[Haskell-cafe] Re: [Haskell] View patterns in GHC: Request?for?feedback

[David Roundy]

On Tue, Jul 31, 2007 at 04:04:17PM -0700, Stefan O'Rear wrote:
> On Tue, Jul 31, 2007 at 03:31:54PM -0700, David Roundy wrote:
> > On Mon, Jul 30, 2007 at 11:47:46AM +0100, Jon Fairbairn wrote:
> > > ChrisK <haskell at list.mightyreason.com> writes:
> > > 
> > > > And the readability is destroyed because you cannot do any type inference in
> > > > your head.
> > > > 
> > > > If you see
> > > > 
> > > > {
> > > >  Matrix m = ....;
> > > >  Matrix x = m * y;
> > > >  ...;
> > > > }
> > > > 
> > > > Then you know very little about the possible types of y
> > > > since can only conclude that:
> > > 
> > > [snippage] This is all very horrid, but as far as I can tell
> > > what I was proposing wouldn't lead to such a mess, except
> > > possibly via defaulting, which, as the least important
> > > aspect of the idea could easily be abandoned.
> > 
> > What your suggestion would do would be to make the type inferred for every
> > pattern-matched function polymorphic, which means that in order to
> > determine the correctness of a function you'd need to examine all other
> > modules.  Similarly, if you fail to include a type signature in some simple
> > pattern-matched function in a where clause, adding an import of another
> > module could make that function fail to compile (with an undeterminable
> > type error).
> 
> Excuse me?  One of the most critical properties of type classes is that
> adding new instances can never make old code that uses old instances
> stop compiling; the worst you could get is a definition conflict.

I see that I was wrong.  I was thinking of something like

foo :: C a => Int -> a
bar :: C a => a -> Int
baz :: Int -> Int

baz = bar . foo

and that this would compile if there was only one instance of class C.  But
I see that in fact it will fail to compile regardless, which makes sense.
-- 
David Roundy
Department of Physics
Oregon State University
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