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

[Roman Cheplyaka]

On 10/06/15 22:21, Nathan Bouscal wrote:
> On Wed, Jun 10, 2015 at 12:11 PM, Roman Cheplyaka <roma at ro-che.info
> <mailto:roma at 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 <ezyang at mit.edu <mailto:ezyang at 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


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