RecursiveTypeClass.hs

{-# LANGUAGE FlexibleInstances #-}

import GHC.Float

data LangVariable = LangString String | LangNumber Double | LangNull deriving Show

class LangCoercion a where
    toLangVariable :: a -> LangVariable

instance LangCoercion String where
    toLangVariable a = LangString a

instance LangCoercion Double where
    toLangVariable a = LangNumber a

instance LangCoercion Float where
    toLangVariable a = LangNumber $ float2Double a

instance  LangCoercion (Maybe a) where
    toLangVariable (Just n) = toLangVariable n
    --                        ^ Error
    -- No instance for (LangCoercion a)
    --   arising from a use of `toLangVariable'
    -- Possible fix:
    --   add (LangCoercion a) to the context of the instance declaration
    -- In the expression: toLangVariable n
    -- In an equation for `toLangVariable':
    --     toLangVariable (Just n) = toLangVariable n
    -- In the instance declaration for `LangCoercion (Maybe a)'
    toLangVariable (Nothing) = LangNull