#| |
<expr> ::= <num> |
| {+ <expr> <expr>} |
| {- <expr> <expr>} |
| {if0 <expr> <expr> <expr>} |
| {with {<sym> <expr>} <expr>} |
| <id> |
| {<sym> <expr>} |
|# |
(deftype Expr |
(num n) |
(add l r) |
(sub l r) |
(if0 c t f) |
(with id named-expr body) |
(id s) |
(app f-name arg-expr)) |
|
;; parse :: s-expr -> Expr |
#| where |
<s-expr> ::= <num> |
| <sym> |
| (list '+ <s-expr> <s-expr>) |
| (list '- <s-expr> <s-expr>) |
| (list 'if0 <s-expr> <s-expr> <s-expr>) |
| (list 'with (list <sym> <s-expr>) <s-expr>) |
| (list <sym> <s-expr>) |
|# |
(define (parse s-expr) |
(match s-expr |
[(? number?) (num s-expr)] |
[(? symbol?) (id s-expr)] |
[(list '+ l r) (add (parse l) (parse r))] |
[(list '- l r) (sub (parse l) (parse r))] |
[(list 'if0 c t f) (if0 (parse c) |
(parse t) |
(parse f))] |
[(list 'with (list x e) b) |
(with x (parse e) (parse b))] |
[(list f a) (app f (parse a))])) |
|
;; function definition |
(deftype FunDef |
(fundef name arg body)) |
|
;; lookup-fundef :: sym Listof(FunDef) -> FunDef |
(define (lookup-fundef f funs) |
(match funs |
['() (error 'lookup-fundef "function not found: ~a" f)] |
[(cons (and fd (fundef fn _ _)) rest) |
(if (symbol=? fn f) |
fd |
(lookup-fundef f rest))])) |
|
;; interp :: Expr Listof(FunDef) Env -> number |
(define (interp expr funs env) |
(match expr |
[(num n) n] |
[(add l r) (+ (interp l funs env) (interp r funs env))] |
[(sub l r) (- (interp l funs env) (interp r funs env))] |
[(if0 c t f) |
(if (zero? (interp c funs env)) |
(interp t funs env) |
(interp f funs env))] |
[(with bound-id named-expr bound-body) |
(interp bound-body |
funs |
(extend-env bound-id |
(interp named-expr funs env) |
env))] |
[(id x) (env-lookup x env)] |
[(app f arg-expr) |
(def (fundef _ farg fbody) (lookup-fundef f funs)) |
(interp fbody |
funs |
(extend-env farg |
(interp arg-expr funs env) |
empty-env))])) |
|
;; run :: s-expr [listof(FunDef)] -> number |
(define (run prog [funs '()]) |
(interp (parse prog) funs empty-env)) |