Re: [Haskell-cafe] Instance deriving for Void types.

29 Oct 2013

      Hi.

On Tue, Oct 29, 2013 at 5:15 AM, Lyndon Maydwell  wrote:
...
I was wondering if it's possible to have automatic deriving for some classes
for the Void type. This would be classes that require a parameter of the
type in its functions. This should obviously be trivial as the definition
can be undefined since there's no way to supply a value of the type.
I guess the obvious one would be Show.
data Nada deriving Show
{*
instance Show Nada where
  show _ = undefined
*}
This actually works with StandaloneDeriving:

{-# LANGUAGE StandaloneDeriving #-}

data Nada
deriving instance Show Nada
deriving instance Eq Nada
deriving instance Ord Nada

And I agree, all of these are useful, in particular if Nada is used as
an argument to a parameterized datatype. Trying to derive Read
triggers a compiler bug with 7.6.3, but this seems to be fixed in
HEAD.

Cheers,
  Andres

-- 
Andres Löh, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com