instance union proposal
Serge D. Mechveliani
mechvel at botik.ru
Sat Nov 12 11:50:46 CET 2011
Dear Haskell implementors,
I suggest the following small extension to the instance declaration in
the language. So far -- for Haskell + glasgow-ext.
I think that they are easy to implement.
This is the "instance union" proposal.
It is needed to write shorter several `old' instance declarations.
This will make programs easier to read.
It suggests the so-called
inherited decl and, more general, union decl.
Inherited instance decl proposal
Union several instance declarations with the same condition part and such
that among the conclusion classes there exists some which inherits all
Example 1. My program uses the class tower
Field a Picture 1.
AddGroup a MulSemigroup a
AddSemigroup a |
-- "|" means that the upper inherits from the lower.
Now, by the application meaning, I need to write
instance (Show a, CRing a) => CRing (Pol a)
<implement operations of Set>
<implement operations of AddGroup>
<implement operations of CRing>
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