Re: [Haskell-cafe] The evil GADTs extension in ghci 7.8.4 (maybe in other versions too?)

10 Jun 2015


      On 10/06/15 22:21, Nathan Bouscal wrote:
...
On Wed, Jun 10, 2015 at 12:11 PM, Roman Cheplyaka mailto:roma@ro-che.info> wrote:
On 10/06/15 21:50, wren romano wrote:
    > On Thu, Jun 4, 2015 at 11:43 PM, Edward Z. Yang mailto:ezyang@mit.edu> wrote:
    >> GHC used to always generalize let-bindings, but our experience
    >> with GADTs lead us to decide that let should not be generalized
    >> with GADTs.  So, it's not like we /wanted/ MonoLocalBinds, but
    >> that having them makes the GADT machinery simpler.
    >>
    >> This blog post gives more details on the matter:
    >>     https://ghc.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7
    >
    > The fact that -XGADTs (in isolation) implies -XMonoLocalBinds isn't
    > the problem. The problem is, the order in which language pragma are
    > offered should not matter. Whether I say {-# LANGUAGE GADTs,
    > NoMonoLocalBinds #-} or {-# LANGUAGE NoMonoLocalBinds, GADTs #-}
    > shouldn't matter. Both should mean the same thing, regardless of how
    > annoying it may be to work in that language.
The current behavior may be surprising if you are not aware of it, but
    it's the only sensible one. Otherwise, what should the meaning of
{-# LANGUAGE MonoLocalBinds, NoMonoLocalBinds #-}
be?
Roman
Arguably it should be a compiler warning.
You could have enabled one of them project-wise and the other one for a
sepcific module. Being able to override extensions is useful.

Roman