[Haskell-cafe] Re: Typeclass vs. Prolog programming
I previously 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}.
I should have been more precise and said:
The former _eventually_ leads to the substitution {x-Int}.
Your analysis is right on the mark. That's exactly how I think of
TypeCast.
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.
One of the drafts of the HList paper, when describing TypeCast,
literally had a phrase about `fooling the typechecker'...
Well, it seems things like TypeCast were not envisioned by the
Founding Fathers. In this respect, the story of C++ templates come to
mind. My feeling is that we're still discovering the capabilities of
the Haskell typechecker and the right abstractions. We should view
TypeCast as an ungainly _encoding_ of a simple abstraction, or just as
a tool for implementing type-level typecase and local improvement
rules. When the right abstraction emerges (and perhaps it already has:
CHR), GHC might implement it directly. Until then, we can use the
encoding...
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[Haskell-cafe] Re: Typeclass vs. Prolog programming
On 9/27/06, [EMAIL PROTECTED] [EMAIL PROTECTED] wrote: This message is intended as a long answer to Michael Shulman's question (Re: variadic functions and typeCast) and Jason Dagit's question (Re: Duplicate Instance problem). Incidentally, the short answer to Jason Dagit's question is `constraints are disregarded during instance selection'. The answer to Michael Shulman is yes. Hmm...Perhaps I'm just sleepy but I read through the descriptions below and I'm still not sure if what I want to do is easy or extremely difficult. I think you're suggesting that I should use something like TypeCast to control the unification if I want to get rid of my duplicate instances problem? I'll re-read this a few times and look at the TypeCast stuff. I feel like this is quite a bit over my head at the moment, but your description is very good and hopefully it will make more sense in the morning. Thanks, Jason ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Typeclass vs. Prolog programming
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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