Can't define a lens for a field with a constraint
{-# LANGUAGE RankNTypes #-}
type Lens' s a = forall f. Functor f => (a -> f a) -> s -> f s
data T a = MkT { _tfield :: Eq a => a }
tfield :: Eq a => Lens' (T a) a
tfield f t = MkT <$> f (_tfield t)
This code compiles with GHC 8.2.1. On GHC 8.0.2 the following error is reported:
tfield.hs:8:22: error:
• Couldn't match type ‘a’ with ‘Eq a => a’
‘a’ is a rigid type variable bound by
the type signature for:
tfield :: forall a. Eq a => Lens' (T a) a
at tfield.hs:7:11
Expected type: f (Eq a => a)
Actual type: f a
• In the second argument of ‘(<$>)’, namely ‘f (_tfield t)’
In the expression: MkT <$> f (_tfield t)
In an equation for ‘tfield’: tfield f t = MkT <$> f (_tfield t)
• Relevant bindings include
t :: T a (bound at tfield.hs:8:10)
f :: a -> f a (bound at tfield.hs:8:8)
tfield :: Lens' (T a) a (bound at tfield.hs:8:1)
I could not find a relevant GHC ticket. Unless it's a known issue, I am going to create a Phab Diff with this code as a test case to avoid a regression in the future.
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |