G'day all.
On Wed, Jun 25, 2003 at 08:48:12AM -0700, Hal Daume wrote:
> I'm not sure this is really necessary (at least syntax-wise).
Well, of course, no extension is absolutely necessary in a Turing-hard
language. :-)
For the record, here are a couple of other solutions which avoid the
problem. You could, for example, make the return type a phantom
type:
newtype T a = T Int
class Trait a where { trait :: T a }
instance Trait Int where { trait = T 0 }
instance Trait Char where { trait = T 1 }
Or construct the type dictionary explicitly:
data Traits a
= Traits {
trait1 :: a,
trait2 :: Int
}
class Trait a where
traits :: Traits a
instance Trait Char where
traits = Traits {
trait1 = 'a',
trait2 = 16
}
Neither solution seems as nice, though, particularly as the traits
typeclass idiom is already entrenched (e.g. Bounded).
> As far as I can tell with the various --ddump-* flags, the compiler
> hasn't yet figured out that the argument to trait is useless (i.e., it
> keeps it in there).
Nor can it, because you could easily declare an instance in another
module for which the argument is _not_ useless. This is impossible to
detect at compile time.
> Of course, the powers that be can weight in on this, and I'm sure that
> you're aware of the phantom type solution, but I figured I'd post anyway
> so that others can get a look at types like this for their own
> benefit...
Sure. Any input is good. I'm not convinced that my proposed solution
is the best one. (I'm pretty sure that it's the minimal extension
required, though.)
Cheers,
Andrew Bromage
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