letbindings should desuger into nested pattern matching

[swr1bm86]

e.g.

(let [p1 v1
      p2 v2]
  body)

desuger to

(match v1
  (p1 → (match v2
          (p2 → body))))

this is quite useful when v1 and v2 are just called recursively inside the function itself.

[swr1bm86]

more generally, to support recursive letbindings, should introduce a new special form letrec

(letrec [ev? (λ n → (if (zero? n) true (od? (dec n))))
         od? (λ n → (if (zero? n) false (ev? (dec n))))]
  (begin (print (bool2str (ev? 11)))
         (print (bool2str (ev? 12)))
         (print (bool2str (od? 11)))
         (print (bool2str (od? 12)))))

desuger to

(let [ev? (λ n →
            (match (λ n → (if (zero? n) false (ev? (dec n))))
              (od? → (if (zero? n) true (od? (dec n))))))
      od? (λ n →
            (match (λ n → (if (zero? n) true (od? (dec n))))
              (ev? → (if (zero? n) false (ev? (dec n))))))]
  (begin (print (bool2str (ev? 11)))
         (print (bool2str (ev? 12)))
         (print (bool2str (od? 11)))
         (print (bool2str (od? 12)))))