diff options
Diffstat (limited to 'compiler/GHC/Runtime/Eval.hs')
| -rw-r--r-- | compiler/GHC/Runtime/Eval.hs | 42 |
1 files changed, 1 insertions, 41 deletions
diff --git a/compiler/GHC/Runtime/Eval.hs b/compiler/GHC/Runtime/Eval.hs index 0891da5808..aad557579b 100644 --- a/compiler/GHC/Runtime/Eval.hs +++ b/compiler/GHC/Runtime/Eval.hs @@ -81,7 +81,6 @@ import GHC.ByteCode.Types import GHC.Runtime.Linker as Linker import GHC.Driver.Session import GHC.Driver.Ppr -import GHC.LanguageExtensions import GHC.Types.Unique import GHC.Types.Unique.Supply import GHC.Utils.Monad @@ -1283,10 +1282,7 @@ obtainTermFromVal hsc_env _bound _force _ty _x = withInterp hsc_env $ \case obtainTermFromId :: HscEnv -> Int -> Bool -> Id -> IO Term obtainTermFromId hsc_env bound force id = do hv <- Linker.getHValue hsc_env (varName id) - cvObtainTerm (updEnv hsc_env) bound force (idType id) hv - where updEnv env = env {hsc_dflags = -- #14828 - xopt_set (hsc_dflags env) ImpredicativeTypes} - -- See Note [Setting ImpredicativeTypes for :print command] + cvObtainTerm hsc_env bound force (idType id) hv -- Uses RTTI to reconstruct the type of an Id, making it less polymorphic reconstructType :: HscEnv -> Int -> Id -> IO (Maybe Type) @@ -1296,39 +1292,3 @@ reconstructType hsc_env bound id = do mkRuntimeUnkTyVar :: Name -> Kind -> TyVar mkRuntimeUnkTyVar name kind = mkTcTyVar name kind RuntimeUnk - - -{- -Note [Setting ImpredicativeTypes for :print command] - -If ImpredicativeTypes is not enabled, then `:print <term>` will fail if the -type of <term> has nested `forall`s or `=>`s. -This is because the GHCi debugger's internals will attempt to unify a -metavariable with the type of <term> and then display the result, but if the -type has nested `forall`s or `=>`s, then unification will fail. -As a result, `:print` will bail out and the unhelpful result will be -`<term> = (_t1::t1)` (where `t1` is a metavariable). - -Beware: <term> can have nested `forall`s even if its definition doesn't use -RankNTypes! Here is an example from #14828: - - class Functor f where - fmap :: (a -> b) -> f a -> f b - -Somewhat surprisingly, `:print fmap` considers the type of fmap to have -nested foralls. This is because the GHCi debugger sees the type -`fmap :: forall f. Functor f => forall a b. (a -> b) -> f a -> f b`. -We could envision deeply instantiating this type to get the type -`forall f a b. Functor f => (a -> b) -> f a -> f b`, -but this trick wouldn't work for higher-rank types. - -Instead, we adopt a simpler fix: enable `ImpredicativeTypes` when using -`:print` and friends in the GHCi debugger. This allows metavariables -to unify with types that have nested (or higher-rank) `forall`s/`=>`s, -which makes `:print fmap` display as -`fmap = (_t1::forall a b. Functor f => (a -> b) -> f a -> f b)`, as expected. - -Although ImpredicativeTypes is a somewhat unpredictable from a type inference -perspective, there is no danger in using it in the GHCi debugger, since all -of the terms that the GHCi debugger deals with have already been typechecked. --} |