OverloadedRecordFields extension for GHC allows multiple record datatypes to share the same field names, and uses type information to disambiguate them. For more information, see:
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Lennart: I've implemented 2&3 in the Mu compiler, and I'll add some comments about it.
Lennart: The MagicClasses proposal is fundamentally broken, because it breaks abstraction.
If we have a data type
R with a field
foo of type
T, then we generate
instance HasField "foo" R T.
Since instances are silently exported and imported it means that this instance is now available in every module that somehow depends on the defining module. This means that the
foo field of
R is now accessible everywhere. There is no way to limit the scope of
foo anymore. This is really terrible. Any record proposal that no longer allows abstract data types to be defined is broken.
Adam: I don't think so. We will only solve a
HasField constraint automatically in modules that have the field in scope, and thus we can retain the same abstraction rules as normal Haskell. This is just like
Coercible. We don't really generate and export instances, rather there is special behaviour in the constraint solver. See discussion on representation hiding (now updated to the latest design).
Phab:D761, Phab:D1391, Phab:D1486, Phab:D1586, Phab:D1600:
- Prototype implementation of the magic typeclasses
The extension was implemented in 2013 as a Google Summer of Code project, by Adam Gundry under the mentorship of Simon Peyton Jones.