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• kind signatures 1 answer

can explain or give link explaining * -> * part of code below?

data binarytree t :: * -> *    leaf :: binarytree empty    branch :: -> binarytree isempty -> binarytree isempty' -> binarytree nonempty 

thanks lot.

kind signatures. can used explicitly define arity of type constructor. * means "any type", , rest works kind of normal function types.in case, have partial application:

binarytree t :: * -> * 

means "binarytree t function types types", makes sense, since can apply 1 other type:

f :: (binarytree t) -> (binarytree t) b          * -> *     *        * -> *     * 

the binarytree t part applied type a, giving type binarytree t :: *. (binarytree t) alone not applied yet, having kind * -> *, , still need apply simple type. (this works same way when f 1 still has type int -> int when f :: int -> int -> int)

note can mix normal "arity declarations", mentioning type's parameters, implicit, , kind signatures, done here. have written binarytree follows:

data binarytree t  -- normal variant 

or this:

data binarytree :: * -> * -> *  -- "kinded" 

in both cases, compiler knows binarytree going take 2 type parameters.

and for? first, is described in link, need or want explicitly declare how many type parameters type should take. maybe more interesting case occurs when use diffent kinds * (aka datakinds). @ this:

data empty = empty | nonempty data binarytree (t :: empty) (a :: *) :: *     -- ... 

oh, , ghci lets @ kinds, in case unsure. works same way :t:

prelude> :k either either :: * -> * -> *