Re: [Haskell-cafe] GADTs and pattern matching

19 Jun 2013


      Good point.  I stand corrected.

-Brent

On Wed, Jun 19, 2013 at 11:42:23AM -0300, Felipe Almeida Lessa wrote:
...
Brent, maybe I'm misunderstanding what you're saying, but I don't
think that the order of the arguments is playing any role here besides
defining the order in which the pattern matches are desugared.
To illustrate,
-- This does work
  foo1' :: a -> Foo a -> Int
  foo1' m Foo = case m of
                  Nothing -> undefined
                  Just _  -> undefined
Despite having the same type as foo1, foo1' does work because now I've
pattern matched on the GADT first.  As soon as I do that, its equality
constraint of (a ~ Maybe v) enters into scope of the case branches.
Cheers,
On Wed, Jun 19, 2013 at 7:59 AM, Brent Yorgey  wrote:
...
On Wed, Jun 19, 2013 at 11:11:16AM +0100, Francesco Mazzoli wrote:
...
At Wed, 19 Jun 2013 10:03:27 +0000 (UTC),
AntC wrote:
...
Hi Francesco, I think you'll find that the 'annoyance' is nothing to do
with GADTs. I suggest you take the type signature off of foo1, and see
what type ghc infers for it. It isn't :: a -> Foo a -> Int.
[...]
Yep, that message explains what's going on well enough for me.
Did you read the rest of the code?  That ought to work, because GHC
infers and uses the type equality (something like ‘v ~ Var v1’) and uses
it to coerce the ‘x’.
And, surprise surprise, if the argument order is switched, it works!
data Foo v where
        Foo :: forall v. Foo (Maybe v)
foo1 :: Foo a -> a -> Int
    foo1 Foo Nothing  = undefined
    foo1 Foo (Just x) = undefined
Yes, I was going to suggest switching the argument order before
reading your message.  This is an interesting way in which you can
observe that Haskell does not really have "multi-argument functions".
All multi-argument functions are really one-argument functions which
return functions.  So a function of type
foo1 :: a -> (Foo a -> Int)
must take something of type a (for *any* choice of a, which the caller
gets to choose) and return a function of type (Foo a -> Int).  *Which*
function is returned (e.g. one that tries to pattern match on the Foo)
makes no difference to whether foo1 typechecks.
On the other hand, a function of type
foo2 :: Foo a -> (a -> Int)
receives something of type Foo a as an argument.  It may pattern-match
on the Foo a, thus bringing into scope the fact that (a ~ Maybe v).
Now when constructing the output function of type (a -> Int) it may
make use of this fact.
-Brent
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-- 
Felipe.
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