diff options
Diffstat (limited to 'docs/users_guide')
| -rw-r--r-- | docs/users_guide/exts/implicit_parameters.rst | 23 |
1 files changed, 21 insertions, 2 deletions
diff --git a/docs/users_guide/exts/implicit_parameters.rst b/docs/users_guide/exts/implicit_parameters.rst index a8da87e416..ee6234e55a 100644 --- a/docs/users_guide/exts/implicit_parameters.rst +++ b/docs/users_guide/exts/implicit_parameters.rst @@ -180,6 +180,27 @@ parameter. So we get the following results in GHCi: Adding a type signature dramatically changes the result! This is a rather counter-intuitive phenomenon, worth watching out for. +Implicit parameters scoping guarantees +------------------------------------- + +GHC always takes the most nested implicit parameter binding from the +context to find the value. Consider the following code:: + + let ?f = 1 in let ?f = 2 in ?f + +This expression will always return 2. + +Another example of this rule is matching over constructors with constraints. +For example:: + + data T where + MkT :: (?f :: Int) => T + + f :: T -> T -> Int + f MkT MkT = ?f + +Here GHC will always take ``?f`` from the last match. + Implicit parameters and monomorphism ------------------------------------ @@ -199,5 +220,3 @@ a type signature for ``y``, then ``y`` will get type ``(?x::Int) => Int``, so the occurrence of ``y`` in the body of the ``let`` will see the inner binding of ``?x``, so ``(f 9)`` will return ``14``. - - |