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’ > _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
