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Diffstat (limited to 'docs/users_guide')
| -rw-r--r-- | docs/users_guide/using-warnings.rst | 40 |
1 files changed, 39 insertions, 1 deletions
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst index b72ae4250b..87ddcdabf7 100644 --- a/docs/users_guide/using-warnings.rst +++ b/docs/users_guide/using-warnings.rst @@ -33,6 +33,7 @@ generally likely to indicate bugs in your program. These are: * :ghc-flag:`-Wunsupported-llvm-version` * :ghc-flag:`-Wtabs` * :ghc-flag:`-Wunrecognised-warning-flags` + * :ghc-flag:`-Winaccessible-code` The following flags are simple ways to select standard "packages" of warnings: @@ -685,7 +686,7 @@ of ``-W(no-)*``. Similar warnings are given for a redundant constraint in an instance declaration. - When turning on, you can suppress it on a per-module basis with + When turning on, you can suppress it on a per-module basis with :ghc-flag:`-Wno-redundant-constraints <-Wredundant-constraints>`. Occasionally you may specifically want a function to have a more constrained signature than necessary, perhaps to leave yourself @@ -1088,6 +1089,43 @@ of ``-W(no-)*``. second pattern overlaps it. More often than not, redundant patterns is a programmer mistake/error, so this option is enabled by default. +.. ghc-flag:: -Winaccessible-code + :shortdesc: warn about inaccessible code + :type: dynamic + :reverse: -Wno-inaccessible-code + :category: + + .. index:: + single: inaccessible code, warning + single: inaccessible + + By default, the compiler will warn you if types make a branch inaccessible. + This generally requires GADTs or similar extensions. + + Take, for example, the following program :: + + {-# LANGUAGE GADTs #-} + + data Foo a where + Foo1 :: Foo Char + Foo2 :: Foo Int + + data TyEquality a b where + Refl :: TyEquality a a + + checkTEQ :: Foo t -> Foo u -> Maybe (TyEquality t u) + checkTEQ x y = error "unimportant" + + step2 :: Bool + step2 = case checkTEQ Foo1 Foo2 of + Just Refl -> True -- Inaccessible code + Nothing -> False + + The ``Just Refl`` case in ``step2`` is inaccessible, because in order for + ``checkTEQ`` to be able to produce a ``Just``, ``t ~ u`` must hold, but + since we're passing ``Foo1`` and ``Foo2`` here, it follows that ``t ~ + Char``, and ``u ~ Int``, and thus ``t ~ u`` cannot hold. + .. ghc-flag:: -Wsimplifiable-class-constraints :shortdesc: 2arn about class constraints in a type signature that can be simplified using a top-level instance declaration. |