[project @ 2002-10-18 13:41:50 by simonpj]

[project @ 2002-10-18 13:41:50 by simonpj]
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   Fix a serious error in the "newtype deriving" feature
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The "newtype deriving" feature lets you derive arbitrary classes for
a newtype, not just the built-in ones (Read, Show, Ix etc).  It's very
cool, but Hal Duame discovered that it did utterly the Wrong Thing
for superclasses.  E.g.

	newtype Foo = MkFoo Int deriving( Show, Num, Eq )

You'd get a Num instance for Foo that was *identical* to the
Num instance for Int, *including* the Show superclass. So the
superclass in the Num dictionary would show a Foo just like an
Int, which is wrong... it should show as "Foo n".

This commit fixes the problem, by building a new dictionary every time,
but using the methods from the dictionary for the representation type.

I also fixed a bug that prevented it working altogether when the
representation type was not the application of a type constructor.
For example, this now works

	newtype Foo a = MkFoo a deriving( Num, Eq, Show )

I also made it a bit more efficient in the case where the type is
not parameterised.  Then the "dfun" doesn't need to be a function.