#5677: Allow the definition of extra functions in typeclass instances
---------------------------------+------------------------------------------
Reporter: FUZxxl | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 7.2.1 | Keywords: instance typeclass extension
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: GHC rejects valid program
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It would be nice, if one could define functions in an instance declaration
that don't exist in the corresponding typeclass. Such a function should be
only visible to the functions in the instance it is defined in.
I can imagine two purposes for such functions:
Consider a library MyLib that has a typeclass Foo. In version 1.0 of the
library, Foo is defined as:
{{{
class Foo a where
foo :: ...
bar :: ...
}}}
but in version 2.0, an additional member baz was added. If you want to
write an instance of Foo that works both on 1.0 and 2.0, you currently
have to use the preprocessor to hide the definition of baz if linked
against library version 1.0. It would make writing code that works with
multiple versions of other libraries much easier, if an extranous function
in an instance is allowed.
The other purpose is that one can put common code of member functions in
an extra function without moving the code out of the instance body.
Together with ScopedTypeVariables, this might turn out to be pretty
useful.
As an addition, I would recommend to add a warning for the case, that an
instance declares a function that is neither present in the class
declaration nor used in the instance itself. This would catch most simple
errors.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5677>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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