diff options
Diffstat (limited to 'compiler/GHC/Core/TyCon.hs')
| -rw-r--r-- | compiler/GHC/Core/TyCon.hs | 30 |
1 files changed, 18 insertions, 12 deletions
diff --git a/compiler/GHC/Core/TyCon.hs b/compiler/GHC/Core/TyCon.hs index 56fe29cb7e..d972752e9a 100644 --- a/compiler/GHC/Core/TyCon.hs +++ b/compiler/GHC/Core/TyCon.hs @@ -1337,17 +1337,22 @@ Note [Newtype eta] ~~~~~~~~~~~~~~~~~~ Consider newtype Parser a = MkParser (IO a) deriving Monad -Are these two types equal (that is, does a coercion exist between them)? +Are these two types equal? That is, does a coercion exist between them? Monad Parser Monad IO -which we need to make the derived instance for Monad Parser. +(We need this coercion to make the derived instance for Monad Parser.) Well, yes. But to see that easily we eta-reduce the RHS type of -Parser, in this case to IO, so that even unsaturated applications -of Parser will work right. This eta reduction is done when the type -constructor is built, and cached in NewTyCon. +Parser, in this case to IO, so that even unsaturated applications of +Parser will work right. So instead of + axParser :: forall a. Parser a ~ IO a +we generate an eta-reduced axiom + axParser :: Parser ~ IO -Here's an example that I think showed up in practice +This eta reduction is done when the type constructor is built, in +GHC.Tc.TyCl.Build.mkNewTyConRhs, and cached in NewTyCon. + +Here's an example that I think showed up in practice. Source code: newtype T a = MkT [a] newtype Foo m = MkFoo (forall a. m a -> Int) @@ -1359,14 +1364,15 @@ Source code: w2 = MkFoo (\(MkT x) -> case w1 of MkFoo f -> f x) After desugaring, and discarding the data constructors for the newtypes, -we get: - w2 = w1 `cast` Foo CoT -so the coercion tycon CoT must have - kind: T ~ [] - and arity: 0 +we would like to get: + w2 = w1 `cast` Foo axT -This eta-reduction is implemented in GHC.Tc.TyCl.Build.mkNewTyConRhs. +so that w2 and w1 share the same code. To do this, the coercion axiom +axT must have + kind: axT :: T ~ [] + and arity: 0 +See also Note [Newtype eta and homogeneous axioms] in GHC.Tc.TyCl.Build. ************************************************************************ * * |