The fix datatype is the simplest way to model birecursion via a direct encoding of the project and embed functions.

data class Fix<out A>(val unfix: Kind<A, Eval<FixOf<A>>>) : FixOf<A>

embed is isomorphic to the Fix constructor, and project is isomorphic to unfix.

Comparison to Mu and Nu

If Mu is A and Nu is A?, then fix is A!. Fix provides no information about whether or not a datatype is infinite, and is generally less safe that Mu or Nu. However, due to its simplicity, both Recursive and Corecursive instances for fix are very fast.

Supported Type Classes

Module Type classes
arrow.recursion.typeclasses Birecursive, Corecursive, Recursive