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System F is Amazing

I've been working with System F quite a bit lately, as it has a complicated enough typing system to be interesting, without having the full insanity of dependent types. I've been working with System F including sums products and recursive types. However, all of these can actually be encoded directly in System F. At first I thought this was merely a curiosity. It turns out however that for recursive types, the built in variety has some serious advantages.

System F allows you to take types as arguments as well as terms. From this system you can weave together an impressive array of very expressive types. First I'll define a few of these, and then I'll further describe what I find really fascinating.

The basic idea is that we can apply a function such as (Λ X . λ f : X→X, x : X. f x) to some types and terms and we evaluate in a way analogous to the λ-Calculus:

(Λ X . λ f : X→X, x : X. f x) [T] g t ⇒ g t

Now lets look at how we can spin some basic datatypes directly…