[Haskell-beginners] am I wording this right?
Peter Verswyvelen
bugfact at gmail.com
Tue Aug 4 20:15:44 EDT 2009
That sound okay to me.
Usually when we have type constructor "T a" and an "instance Functor T
where..." we just say that "T is a functor"
Note that the signature of a type constructor is called the "kind" of
the type constructor.
For example, the following code
data NotSoKind = X
instance Functor NotSoKind where
would give the error:
Kind mis-match
Expected kind `* -> *', but `NotSoKind' has kind `*'
In the instance declaration for `Functor NotSoKind'
and
instance Functor (,) where
gives the error
(,)' is not applied to enough type arguments
Expected kind `* -> *', but `(,)' has kind `* -> * -> *'
In the instance declaration for `Functor (,)'
Note however that the following is correct:
instance Functor ((,) a) where
fmap f (x,y) = (x, f y)
and even:
instance Functor ((->) a) where
fmap f g = f . g
You can ask GHCi to show the kind of a type constructor:
:kind (,)
(,) :: * -> * -> *
:kind ((,) 1)
((,) 1) :: * -> *
:kind Char
*
On Wed, Aug 5, 2009 at 1:07 AM, Michael P Mossey <mpm at alumni.caltech.edu> wrote:
>
> Is this the right way of saying what I'm trying to say?
>
> "Functor is a typeclass of type constructors which take one argument."
>
> Thanks,
> Mike
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