Hmm.. okay, here is a rough draft that covers all the important cases I
think.
assume the following declarations:
class C1 a where
f1 :: t1
f1 = d1
class C2 a where
f2 :: t2
f2 = d2
f3 :: t3
f3 = d3
class alias S a => A a = (C1 a, C2 a) where
f1 = nd1
okay, the desugaring is as follows:
there are two different desugaring rules, one for instances, one for the
alias appearing anywhere other than an instance declaration:
g :: A a => a -> b
g = ...
translates to
g :: (S a, C1 a, C2 a) => a -> b
g = ...
the triplet of (S a, C1 a, C2 a) is completely equivalent to (A a) in
all ways and all places (other than instance heads), one is just a
different way to express the other, just like type synonyms. An alias
just expands to the union of the classes it is an alias for as well as
its class constraints (superclasses).
now for instance declarations
instance A a where
f2 = bf2
expands to
instance (S a) => C1 a where
f1 = nd1
instance (S a) => C2 a where
f2 = bf2
f3 = d3
Note that when declaring an instance of a concrete type, like Int, the
constraint (S Int) will be trivially satisfied or not at compile time.
(bf2 is free to use methods of 'S' of course).
this translation is also a bijection, declaring those two instances
manually as above is indistinguishable from declaring instances via the
alias in all ways.
Hopefully the generalization to arbitrary numbers of classes is clear...
What about multiple parameters? Can A have more parameters than the Ci?
Should they be in the same order? Should they overlap?
What about instance contexts, like:
instance I a => A a where ...
(What about functional dependencies?)
Tom
--
Tom Schrijvers
Department of Computer Science
K.U. Leuven
Celestijnenlaan 200A
B-3001 Heverlee
Belgium
tel: +32 16 327544
e-mail: [EMAIL PROTECTED]
url: http://www.cs.kuleuven.be/~toms/
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