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| author | vivid-synth <vivid.haskell@gmail.com> | 2017-02-14 09:51:54 -0500 |
|---|---|---|
| committer | Ben Gamari <ben@smart-cactus.org> | 2017-02-14 10:53:01 -0500 |
| commit | c3bbd1afc85cd634d8d26e27bafb92cc7481667b (patch) | |
| tree | 729e78fe6b02eecef0035d4220cdad44d3b401a0 /docs/users_guide/8.2.1-notes.rst | |
| parent | 2f1017b924740e66f093b0baba62ac0b1528abf8 (diff) | |
| download | haskell-c3bbd1afc85cd634d8d26e27bafb92cc7481667b.tar.gz | |
Allow type defaulting for multi-param type classes with ExtendedDefaultRules
Expressions like the following will now typecheck:
```
data A x = A deriving Show
class ToA a x where
toA :: a -> A x
instance ToA Integer x where
toA _ = A
main = print (toA 5 :: A Bool)
```
The new defaulting rules are
Find all the unsolved constraints. Then:
* Find those that have exactly one free type variable, and partition
that subset into groups that share a common type variable `a`.
* Now default `a` (to one of the types in the default list) if at least
one of the classes `Ci` is an interactive class
Reviewers: goldfire, bgamari, austin, mpickering, simonpj
Reviewed By: bgamari, simonpj
Subscribers: mpickering, simonpj, goldfire, thomie
Differential Revision: https://phabricator.haskell.org/D2822
Diffstat (limited to 'docs/users_guide/8.2.1-notes.rst')
| -rw-r--r-- | docs/users_guide/8.2.1-notes.rst | 3 |
1 files changed, 3 insertions, 0 deletions
diff --git a/docs/users_guide/8.2.1-notes.rst b/docs/users_guide/8.2.1-notes.rst index 00e6c7c4b5..d70dc50e42 100644 --- a/docs/users_guide/8.2.1-notes.rst +++ b/docs/users_guide/8.2.1-notes.rst @@ -154,6 +154,9 @@ Compiler allocation and a potential space leak when deriving ``Functor`` for a recursive type. +- The :ghc-flag:`-XExtendedDefaultRules` extension now defaults multi-parameter + typeclasses. See :ghc-ticket:`12923`. + GHCi ~~~~ |