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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/exts/poly_kinds.rst | 31 |
1 files changed, 31 insertions, 0 deletions
diff --git a/docs/users_guide/exts/poly_kinds.rst b/docs/users_guide/exts/poly_kinds.rst index 5b34ae10e4..37068125de 100644 --- a/docs/users_guide/exts/poly_kinds.rst +++ b/docs/users_guide/exts/poly_kinds.rst @@ -437,6 +437,37 @@ parameters:: Note that ``F0``, ``F1``, ``F2``, ``FD1``, and ``FD2`` all have identical standalone kind signatures. The arity is inferred from the type family header. +The kind variables bound by an outermost ``forall`` in a standalone kind +signature scope only over the kind in that signature. Unlike term-level type +signatures (see :ref:`decl-type-sigs`), the outermost kind variables do *not* +scope over the corresponding declaration. For example, given this class +declaration: :: + + class C (a :: k) where + m :: Proxy k -> Proxy a -> String + +The following would *not* be an equivalent definition of ``C``: :: + + type C :: forall k. k -> Constraint + class C a where + m :: Proxy k -> Proxy a -> String + +Because the ``k`` from the standalone kind signature does not scope over +``C``'s definition, the ``k`` in ``m``'s type signature is no longer the kind +of ``a``, but rather a completely distinct kind. It's as if you had written +this: :: + + type C :: forall k. k -> Constraint + class C (a :: kindOfA) where + m :: forall k. Proxy k -> Proxy (a :: kindOfA) -> String + +To avoid this issue, ``C``'s definition must be given an inline kind annotation +like so: :: + + type C :: forall k. k -> Constraint + class C (a :: k) where + m :: Proxy k -> Proxy a -> String + Standalone kind signatures and declaration headers -------------------------------------------------- |