[Haskell-cafe] Re: [Haskell] View patterns in GHC: Request for
drl at cs.cmu.edu
Mon Jul 30 05:31:40 EDT 2007
With the functional dependency, you can't work with the view datatypes
at all. Once you write
data TypView = Unit | Arrow Typ Typ
instance View Typ TypView where
view = ...
you're no longer allowed to take apart a TypView at all!
E.g. you can't write
outUnit :: TypView -> Bool
outUnit Unit = True
outUnit _ = False
because the implicit application of the view function will mean that
outUnit must consume a Typ.
Personally, I'd rather have special syntax in the pattern (-> pat) than
have these global effects on what you can do with certain types.
On Jul27, Stefan O'Rear wrote:
> On Fri, Jul 27, 2007 at 05:22:37AM -0400, Dan Licata wrote:
> > On Jul26, Stefan O'Rear wrote:
> > > > So, this syntax affects a lot of code, existing or otherwise, that
> > > > doesn't use view patterns, which is something we're trying to avoid.
> > >
> > > Eh? I *think* the typing rules are the same for the no-view case. If
> > > the auto-deriving hack isn't implemented, you only need a
> > > deriving(View), otherwise there should be no change at all...
> > Assuming you don't have the functional dependency: "affects" in the
> > sense that any code you write has a generalized type, so you have to
> > explain view patterns to beginners right out of the gate, etc. If you
> > write
> > length  = 
> > length (h : t) = 1 + length t
> > we don't want to have to explain to beginners why it has type
> > length :: forall a,b,c. View a [b] -> a -> Num c
> Right, which is why I think the functional dependency is good. If we
> have it, and the auto-deriving hack, what breaks?
> length  = 
> length (h : t) = 1 + length t
> length :: forall a b c. (View a [b], Num c) => a -> c
> ==> (one of the FD rules)
> length :: forall a b c. (View [a] [b], Num c) => [a] -> c
> ==> (plain context reduction, the first constraint is tautological)
> length :: forall a c. Num c => [a] -> c
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