Re: [Haskell-cafe] Typeclass default implementation in subclasses

This post is also literate haskell. By enabling these potentially dangerous extensions you'll get the behaviour you want.

{-# LANGUAGE TypeSynonymInstances #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-}

module Thing where

import Text.PrettyPrint.HughesPJ

ppQuote :: Show a => a -> Doc ppQuote = doubleQuotes . text . show

x :: Int x = 1

y :: String y = "hello" z :: Char z = 'a'

class (Show a) => Quotable a where quote :: a -> Doc quote = ppQuote

instance (Show a) => Quotable a

instance Quotable String where quote = text . show -- don't need the doubleQuotes call for String

instance Quotable Char where quote c = quote [c] -- just lift it to String

Example: *Thing> quote "pi" "pi" *Thing> quote 3.14159 "3.14159"