diff options
| -rw-r--r-- | compiler/GHC/Tc/Module.hs | 37 | ||||
| -rw-r--r-- | docs/users_guide/ghci.rst | 8 | ||||
| -rw-r--r-- | testsuite/tests/ghci/scripts/T20974.hs | 15 | ||||
| -rw-r--r-- | testsuite/tests/ghci/scripts/T20974.script | 2 | ||||
| -rw-r--r-- | testsuite/tests/ghci/scripts/T20974.stdout | 1 | ||||
| -rwxr-xr-x | testsuite/tests/ghci/scripts/all.T | 1 |
6 files changed, 56 insertions, 8 deletions
diff --git a/compiler/GHC/Tc/Module.hs b/compiler/GHC/Tc/Module.hs index 40bc5188f6..11278d6bc7 100644 --- a/compiler/GHC/Tc/Module.hs +++ b/compiler/GHC/Tc/Module.hs @@ -2576,14 +2576,12 @@ tcRnExpr hsc_env mode rdr_expr mkPhiTy (map idType dicts) res_ty } ; ty <- zonkTcType all_expr_ty ; - -- We normalise type families, so that the type of an expression is the - -- same as of a bound expression (GHC.Tc.Gen.Bind.mkInferredPolyId). See Trac - -- #10321 for further discussion. + -- See Note [Normalising the type in :type] fam_envs <- tcGetFamInstEnvs ; - -- normaliseType returns a coercion which we discard, so the Role is - -- irrelevant - return (reductionReducedType (normaliseType fam_envs Nominal ty)) - } + let { normalised_type = reductionReducedType $ normaliseType fam_envs Nominal ty + -- normaliseType returns a coercion which we discard, so the Role is irrelevant. + ; final_type = if isSigmaTy res_ty then ty else normalised_type } ; + return final_type } where -- Optionally instantiate the type of the expression -- See Note [TcRnExprMode] @@ -2608,6 +2606,31 @@ and not forall {b}. Int -> b -> Int Solution: use tcInferSigma, which in turn uses tcInferApp, which has a special case for application chains. + +Note [Normalising the type in :type] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +In :t <expr> we usually normalise the type (to simplify type functions) +before displaying the result. Reason (see #10321): otherwise we may show +types like + <expr> :: Vec (1+2) Int +rather than the simpler + <expr> :: Vec 3 Int +In GHC.Tc.Gen.Bind.mkInferredPolyId we normalise for a very similar reason. + +However this normalisation is less helpful when <expr> is just +an identifier, whose user-written type happens to contain type-function +applications. E.g. (#20974) + test :: F [Monad, A, B] m => m () +where F is a type family. If we say `:t test`, we'd prefer to see +the type family un-expanded. + +We adopt the following ad-hoc solution: if the type inferred for <expr> +(before generalisation, namely res_ty) is a SigmaType (i.e. is not +fully instantiated) then do not normalise; otherwise normalise. +This is not ideal; for example, suppose x :: F Int. Then + :t x +would be normalised because `F Int` is not a SigmaType. But +anything here is ad-hoc, and it's a user-sought improvement. -} -------------------------- diff --git a/docs/users_guide/ghci.rst b/docs/users_guide/ghci.rst index cb98b15f6a..c26dedb5c3 100644 --- a/docs/users_guide/ghci.rst +++ b/docs/users_guide/ghci.rst @@ -2981,13 +2981,19 @@ commonly used commands. Infers and prints the type of ⟨expression⟩. For polymorphic types it instantiates the 'inferred' forall quantifiers (but not the - 'specified' ones; see :ref:`inferred-vs-specified`), solves constraints, and re-generalises. + 'specified' ones; see :ref:`inferred-vs-specified`), solves constraints, + re-generalises, and then reduces type families as much as possible. .. code-block:: none *X> :type length length :: Foldable t => t a -> Int + Type family reduction is skipped if the function is not fully instantiated, + as this has been observed to give more intuitive results. + You may want to use :ghci-cmd:`:info` if you are not applying any arguments, + as that will return the original type of the function without instantiating. + .. ghci-cmd:: :type +d; ⟨expression⟩ Infers and prints the type of ⟨expression⟩, instantiating *all* the forall diff --git a/testsuite/tests/ghci/scripts/T20974.hs b/testsuite/tests/ghci/scripts/T20974.hs new file mode 100644 index 0000000000..6d02dea7cf --- /dev/null +++ b/testsuite/tests/ghci/scripts/T20974.hs @@ -0,0 +1,15 @@ +{-# LANGUAGE DataKinds, KindSignatures, TypeFamilies, TypeOperators #-} + +module T20974 where + +import Data.Kind + +class A (m :: Type -> Type) +class B (m :: Type -> Type) + +type family F cs (m :: Type -> Type) :: Constraint where + F '[] m = () + F (c : cs) m = (c m, F cs m) + +test :: F [Monad, A, B] m => m () +test = pure () diff --git a/testsuite/tests/ghci/scripts/T20974.script b/testsuite/tests/ghci/scripts/T20974.script new file mode 100644 index 0000000000..730444a54b --- /dev/null +++ b/testsuite/tests/ghci/scripts/T20974.script @@ -0,0 +1,2 @@ +:l T20974 +:type test diff --git a/testsuite/tests/ghci/scripts/T20974.stdout b/testsuite/tests/ghci/scripts/T20974.stdout new file mode 100644 index 0000000000..6cfee4e9a4 --- /dev/null +++ b/testsuite/tests/ghci/scripts/T20974.stdout @@ -0,0 +1 @@ +test :: F '[Monad, A, B] m => m () diff --git a/testsuite/tests/ghci/scripts/all.T b/testsuite/tests/ghci/scripts/all.T index e35d3b804d..bccfa977e5 100755 --- a/testsuite/tests/ghci/scripts/all.T +++ b/testsuite/tests/ghci/scripts/all.T @@ -362,3 +362,4 @@ test('T20587', [extra_files(['../shell.hs'])], ghci_script, ['T20587.script']) test('T20909', normal, ghci_script, ['T20909.script']) test('T20150', normal, ghci_script, ['T20150.script']) +test('T20974', normal, ghci_script, ['T20974.script']) |