puffnfresh

How do we add extra information to a tree? This has been called [The
AST Typing
Problem](http://blog.ezyang.com/2013/05/the-ast-typing-problem/).

After being hit with this problem in Roy's new type-inference engine,
I tried figuring out how to represent the algorithm. I eventually
realised that it looked like a comonadic operation. Turns out it's
been done before but I couldn't find any complete example.

Below is some literate Haskell to show how to use the Cofree Comonad

lexi-lambda

Monads and delimited control are very closely related, so it isn’t too hard to understand them in terms of one another. From a monadic point of view, the big idea is that if you have the computation m >>= f, then f is m’s continuation. It’s the function that is called with m’s result to continue execution after m returns.

If you have a long chain of binds, the continuation is just the composition of all of them. So, for example, if you have

m >>= f >>= g >>= h

then the continuation of m is f >=> g >=> h. Likewise, the continuation of m >>= f is g >=> h.

wilbowma

#define NULL 0

struct pair {
  int first;
  struct pair* rest;
};

void display(struct pair* ls) {
  if (ls == NULL) {
    printf("()");