type family Id a
type instance Id Int = Int
foo :: Id a -> Id a
foo = id n
foo' :: Id a -> Id a
foo' = foo
type function notation is slightly misleading, as it presents
qualified polymorphic types in a form usually reserved for
unqualified polymorphic types.
rewriting foo's type helped me to see the ambiguity that
others have pointed out:
foo :: r ~ Id a => r -> r
there's nowhere to get the 'a' from. so unless 'Id' itself is
fully polymorphic in 'a' (Id a = a, Id a = Int, ..), 'foo' can't
really be used. for each type variable, there needs to be
at least one position where it is not used as a type family
parameter.
it might be useful to issue an ambiguity warning for
such types?
perhaps, with closed type families, one might be able to
reason backwards (is there a unique 'a' such that 'Id a ~ Int'?),
as with bidirectional FDs. but if you want to flatten all
'Maybe'-nests, even that direction isn't unique.
The type checking problem for foo' boils down to verifying the formula
forall a. exists b. Id a ~ Id b
naively, i'd have expected to run into this instead/first:
(forall a. Id a -> Id a) ~ (forall b. Id b -> Id b)
which ought to be true modulo alpha conversion, without guesses,
making 'foo'' as valid/useless as 'foo' itself.
where am i going wrong?
claus
ps. i find these examples and discussions helpful, if often initially
puzzling - they help to clear up weak spots in the practice
of type family use, understanding, and implementation,
thereby improving all while making families more familiar!-)
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