Intentionally omitted instance
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Consider you write the following in GHCi:
Prelude> 2+3::Char
<interactive>:2:2:
No instance for (Num Char)
arising from a use of `+'
Possible fix: add an instance declaration for (Num Char)
In the expression: 2 + 3 :: Char
In an equation for `it': it = 2 + 3 :: Char
In many cases users now actually try to implement instance Num Char,
but this is certainly not a good idea.
Is there a way to tell the user
that the instance was left unimplemented by intention?
Actually, this is possible with current Haskell extensions
by implementing an instance with a depending instance that a user cannot fulfill:
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE UndecidableInstances #-}
module Data.Char (Char) where
class Unimplementable a where
instance Unimplementable Char => Num Char where
(+) = undefined
(*) = undefined
abs = undefined
signum = undefined
fromInteger = undefined
The important point is to not export the Unimplementable class.
It is ugly that you have to enable UndecidableInstances for this trick
and you still have to suppress warnings about unimplemented methods.
Thus it might be better have language support for prohibition of certain instances like in GHC ticket 9334: "Instance chains".