Type wildcard infers differently than no type signature
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE PartialTypeSignatures #-}
module Main where
import Optics
main :: IO ()
main = putStrLn "Hello, Haskell!"
g :: Lens' String Char
g = undefined
f :: _
f o = "foo" & (g%o) .~ 10Full source code of the test case at: https://gitlab.haskell.org/dminuoso/ghc-wildcard-bug/
The above code fails to type check with:
Without the wildcard type annotation on f it compiles and infers the following type for f:
*Main> :t f
f :: (Is (Join A_Lens l) A_Setter, Is l (Join A_Lens l),
Is A_Lens (Join A_Lens l), Num b) =>
Optic l js Char Char a b -> String
*Main> Leaving the wildcard in, the type checker bails out.
[nix-shell:~/git/ghc-wildcard-bug]$ cabal build
Build profile: -w ghc-8.8.4 -O1
In order, the following will be built (use -v for more details):
- ghc-wildcard-bug-0.1.0.0 (exe:ghc-wildcard-bug) (file Main.hs changed)
Preprocessing executable 'ghc-wildcard-bug' for ghc-wildcard-bug-0.1.0.0..
Building executable 'ghc-wildcard-bug' for ghc-wildcard-bug-0.1.0.0..
[1 of 1] Compiling Main ( Main.hs, /home/dminuoso/git/ghc-wildcard-bug/dist-newstyle/build/x86_64-linux/ghc-8.8.4/ghc-wildcard-bug-0.1.0.0/x/ghc-wildcard-bug/build/ghc-wildcard-bug/ghc-wildcard-bug-tmp/Main.o )
Main.hs:15:15: error:
• Overlapping instances for Is (Join A_Lens l0) A_Setter
arising from a use of ‘.~’
Matching instances:
instance [overlappable] (TypeError ...) => Is k l
-- Defined in ‘optics-core-0.3.0.1:Optics.Internal.Optic.Subtyping’
instance Is k k
-- Defined in ‘optics-core-0.3.0.1:Optics.Internal.Optic.Subtyping’
instance Is A_Lens A_Setter
-- Defined in ‘optics-core-0.3.0.1:Optics.Internal.Optic.Subtyping’
...plus four others
(use -fprint-potential-instances to see them all)
(The choice depends on the instantiation of ‘l0’
To pick the first instance above, use IncoherentInstances
when compiling the other instance declarations)
• In the second argument of ‘(&)’, namely ‘(g % o) .~ 10’
In the expression: "foo" & (g % o) .~ 10
In an equation for ‘f’: f o = "foo" & (g % o) .~ 10
|
15 | f o = "foo" & (g%o) .~ 10
| ^^^^^^^^^^^
Main.hs:15:16: error:
• Overlapping instances for Is l0 (Join A_Lens l0)
arising from a use of ‘%’
Matching instances:
instance [overlappable] (TypeError ...) => Is k l
-- Defined in ‘optics-core-0.3.0.1:Optics.Internal.Optic.Subtyping’
instance Is k k
-- Defined in ‘optics-core-0.3.0.1:Optics.Internal.Optic.Subtyping’
instance Is A_Getter A_Fold
-- Defined in ‘optics-core-0.3.0.1:Optics.Internal.Optic.Subtyping’
...plus 35 others
(use -fprint-potential-instances to see them all)
(The choice depends on the instantiation of ‘l0’
To pick the first instance above, use IncoherentInstances
when compiling the other instance declarations)
• In the first argument of ‘(.~)’, namely ‘(g % o)’
In the second argument of ‘(&)’, namely ‘(g % o) .~ 10’
In the expression: "foo" & (g % o) .~ 10
|
15 | f o = "foo" & (g%o) .~ 10
| Also tried enabling NoMonomorphismRestriction to no avail.