[Haskell-cafe] Signature for non-empty filter

[Henning Thielemann]

On Tue, 5 Feb 2008, Bulat Ziganshin wrote:

> Hello Henning,
>
> Tuesday, February 5, 2008, 6:01:27 PM, you wrote:
>
> > Is Haskell's type system including extensions strong enough for describing
> > a function, that does not always return a trivial value? E.g.
> >    (filter (\x -> x==1 && x==2))
>
> such things may be detected by (too) smart compiler, but in general
> it's undecidable: filter (if LifeHasMeaning then const True else odd) ;)

As I said, if the programmer could specify an input on the type level for
which the output is non-trivial, then this would solve the problem.