diff options
Diffstat (limited to 'compiler/GHC/Core/DataCon.hs')
| -rw-r--r-- | compiler/GHC/Core/DataCon.hs | 33 |
1 files changed, 30 insertions, 3 deletions
diff --git a/compiler/GHC/Core/DataCon.hs b/compiler/GHC/Core/DataCon.hs index 064cdc866f..01ab6414c3 100644 --- a/compiler/GHC/Core/DataCon.hs +++ b/compiler/GHC/Core/DataCon.hs @@ -250,6 +250,8 @@ in wrapper_reqd in GHC.Types.Id.Make.mkDataConRep. * Type variables may be permuted; see MkId Note [Data con wrappers and GADT syntax] +* Datatype contexts require dropping some dictionary arguments. + See Note [Instantiating stupid theta]. Note [The stupid context] ~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -282,7 +284,7 @@ gets inferred type I say the context is "stupid" because the dictionaries passed are immediately discarded -- they do nothing and have no benefit. -(See Note [Instantiating stupid theta] in GHC.Tc.Gen.Head.) +(See Note [Instantiating stupid theta].) It's a flaw in the language. GHC has made some efforts to correct this flaw. In GHC, datatype contexts @@ -326,6 +328,30 @@ Some other notes about stupid contexts: result, dcStupidTheta is always empty for data types defined using GADT syntax. +Note [Instantiating stupid theta] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider a data type with a "stupid theta" (see +Note [The stupid context]): + + data Ord a => T a = MkT (Maybe a) + +We want to generate an Ord constraint for every use of MkT; but +we also want to allow visible type application, such as + + MkT @Int + +To achieve this, the wrapper for a data (or newtype) constructor +with a datatype context contains a lambda which drops the dictionary +argments corresponding to the datatype context: + + /\a \(_d:Ord a). MkT @a + +Notice that the wrapper discards the dictionary argument d. +We don't need it; it was only there to generate a Wanted constraint. +(That is why it is stupid.) + +This all happens in GHC.Types.Id.Make.mkDataConRep. + ************************************************************************ * * \subsection{Data constructors} @@ -1449,9 +1475,10 @@ dataConWrapperType :: DataCon -> Type -- mentions the family tycon, not the internal one. dataConWrapperType (MkData { dcUserTyVarBinders = user_tvbs, dcOtherTheta = theta, dcOrigArgTys = arg_tys, - dcOrigResTy = res_ty }) + dcOrigResTy = res_ty, + dcStupidTheta = stupid_theta }) = mkInvisForAllTys user_tvbs $ - mkInvisFunTysMany theta $ + mkInvisFunTysMany (stupid_theta ++ theta) $ mkVisFunTys arg_tys $ res_ty |