Thank you Oleg! That explanation is very clear.
On 9/28/06, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote:
The typechecker commits to the instance
and adds to the current constraints
TypeCast x Int, Ord Bool, Eq Bool
The latter two are obviously satisfied and so discharged. The former
leads to the substitution {x->Int}.
By the rules just enumerated, this substitution would be forbidden,
since the type variable x is not considered variable, right? So this
happens via the magic of functional dependencies? The typechecker
encounters (eventually) the instance
instance TypeCast'' () x x
and since TypeCast'' is declared with a functional dependency:
class TypeCast'' t x y | t x -> y, t y -> x
it concludes that the first x in the above instance is uniquely
determined by the second, which in our case is Int, and therefore x
must be Int. Is that right?
This is all very beautiful, but it's a little annoying that the
cornerstone "silver bullet" TypeCast has to be defined in a way that
fools the typechecker into doing the right thing in spite of itself.
Is this all part of the general "fundeps are very hard to get right"
problem?
Mike
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