Throw & Catch

I didn't implement exceptions right, yet. Throws and catches are factored out but catches catch the exception local in the term. I looked at call-cc and reset-shift, couldn't decide, and, once again, I am going for my own most straightforward implementation described below.

A ((try E₀ catch E₁) exc) block is roughly translated to (R (λs. E₀ (λx.s (E₁ x))))  which evaluates to ((λs. E₀ (λx. s (E₁ x))) S_k) , where S_k is a control object holding the continuation of the original block. It looks the most like reset-shift I guess. The difference is that, as far as I know, reset-shift is a syntactical addition to lambda calculus, whereas these combinators model roughly the same, but depend on call-by-value evaluation to express a control operator which you can't model directly syntactically using common combinators.

R takes a function f and rewrites to f S_k. S_k takes a value v and rewrites to v, but escapes the local context up to the point R was applied.

I didn't implement it yet. Worried a bit about the shift combinator escaping the local context.

013110: Convinced myself, and implemented it. Since the try part of a try-catch can only evaluate to a value or an exception, the exception handler is either applied, or not, so should be properly discarded.

Food is a nice idea sometimes too.