[Haskell-cafe] Automatic derivation (TemplateHaskell?)

[Joel Reymont]

Here's a complete working example. There seems to be an error in the  
parser but everything is derived fine.

*Main> run fooParser "Foo(10)"
Foo (Int 10)
*Main> run fooParser "Foo(10.5)"
parse error at (line 1, column 7):
unexpected "."
expecting digit or ")"

FunParser.hs:

Replace "show seen" with "show (seen + 1)" in args'

baz.hs:

import Text.ParserCombinators.Parsec hiding ( parse )
import qualified Text.ParserCombinators.Parsec as P
import qualified Text.ParserCombinators.Parsec.Token as T
import Text.ParserCombinators.Parsec.Language( emptyDef )
import Data.Derive.TH
import FunParser

data NumExpr
     = Int Integer
     | Num Double
     deriving Show

instance FunParser NumExpr where
     parse = numExpr

instance FunParser Integer where
     parse = T.integer lexer

instance FunParser Int where
     parse = T.integer lexer >>= return . fromInteger

data Foo
     = Foo NumExpr
     | Bar Int NumExpr
     deriving Show

lexer = T.makeTokenParser emptyDef

identifier = T.identifier lexer
reserved = T.reserved lexer
integer = T.integer lexer
float = T.float lexer

numExpr :: GenParser Char a NumExpr
numExpr =
     choice [ integer >>= return . Int
            , float >>= return . Num
            ]

$( derive makeFunParser ''Foo )

fooParser :: GenParser Char a Foo
fooParser = parse

run p input =
     case (P.parse p "" input) of
       Left  err -> putStr "parse error at " >> print err
       Right x   -> print x