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

[Edward Z. Yang]

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’
>