers (handy to try various target types)

http://code.haskell.org/~slyfox/M.y

and even succeded in it with the following hack:

{-# OPTIONS  -cpp #-}
...
#define HappyAbsSyn (HappyAbsSyn_ ta tb)
...
%tokentype { Token ta tb }

What do you think of adding direct support for polymorphic token types to happy?

Thanks!

[Ericson2314]

Unfortunately http://code.haskell.org/~slyfox/M.y is dead now, so it's a bit hard for me to see what this is about.

[trofi]

Let's inline it here:

{

module M
    ( parse_regexps
    ) where

import           Data.Char

#define HappyAbsSyn (HappyAbsSyn_ ta tb)

}

%name      parser
%tokentype { Token ta tb }
%error     { parseError }
%token
    name          { TokenName $$ }
    int           { TokenInt $$ }

%%

Expr :: { [Token ta tb] }
    : name    { [TokenName $1] }
    | int     { [TokenInt  $1] }


{

data Token ta tb
    = TokenName ta
    | TokenInt  tb


instance (Show ta, Show tb) => Show (Token ta tb) where
    show (TokenName ta) = show ta
    show (TokenInt  tb) = show tb


class Eatable ta where
    fromString :: String -> ta


parseError :: [Token ta tb] -> c
parseError e = error "Parse error"


lexer :: (Eatable ta, Eatable tb) => String -> [Token ta tb]
lexer [] = []
lexer (c : cs)
      | isSpace c = lexer cs
      | isAlpha c = lex_name (c : cs)
      | isDigit c = lex_int  (c : cs)


lex_name :: (Eatable ta, Eatable tb) => String -> [Token ta tb]
lex_name cs =
    let (nm, rest) = span isAlpha cs
    in  TokenName (fromString nm) : lexer rest


lex_int :: (Eatable ta, Eatable tb) => String -> [Token ta tb]
lex_int cs =
    let (num, rest) = span isDigit cs
    in  TokenInt (fromString num) : lexer rest


--------------------------------------------------------------------------------
parse_regexps :: (Eatable ta, Eatable tb) => String -> [Token ta tb]
parse_regexps = parser . lexer

}

[Ericson2314]

OK thanks. I will try to investigate that soon.