diff options
Diffstat (limited to 'compiler/GHC/Core/TyCon.hs')
| -rw-r--r-- | compiler/GHC/Core/TyCon.hs | 30 |
1 files changed, 12 insertions, 18 deletions
diff --git a/compiler/GHC/Core/TyCon.hs b/compiler/GHC/Core/TyCon.hs index aa5c3460b0..e4f31e9fe0 100644 --- a/compiler/GHC/Core/TyCon.hs +++ b/compiler/GHC/Core/TyCon.hs @@ -293,7 +293,14 @@ See also Note [Wrappers for data instance tycons] in GHC.Types.Id.Make Indeed the latter type is unknown to the programmer. - There *is* an instance for (T Int) in the type-family instance - environment, but it is only used for overlap checking + environment, but it is looked up (via tcLookupDataFamilyInst) + in can_eq_nc (via tcTopNormaliseNewTypeTF_maybe) when trying to + solve representational equalities like + T Int ~R# Bool + Here we look up (T Int), convert it to R:TInt, and then unwrap the + newtype R:TInt. + + It is also looked up in reduceTyFamApp_maybe. - It's fine to have T in the LHS of a type function: type instance F (T a) = [a] @@ -1251,34 +1258,21 @@ example, newtype T a = MkT (a -> a) -the NewTyCon for T will contain nt_co = CoT where CoT t : T t ~ t -> t. - -In the case that the right hand side is a type application -ending with the same type variables as the left hand side, we -"eta-contract" the coercion. So if we had - - newtype S a = MkT [a] - -then we would generate the arity 0 axiom CoS : S ~ []. The -primary reason we do this is to make newtype deriving cleaner. +the NewTyCon for T will contain nt_co = CoT where CoT :: forall a. T a ~ a -> a. -In the paper we'd write - axiom CoT : (forall t. T t) ~ (forall t. [t]) -and then when we used CoT at a particular type, s, we'd say - CoT @ s -which encodes as (TyConApp instCoercionTyCon [TyConApp CoT [], s]) +We might also eta-contract the axiom: see Note [Newtype eta]. Note [Newtype eta] ~~~~~~~~~~~~~~~~~~ Consider newtype Parser a = MkParser (IO a) deriving Monad -Are these two types equal (to Core)? +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. Well, yes. But to see that easily we eta-reduce the RHS type of -Parser, in this case to ([], Froogle), so that even unsaturated applications +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. |