Adding Applicative/Functor instances to all Monads in GHC

[Bas van Dijk]

Note that I already did this once as part of:

http://hackage.haskell.org/trac/ghc/ticket/4834

However, that never got merged.

So +1.

Bas

On 16 May 2013 13:33, David Luposchainsky <dluposchainsky at googlemail.com> wrote:
> Hello everyone,
>
> I recently went through GHC's source, and discovered there are quite a
> few instances of Monad that don't have Functor or Applicative instances.
> Fixing this is very easy by adding the standard instances (pure = return
> etc.), and will not break code.
>
> There is one exception to this: Hoopl defines its own (<*>). However,
> since it does not seem to have any packages depend on it otherwise,
> renaming this operator is also easily done (the thing is only used ten
> times or so). I called it (<*|*>) to complement (|*><*|), but naming
> should be the least important issue here.
>
> Small piece of backstory:
> The idea behind this is making GHC future-proof for changing Applicative
> to be a superclass of Monad. However, I think adding the Applicative
> instances is a good idea regardless of whether this will ever happen, so
> that's the only thing I'm proposing right now.
>
> Greetings,
> David
>
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