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 <[EMAIL PROTECTED]> 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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