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

[Ki Yung Ahn]

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’
>>
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