PROPOSAL: re-export 'Typeable' type-class from Prelude

[Michael Snoyman]

The one case I think Typeable should *always* be around for is creating new
instances of Exception. The fact that AutoDeriveTypeable doesn't require
Typeable in Prelude lessens my +1 a bit down to a +0.5, but I really want
it to be easier for people to define their own exception types.

On Thu, Sep 11, 2014 at 4:57 PM, Anthony Cowley <acowley at seas.upenn.edu>
wrote:

> I'm -1 on this as I'm not yet convinced that reaching for Typeable should
> be such a quick decision that it deserves to always be in scope.
>
> Anthony
>
> > On Sep 11, 2014, at 6:20 AM, Herbert Valerio Riedel <hvr at gnu.org> wrote:
> >
> >
> > TL;DR
> > =====
> >
> > Re-export 'Data.Typeable.Typeable' from 'Prelude'
> >
> > Motivation
> > ==========
> >
> > Since GHC 7.8 the ubiquitous 'Typeable' instances can only be
> > auto-derived via `... deriving Typeable`, moreover there's a new
> > extension `-XAutoDeriveTypeable` which implicitly auto-derives
> > 'Typeable' for all defined types in modules for which that extension is
> > enabled.
> >
> > However, even if you enable `-XAutoDeriveTypeable` you still need to
> > explicitly bring the 'Typeable' class into scope, with e.g.
> >
> >    import Data.Typeable (Typeable)
> >
> > otherwise GHC complains with
> >
> >    Not in scope: type constructor or class ‘Typeable’
> >
> > Since at this point it's become current practice to have 'Typeable'
> > instances for most types, it would be beneficial to save an 'import
> > Data.Typeable (Typeable)' line for the sole purpose of deriving such
> > instances.
> >
> > By having 'Prelude' re-export 'Typeable' from GHC 7.10 on (should this
> > proposal be implemented) it would suffice to have a
> >
> >  default-extensions: -XAutoDeriveTypeable`
> >
> > in the Cabal file and have 'Typeable' instance auto-derived for all
> > types defined in a package w/o any source changes (unless there's a
> > name-clash with 'Typeable')
> >
> > Discussion period
> > =================
> >
> > The usual 2 weeks
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