On Sun, Oct 19, 2014 at 4:55 PM, Malcolm Wallace <malcolm.wallace@me.com> wrote:

On 19 Oct 2014, at 11:20, Herbert Valerio Riedel wrote:

> -- This module deliberately declares orphan instances:
> {-# OPTIONS_GHC -fno-warn-orphans #-}
>
> -- | Optional instance of 'Text.Show.Show' for functions:
> --
> -- > instance Show (a -> b) where
> -- > showsPrec _ _ = showString \"\<function\>\"
> --
> -----------------------------------------------------------------------------
> Should this module be
> deprecated, removed, alternatively, should the `Show` instance be made a
> non-orphan (e.g. by importing it by the `Prelude` module), or shall this
> curiousity be just left untouched in `base` in its current form?

Under no circumstances should this instance ever be visible from the Prelude.

It was deliberately made an orphan because the instance is non-conformant to the intended semantics of the Show class. Having a default instance of Show for functions would be a disaster, by causing static type error messages that indicate an arity problem, to become erratic runtime behaviours instead (without even throwing an exception).

While I'm sensitive to this position (thus my very reluctant +0.5 on merging into Prelude), I don't see the "disaster" you're referring to. The only case I can picture where this might happen is when you have a value that you are *only* ever displaying, not using in any other way. But won't it become painfully obvious very quickly that you messed up?

Can you describe concretely a scenario where the presence of this instance in Prelude would cause a disaster?

And if the instance is really as bad as that, I think having it in base at all is a mistake. All it takes it one library upstream from you to import that module, and you've been infected.

Michael