[[PageOutline]]
= Kind Inference =

== Brief Explanation ==

Haskell 98 lacks kind polymorphism, and performs kind inference over dependency groups with polymorphic kinds defaulted to `*`. The Report gives the following example of illegal declarations:
{{{
data Tree a = Leaf | Fork (Tree a) (Tree a)

type TreeList = Tree []     -- illegal
}}}
(because `Tree` has been assigned the kind {{{* -> *}}}).

A more meaningful example is the following attempt to compose monad transformers:
{{{
newtype StateT s m a = S (s -> m (a,s))

newtype ReaderT r m a = R (r -> m a)

newtype Compose f g m a = C (f (g m) a)

type SR s r = Compose (StateT s) (ReaderT r)     -- illegal
}}}
GHC users have been able to work around this by adding KindAnnotations to the above definition of `Compose`.

== References ==

 * [http://www.haskell.org/onlinereport/decls.html#kindinference Haskell 98 Kind Inference]

== Tickets ==
[[TicketQuery(description~=KindInference)]]

== Alternatives ==

Listed in order of decreasing need for KindAnnotations
(even if you think those are useful in their own right):

=== GHC 6.4 ===

Kind inference is performed across all the `data`, `newtype`, `type` and `class` declarations of a module before defaulting, and so the above examples are accepted.

=== Monomorphic kinds, using all available information ===

Kind inference is performed across the whole module, using all occurrences of type constructors and classes, i.e. the above sources plus instance declarations, type signature declarations and expression type signatures.

=== Polymorphic kinds ===

Type constructors and classes have polymorphic kinds, inferred over dependency groups.  KindAnnotations would never be required.