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

[Edward Z. Yang]

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

Edward

Excerpts from Ki Yung Ahn's message of 2015-06-04 20:37:27 -0700:
> Such order dependent could be very confusing for the users. I thought I 
> turned off certain feature but some other extension turning it on is 
> strange. Wouldn't it be better to decouple GADT and MonoLocalBinds?
> 
> 2015년 06월 04일 20:31에 Edward Z. Yang 이(가) 쓴 글:
> > This is because -XGADTs implies -XMonoLocalBinds.
> >
> > Edward
> >
> > Excerpts from Ki Yung Ahn's message of 2015-06-04 20:29:50 -0700:
> >> \y -> let x = (\z -> y) in x x
> >>
> >> is a perfectly fine there whose type is  a -> a.
> >> (1) With no options, ghci infers its type correctly.
> >> (2) However, with -XGADTs, type check fails and raises occurs check.
> >> (3) We can remedy this by supplying some additional options
> >> (4) Howver, if you put -XGADTs option at the end, it fails again :(
> >>
> >>
> >> kyagrd at kyahp:~$ ghci
> >> GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
> >> Loading package ghc-prim ... linking ... done.
> >> Loading package integer-gmp ... linking ... done.
> >> Loading package base ... linking ... done.
> >> Prelude> :t \y -> let x = (\z -> y) in x x
> >> \y -> let x = (\z -> y) in x x :: t -> t
> >> Prelude> :q
> >> Leaving GHCi.
> >>
> >>
> >> kyagrd at kyahp:~$ ghci -XGADTs
> >> GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
> >> Loading package ghc-prim ... linking ... done.
> >> Loading package integer-gmp ... linking ... done.
> >> Loading package base ... linking ... done.
> >> Prelude> :t \y -> let x = (\z -> y) in x x
> >>
> >> <interactive>:1:30:
> >>       Occurs check: cannot construct the infinite type: t0 ~ t0 -> t
> >>       Relevant bindings include
> >>         x :: t0 -> t (bound at <interactive>:1:11)
> >>         y :: t (bound at <interactive>:1:2)
> >>       In the first argument of ‘x’, namely ‘x’
> >>       In the expression: x x
> >> Prelude> :q
> >> Leaving GHCi.
> >>
> >>
> >> ~$ ghci -XGADTs -XNoMonoLocalBinds -XNoMonomorphismRestriction
> >> GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
> >> Loading package ghc-prim ... linking ... done.
> >> Loading package integer-gmp ... linking ... done.
> >> Loading package base ... linking ... done.
> >> Prelude> :t \y -> let x = (\z -> y) in x x
> >> \y -> let x = (\z -> y) in x x :: t -> t
> >> Prelude> :q
> >> Leaving GHCi.
> >>
> >>
> >> ~$ ghci -XNoMonoLocalBinds -XNoMonomorphismRestriction -XGADTs
> >> GHCi, version 7.8.4: http://www.haskell.org/ghc/  :? for help
> >> Loading package ghc-prim ... linking ... done.
> >> Loading package integer-gmp ... linking ... done.
> >> Loading package base ... linking ... done.
> >> Prelude> :t \y -> let x = (\z -> y) in x x
> >>
> >> <interactive>:1:30:
> >>       Occurs check: cannot construct the infinite type: t0 ~ t0 -> t
> >>       Relevant bindings include
> >>         x :: t0 -> t (bound at <interactive>:1:11)
> >>         y :: t (bound at <interactive>:1:2)
> >>       In the first argument of ‘x’, namely ‘x’
> >>
> > _______________________________________________
> > Glasgow-haskell-users mailing list
> > Glasgow-haskell-users at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
> >
>