Reparse is a monadic, recursive descent based, comprehensive parser construction library for ocaml.
$ opam install reparse
Add reparse to dune,
(executable # or library
(name hello_world)
(public_name hello_world)
(libraries reparse))
A calculator is the hello world of parsers. Here is an implementation in Reparse which supports +,-,* and / operations.
# #require "reparse";;open Reparse.String
type expr =
| Int of int
| Add of expr * expr
| Sub of expr * expr
| Mult of expr * expr
| Div of expr * expr
let skip_spaces = skip space
let binop : 'a t -> char -> 'b t -> ('a -> 'b -> 'c) -> 'c t =
fun exp1 op exp2 f ->
(exp1, skip_spaces *> char op <* skip_spaces, exp2)
<$$$> fun e1 _ e2 -> f e1 e2
let integer : expr t =
let+ d = skip_spaces *> digits <* skip_spaces in
Int (int_of_string d)
let factor : expr t -> expr t =
fun expr ->
any [ char '(' *> skip_spaces *> expr <* skip_spaces <* char ')'; integer ]
let term : expr t -> expr t =
fun factor ->
recur (fun term ->
let mult = binop factor '*' term (fun e1 e2 -> Mult (e1, e2)) in
let div = binop factor '/' term (fun e1 e2 -> Div (e1, e2)) in
mult <|> div <|> factor)
let expr : expr t =
recur (fun expr ->
let factor = factor expr in
let term = term factor in
let add = binop term '+' expr (fun e1 e2 -> Add (e1, e2)) in
let sub = binop term '-' expr (fun e1 e2 -> Sub (e1, e2)) in
any [ add; sub; term ] <?> "expr")
let rec eval : expr -> int = function
| Int i -> i
| Add (e1, e2) -> eval e1 + eval e2
| Sub (e1, e2) -> eval e1 - eval e2
| Mult (e1, e2) -> eval e1 * eval e2
| Div (e1, e2) -> eval e1 / eval e2# let ast = parse (create_input_from_string "1*2-4+3") expr ;;
val ast : (expr * int, string) result =
Ok (Sub (Mult (Int 1, Int 2), Add (Int 4, Int 3)), 7)
# eval @@ Result.get_ok (parse (create_input_from_string "12+1*10") expr);;
Line 1, characters 9-72:
Error: This expression has type expr * int
but an expression was expected of type expr