[Haskell-cafe] Mysterious factorial
Ralf Hinze
ralf.hinze at comlab.ox.ac.uk
Wed Dec 30 13:15:36 EST 2009
> I would use:
>
> foldm :: Monoid m => [m] -> m
>
> Which is just a better implementation of mconcat / fold. The reason I
> prefer this interface is that foldm has a precondition in order to
> have a simple semantics: the operator you're giving it has to be
> associative. I like to use typeclasses to express laws.
That's a valid point. Unfortunately, Haskell is not Coq, so there
is no guarantee that the monoid laws are actually satisfied. And
using a type-class has the downside that you can have only one
instance per type. Maybe, you want to use both `foldm (+) 0' and
`foldm (*) 1'. So the bottom-line is that you should have both
versions.
Cheers, Ralf
More information about the Haskell-Cafe
mailing list