[Haskell-cafe] Very Small Program

[Jason Dagit]

On 11/2/06, ajb at spamcop.net <ajb at spamcop.net> wrote:
> G'day all.
>
> Quoting Bernie Pope <bjpop at csse.unimelb.edu.au>:
>
> > This is a weird example of a pattern binding, and it is surprising
> > (to me) that the syntax is valid.
>
> Maybe.  But you wouldn't balk at this:
>
>     numzeroes xs = sum [ 1 | 0 <- xs ]
>
> ...even if you wouldn't naturally express it that way.  Patterns like
> this are useful in places other than lets.  I'd find it more surprising
> if lets were treated as a special case.

Actually, I had to try it out to see if it would work.  That's a neat
little trick.  I had known that this similar one worked but it uses
pattern matching on a data constructor so I never questioned it:

  numJusts xs = sum [ 1 | Just x <- xs ]

thanks,
Jason