Peter Gavin wrote:
Has anyone else tried implementing type-level integers using type families?
I tried using a couple of other type level arithmetic libraries
(including type-level on Hackage) and they felt a bit clumsy to use. I
started looking at type families and realized I could pretty much build
an entire Scheme-like language based on them.
In short, I've got addition, subtraction, & multiplication working after
just a days worth of hacking. I'm going to post the darcs archive
sometime, sooner if anyone's interested.
I really like the type-families based approach to this, it's a lot
easier to understand, and you can think about things functionally
instead of relationally. (Switching back and forth between Prolog-ish
thinking and Haskell gets old quick.) Plus you can do type arithmetic
directly in place, instead of using type classes everywhere.
nice, it's been tried before, etc. etc.. And of course it doesn't work
with a released version of GHC, so maybe it's hoping too much that it
would be on Hackage. What I was going to say was, see if there is one
on hackage, otherwise there should be one there to be polished. But I
guess searching haskell-cafe is your man :-) (your way to try to find
any. Or the Haskell blogosphere too.)
One thing that I'd like to be able to do is lazy unification on type
instances, so that things like
...
will work if the non-taken branch can't be unified with anything. Is
this planned? Is it even feasible?
I'm pretty sure it would be possible to implement a Lambda like this,
but I'm not seeing it yet. Any ideas?
Yeah -- that would be neat but generally tends to lead to undecidability
(unless you're really careful making it a lot(?) less useful). That is,
potential nontermination in the type inferencer/checker, not just in
runtime. Then you'll want it to be well-defined when something is
type-level-lazy, so you can reliably write your type-level algorithms.
And *that* is bound to be rather difficult to define and to implement
and maintain.
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