giles
ffa74399fd
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Recursive-descent calculator parses '(1 + 2) * 3 + 4' = 13. Two parser bugs fixed: 1. parse-let now handles inline 'let rec a () = ... and b () = ... in body' via new (:let-rec-mut BINDINGS BODY) and (:let-mut BINDINGS BODY) AST shapes; eval handles both. 2. has-matching-in? lookahead no longer stops at 'and' — 'and' is internal to let-rec, not a decl boundary. Without this fix, the inner 'let rec a () = ... and b () = ...' inside a let-decl rhs would have been treated as the start of a new top-level decl. Baseline exercises mutually-recursive functions, while-loops, ref-cell imperative parsing, and ADT-based AST construction.
77 lines
1.7 KiB
OCaml
77 lines
1.7 KiB
OCaml
(* Baseline: recursive-descent calculator for "+", "*", parens, ints. *)
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type expr =
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| Lit of int
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| Add of expr * expr
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| Mul of expr * expr
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;;
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let parse_input src =
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let pos = ref 0 in
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let peek () = if !pos < String.length src then String.get src !pos else "" in
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let advance () = pos := !pos + 1 in
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let skip_ws () =
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while !pos < String.length src && peek () = " " do advance () done
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in
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let rec parse_atom () =
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skip_ws () ;
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if peek () = "(" then begin
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advance () ;
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let e = parse_expr () in
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skip_ws () ;
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advance () ; (* consume ')' *)
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e
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end
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else
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let start = !pos in
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let rec digits () =
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if !pos < String.length src then
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let c = peek () in
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if c >= "0" && c <= "9" then begin advance () ; digits () end
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else ()
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in
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digits () ;
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let n = Int.of_string (String.sub src start (!pos - start)) in
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Lit n
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and parse_term () =
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skip_ws () ;
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let lhs = ref (parse_atom ()) in
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let rec loop () =
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skip_ws () ;
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if peek () = "*" then begin
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advance () ;
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lhs := Mul (!lhs, parse_atom ()) ;
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loop ()
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end
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in
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loop () ;
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!lhs
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and parse_expr () =
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skip_ws () ;
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let lhs = ref (parse_term ()) in
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let rec loop () =
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skip_ws () ;
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if peek () = "+" then begin
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advance () ;
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lhs := Add (!lhs, parse_term ()) ;
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loop ()
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end
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in
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loop () ;
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!lhs
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in
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parse_expr ()
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;;
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let rec eval e =
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match e with
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| Lit n -> n
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| Add (a, b) -> eval a + eval b
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| Mul (a, b) -> eval a * eval b
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;;
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(* (1 + 2) * 3 + 4 = 9 + 4 = 13 *)
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eval (parse_input "(1 + 2) * 3 + 4")
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