1. And more store-passing interpreters
Desugaring letrec
And more store-passing interpreters

2. Store-passing recap

3. (let loop ([x ivx] [y ivy] …)
body)
(letrec ([loop (lambda (x y …)
body)])
(loop ivx ivy …))

4. (define (sum from to)
(define total 0)
(let loop ()
(set! total (+ total from))
(set! from (+ from 1))
(if (<= from to)
(loop)
total)))

5. (define (sum from to)
(let loop ([i from]
[total 0])
(if (<= i to)
(loop (+ i 1) (+ total i))
total)))

6. env maps variables to addresses
(x, env, st)
env maps variables to addresses
st maps addresses to values
The current size of the store is the next address: | st |

7. (e2, env’[x ↦ |st2|], st2[|st2| ↦ v1]) ⇓ (v2, st3)
(e0, env, st0) ⇓ (((λ (x) e2), env’), st1)
(e1, env, st1) ⇓ (v1, st2)
((e0 e1), env, st0) ⇓ (v2, st3)
((λ (x) e), env, st) ⇓ (((λ (x) e), env), st)
(x, env, st) ⇓ (st(env(x)), st)

8. (letrec* ([x0 e0] …) body)
(let ([x0 ‘undefined] …)
(set! x0 e0)
...
body)

9. (letrec ([x0 e0] …) body)
(let ([x0 ‘undefined] …)
(let ([t0 e0])
(set! x0 t0)
...
body))

10. Let’s code this up.