[Haskell-beginners] type class question from MReader #8

[MH]

Could you please elaborate on how does the constraint (ListConcat xs
ys zs) makes the instance ListConcat (x ::: xs) ys (x ::: zs)
recursive.
Also, I am interested to learn more about various ways to constrain
type classes parameters, is there a paper you would recommend?

Thanks

On Tue, Jul 14, 2009 at 1:12 AM, Brandon S. Allbery
KF8NH<allbery at ece.cmu.edu> wrote:
> On Jul 13, 2009, at 23:43 , MH wrote:
>>
>> class ListConcat a1 a2 a  |  a1 a2 -> a where
>>         listConcat :: a1 -> a2 -> a
>>
>> instance ListConcat Nil a a where listConcat = _|_
>>
>> instance (ListConcat xs ys zs)
>>             => ListConcat (x ::: xs) ys (x ::: zs) where listConcat = _|_
>>
>> 1. How does the last function becomes recursive?
>> 2. Why do we need constraint  '(ListConcat xs ys zs) =>' ?
>
> Your second question is the answer to the first.
>
>> 3. I assume that listConcat function takes three arguments but from
>> example provided, it takes two (see below):
>
> The last type in the (->) chain is the return type, not an argument.
>
>> 4. Does the functional dependency sign (->) implies that the last
>> argument is the return type?
>
> No, it says type a (which is designated as the return type by the definition
> of listConcat) is uniquely determined by the pair of argument types a1, a2
> taken together.  (That is, any given unique combination of argument types
> produces a unique result type.)
>
> --
> brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
> system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
> electrical and computer engineering, carnegie mellon university    KF8NH
>
>
>