Sam Stites

Higher kinded types

December 17, 2015

“higher-kinded types” exist because we have type constructors in haskell.

So, putting my jvm hat back on, imagine interface constructors which take interfaces and types as arguments.

This requires us to expand our language: + type constuctors: *->* + type constants (types with no constructor): *

kind * is the kind of all standard lifted types. Primatives have type # and are unlifted. lifted types are repersented by a pointer (think C) and include most datatypes you’ll ever use. Unlifted types are machine types, represented by a value on the stack rather than a pointer. they cannot be inhabited by bottom.

So “Lifted vs Unlifted” is the same as “Boxed vs Unboxed.”

newtypes are special in that they are a kind (*), but are unlifted because they already represent something that is lifted. So you can only “lift” once – it would be convoluted to have a reference to a reference to a value.

Note that you can’t compose higher-kinded types. Also keep in mind that [] has kind * -> *. This is because the compiler knows the difference between a list of ints and a list of strings. You can do [] Int or [Int] to get that concrete type which would meet your expectations. To this point, you cann’t expect to switch out the a concrete type later on in your code. GHC remembers all.