[Haskell] insufficiently laziness@pattern -- more counterintuitive stuff

[Hal Daume III]

A lot.  If everything were irrefutable, then the following:

> mymap f (x:xs) = f x : xs
> mymap f [] = []

would never get to the second branch and would fail when it reached the 
end of a list.  so would it if you put the lines in the other, "more 
natural," order, though perhaps it's less clear why.

A lot of things would break if this were changed.  Refutable patterns are 
definitely the norm, not the exception.

On Tue, 30 Mar 2004, S. Alexander Jacobson wrote:

> Thanks for the ~ syntax, but my question is really
> why you need it?  What benefit do you get from
> "refutable patterns"?
> 
> Alternatively, would anything break if a future
> Haskell just treated all patterns as irrefutable?
> 
> -Alex-
> 
> _________________________________________________________________
> S. Alexander Jacobson                  mailto:me at alexjacobson.com
> tel:917-770-6565                       http://alexjacobson.com
> 
> 
> On Tue, 30 Mar 2004, Martin [ISO-8859-1] Sjögren wrote:
> 
> > tis 2004-03-30 klockan 17.30 skrev S. Alexander Jacobson:
> > > I would assume that this function:
> > >
> > >   foo list@(h:t) = list
> > >
> > > is equivalent to
> > >
> > >   foo list = list
> > >      where (h:t)=list
> > >
> > > But passing [] to the first generates an error
> > > even though h and t are never used!  Passing [] to
> > > the second works just fine.
> >
> > You can write this as
> >
> > > foo' list@(~(h:t)) = list
> >
> > foo' [] will evaluate to []. The H98 report calls it an "irrefutable
> > pattern", IIRC.
> >
> >
> > Regards,
> > Martin
> >
> 
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-- 
 Hal Daume III                                   | hdaume at isi.edu
 "Arrest this man, he talks in maths."           | www.isi.edu/~hdaume