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@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@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’