[Haskell-cafe] Intro to functional dependencies in Haskell?
marco-oweber at gmx.de
Thu Apr 17 04:05:47 EDT 2008
On Thu, Apr 17, 2008 at 03:21:16PM +1000, Alexis Hazell wrote:
> Hi all,
> i'm having some trouble 'getting' functional dependencies in the Haskell
> context (although my understanding of them in the context of relational
> database theory isn't that great either). Could someone please point me
> to an introduction / tutorial in this regard?
I think you've already tried www.haskell.org -> wiki search ?
You can express this by specifying a functional dependency, or fundep for short:
class Mult a b c | a b -> c where
(*) :: a -> b -> c
This tells Haskell that c is uniquely determined from a and b.
So there you can't implement instances
Mult Int Char Sting
Mult Int Char MyString
of course, your programs will work withou t that | a b -> c as well,
but you need to tell the compiler the result type of
(*) fac1 fac2
wheeras using fundeps it knows the result type becaus there can only be
one instance. (Which is what you want in this case when multiplying
matrizes as well.
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