Haskell' - class aliases
Claus Reinke
claus.reinke at talk21.com
Fri May 2 09:06:24 EDT 2008
> A aliasing of constraints/classes
> (this is the semantic part that could also be explained by
> reduction, or by simple mutual implication encodings)
>
> B aliasing of syntax, especially instance definitions
> (this syntactic part is hard to encode, and simple in
> terms of syntactic macro expansion)
it just occurred to me that there is a precedence for this kind
of translation, in associated types and type functions.
defining an AT in a class is equivalent to defining a TF
outside the class, and connecting the TF to the class with
superclass and instance constraints, right?
class C a where
type CT a
c :: (a,CT a)
instance C a where
type CT a = ..
c = ..
-- vs
type family CT a
type instance CT a = ..
class CT a ~ b => C a where c :: (a,CT a)
instance CT a ~ b => C a where c = ..
though the latter form is not yet supported in GHC (#714).
which leads me to a problem i have with ATs, which applies
to class aliases as well: although the ATs are written as if they
were local to the class, they get lifted out of the class in a naive
manner. in particular, they can only refer to their parameters,
not to other local definitions/types, and their parameters have
to match the class parameters.
however, i assume that the restrictions/translations/implementations
for class aliases are similar to the those for the implementation
of ATs in terms of TFs, which might help?
claus
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