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Diffstat (limited to 'docs/users_guide/exts/explicit_forall.rst')
| -rw-r--r-- | docs/users_guide/exts/explicit_forall.rst | 27 |
1 files changed, 16 insertions, 11 deletions
diff --git a/docs/users_guide/exts/explicit_forall.rst b/docs/users_guide/exts/explicit_forall.rst index 50938e98d0..040145444a 100644 --- a/docs/users_guide/exts/explicit_forall.rst +++ b/docs/users_guide/exts/explicit_forall.rst @@ -56,30 +56,32 @@ The ``forall``-or-nothing rule ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In certain forms of types, type variables obey what is known as the -"``forall``-or-nothing" rule: if a type has an outermost, explicit -``forall``, then all of the type variables in the type must be explicitly -quantified. These two examples illustrate how the rule works: :: +"``forall``-or-nothing" rule: if a type has an outermost, explicit, +invisible ``forall``, then all of the type variables in the type must be +explicitly quantified. These two examples illustrate how the rule works: :: f :: forall a b. a -> b -> b -- OK, `a` and `b` are explicitly bound g :: forall a. a -> forall b. b -> b -- OK, `a` and `b` are explicitly bound h :: forall a. a -> b -> b -- Rejected, `b` is not in scope The type signatures for ``f``, ``g``, and ``h`` all begin with an outermost -``forall``, so every type variable in these signatures must be explicitly -bound by a ``forall``. Both ``f`` and ``g`` obey the ``forall``-or-nothing -rule, since they explicitly quantify ``a`` and ``b``. On the other hand, -``h`` does not explicitly quantify ``b``, so GHC will reject its type -signature for being improperly scoped. +invisible ``forall``, so every type variable in these signatures must be +explicitly bound by a ``forall``. Both ``f`` and ``g`` obey the +``forall``-or-nothing rule, since they explicitly quantify ``a`` and ``b``. On +the other hand, ``h`` does not explicitly quantify ``b``, so GHC will reject +its type signature for being improperly scoped. In places where the ``forall``-or-nothing rule takes effect, if a type does -*not* have an outermost ``forall``, then any type variables that are not -explicitly bound by a ``forall`` become implicitly quantified. For example: :: +*not* have an outermost invisible ``forall``, then any type variables that are +not explicitly bound by a ``forall`` become implicitly quantified. For example: :: i :: a -> b -> b -- `a` and `b` are implicitly quantified j :: a -> forall b. b -> b -- `a` is implicitly quantified k :: (forall a. a -> b -> b) -- `b` is implicitly quantified + type L :: forall a -> b -> b -- `b` is implicitly quantified -GHC will accept ``i``, ``j``, and ``k``'s type signatures. Note that: +GHC will accept ``i``, ``j``, and ``k``'s type signatures, as well as ``L``'s +kind signature. Note that: - ``j``'s signature is accepted despite its mixture of implicit and explicit quantification. As long as a ``forall`` is not an outermost one, it is fine @@ -88,6 +90,9 @@ GHC will accept ``i``, ``j``, and ``k``'s type signatures. Note that: the ``forall`` is not an outermost ``forall``. The ``forall``-or-nothing rule is one of the few places in GHC where the presence or absence of parentheses can be semantically significant! +- ``L``'s signature begins with an outermost ``forall``, but it is a *visible* + ``forall``, not an invisible ``forall``, and therefore does not trigger the + ``forall``-or-nothing rule. The ``forall``-or-nothing rule takes effect in the following places: |