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AndrewLeeQ's solution

to Forth in the OCaml Track

Published at May 25 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

Implement an evaluator for a very simple subset of Forth.

Forth is a stack-based programming language. Implement a very basic evaluator for a small subset of Forth.

Your evaluator has to support the following words:

  • +, -, *, / (integer arithmetic)
  • DUP, DROP, SWAP, OVER (stack manipulation)

Your evaluator also has to support defining new words using the customary syntax: : word-name definition ;.

To keep things simple the only data type you need to support is signed integers of at least 16 bits size.

You should use the following rules for the syntax: a number is a sequence of one or more (ASCII) digits, a word is a sequence of one or more letters, digits, symbols or punctuation that is not a number. (Forth probably uses slightly different rules, but this is close enough.)

Words are case-insensitive.

Getting Started

For installation and learning resources, refer to the exercism help page.

Installation

To work on the exercises, you will need Opam and Base. Consult opam website for instructions on how to install opam for your OS. Once opam is installed open a terminal window and run the following command to install base:

opam install base

To run the tests you will need OUnit. Install it using opam:

opam install ounit

Running Tests

A Makefile is provided with a default target to compile your solution and run the tests. At the command line, type:

make

Interactive Shell

utop is a command line program which allows you to run Ocaml code interactively. The easiest way to install it is via opam:

opam install utop

Consult utop for more detail.

Feedback, Issues, Pull Requests

The exercism/ocaml repository on GitHub is the home for all of the Ocaml exercises.

If you have feedback about an exercise, or want to help implementing a new one, head over there and create an issue. We'll do our best to help you!

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

test.ml

open Base
open OUnit2
open Forth

let print_int_list_option (xs: int list option) = match xs with
| None -> "None"
| Some xs -> "Some [" ^ String.concat ~sep:";" (List.map ~f:Int.to_string xs) ^ "]"
let ae exp got _test_ctxt = assert_equal ~printer:print_int_list_option exp got

let parsing_and_numbers_tests = [
   "numbers just get pushed onto the stack" >::
     ae (Some [1; 2; 3; 4; 5]) (evaluate ["1 2 3 4 5"]);
]


let addition_tests = [
   "can add two numbers" >::
     ae (Some [3]) (evaluate ["1 2 +"]);
   "errors if there is nothing on the stack" >::
     ae None (evaluate ["+"]);
   "errors if there is only one value on the stack" >::
     ae None (evaluate ["1 +"]);
]


let subtraction_tests = [
   "can subtract two numbers" >::
     ae (Some [-1]) (evaluate ["3 4 -"]);
   "errors if there is nothing on the stack" >::
     ae None (evaluate ["-"]);
   "errors if there is only one value on the stack" >::
     ae None (evaluate ["1 -"]);
]


let multiplication_tests = [
   "can multiply two numbers" >::
     ae (Some [8]) (evaluate ["2 4 *"]);
   "errors if there is nothing on the stack" >::
     ae None (evaluate ["*"]);
   "errors if there is only one value on the stack" >::
     ae None (evaluate ["1 *"]);
]


let division_tests = [
   "can divide two numbers" >::
     ae (Some [4]) (evaluate ["12 3 /"]);
   "performs integer division" >::
     ae (Some [2]) (evaluate ["8 3 /"]);
   "errors if dividing by zero" >::
     ae None (evaluate ["4 0 /"]);
   "errors if there is nothing on the stack" >::
     ae None (evaluate ["/"]);
   "errors if there is only one value on the stack" >::
     ae None (evaluate ["1 /"]);
]


let combined_arithmetic_tests = [
   "addition and subtraction" >::
     ae (Some [-1]) (evaluate ["1 2 + 4 -"]);
   "multiplication and division" >::
     ae (Some [2]) (evaluate ["2 4 * 3 /"]);
]


let dup_tests = [
   "copies the top value on the stack" >::
     ae (Some [1; 1]) (evaluate ["1 DUP"]);
   "is case-insensitive" >::
     ae (Some [1; 2; 2]) (evaluate ["1 2 Dup"]);
   "errors if there is nothing on the stack" >::
     ae None (evaluate ["dup"]);
]


let drop_tests = [
   "removes the top value on the stack if it is the only one" >::
     ae (Some []) (evaluate ["1 drop"]);
   "removes the top value on the stack if it is not the only one" >::
     ae (Some [1]) (evaluate ["1 2 drop"]);
   "errors if there is nothing on the stack" >::
     ae None (evaluate ["drop"]);
]


let swap_tests = [
   "swaps the top two values on the stack if they are the only ones" >::
     ae (Some [2; 1]) (evaluate ["1 2 swap"]);
   "swaps the top two values on the stack if they are not the only ones" >::
     ae (Some [1; 3; 2]) (evaluate ["1 2 3 swap"]);
   "errors if there is nothing on the stack" >::
     ae None (evaluate ["swap"]);
   "errors if there is only one value on the stack" >::
     ae None (evaluate ["1 swap"]);
]


let over_tests = [
   "copies the second element if there are only two" >::
     ae (Some [1; 2; 1]) (evaluate ["1 2 over"]);
   "copies the second element if there are more than two" >::
     ae (Some [1; 2; 3; 2]) (evaluate ["1 2 3 over"]);
   "errors if there is nothing on the stack" >::
     ae None (evaluate ["over"]);
   "errors if there is only one value on the stack" >::
     ae None (evaluate ["1 over"]);
]


let user_defined_words_tests = [
   "can consist of built-in words" >::
     ae (Some [1; 1; 1]) (evaluate [": dup-twice dup dup ;"; "1 dup-twice"]);
   "execute in the right order" >::
     ae (Some [1; 2; 3]) (evaluate [": countup 1 2 3 ;"; "countup"]);
   "can override other user-defined words" >::
     ae (Some [1; 1; 1]) (evaluate [": foo dup ;"; ": foo dup dup ;"; "1 foo"]);
   "can override built-in words" >::
     ae (Some [1; 1]) (evaluate [": swap dup ;"; "1 swap"]);
   "can override built-in operators" >::
     ae (Some [12]) (evaluate [": + * ;"; "3 4 +"]);
   "cannot redefine numbers" >::
     ae None (evaluate [": 1 2 ;"]);
   "errors if executing a non-existent word" >::
     ae None (evaluate ["foo"]);
]

let () =
  run_test_tt_main (
    "forth tests" >:::
      List.concat [
        parsing_and_numbers_tests; 
        addition_tests; 
        subtraction_tests; 
        multiplication_tests; 
        division_tests;
        combined_arithmetic_tests; 
        dup_tests; 
        drop_tests;
        swap_tests; 
        over_tests; 
        user_defined_words_tests
        ]
  )
open Base

type operation = 
    | Push of int 
    | Add
    | Sub
    | Mult
    | Div
    | Dup
    | Drop 
    | Swap 
    | Over 
    | Custom of string * operation list
    | Custom_invoke of string
    | Noop
    | Bad_op 

module type Stack = sig 
    type t 
    val empty : t 
    val apply_op : t option -> operation -> t option
    val add_custom : t -> t option 
    val invoke_custom : t option -> string -> t option
    val get_list : t option -> int list option
end 

let (>>=) x f = 
    match x with 
    | None -> None 
    | Some x -> f x

module Stack = struct 
    type t = {
        st: int list;
        c: operation list
    }

    let empty = Some {st=[]; c=[]}
    let push ({st=s;_} as stack) i = Some {stack with st=(i::s)}
    let pop s = 
        match s with 
        | [] -> None 
        | hd::tl -> Some (hd, tl)
    let apply ({st=s;_} as stack) f = 
        pop s >>= (fun (x, s) -> 
            pop s >>= (fun (y, s) -> 
                Some {stack with st=((f y x)::s)}))
    let apply_list ({st=s;_} as stack) f = 
        pop s >>= (fun (x, s) -> 
            pop s >>= (fun (y, s) -> 
                Some {stack with st=((f y x)@s)}))

    let add s = 
        apply s (+)
    let sub s = 
        apply s (-)
    let mult s = 
        apply s ( * )
    let div s = 
        try apply s (/) 
        with Division_by_zero -> None
    let dup ({st=s;_} as stack) = 
        pop s >>= (fun (x, s) -> Some {stack with st=(x::x::s)})
    let drop ({st=s;_} as stack) = 
        if List.length s = 0 then None else Some {stack with st=List.tl_exn s} 
    let swap s = 
        apply_list s (fun a b -> [a; b])
    let over s = 
        apply_list s (fun a b -> [a; b; a])
    let add_custom ({c=c_l;_} as arg) c = 
        Some {arg with c=c::c_l}

    let find_override name lst = 
        match (lst |> List.findi ~f:(fun _ x -> match x with 
                                            | Custom (n, _) -> String.(=) n name
                                            | _ -> false)) with 
        | Some (_, Custom (_, ops)) -> Some ops
        | _ -> None
    
    let rec invoke_custom stack name = 
        stack >>= (fun {st=_; c=c_l} -> 
            match find_override name c_l with 
            | Some ops -> List.fold ~f:(fun acc x -> acc >>= (fun s -> apply_op (Some s) x)) ~init:stack ops 
            | _ -> None)
    and apply_op stack op = 
        stack >>= 
        (fun ({st=_; c=c_l} as s) -> 
            let f c f = 
                match find_override c c_l with 
                | Some _ -> invoke_custom (Some s) c 
                | _ -> f s
            in 
            match op with 
            | Add -> f "+" add 
            | Sub -> f "-" sub 
            | Mult -> f "*" mult
            | Div -> f "/" div
            | Dup -> f "dup" dup
            | Drop -> f "drop" drop
            | Swap -> f "swap" swap
            | Over -> f "over" over 
            | Push x -> push s x
            | Noop -> Some s
            | Custom (_, _) as c -> add_custom s c
            | Custom_invoke name -> invoke_custom (Some s) name
            | Bad_op -> None)
    
    let get_list s = 
        match s with 
        | None -> None 
        | Some x -> Some x.st
end 

let evaluate ops = 
    let get_operations tokens = 
        let is_number n = 
            n |> String.to_list_rev |> List.fold ~f:(fun acc x -> acc && Char.is_digit x) ~init:true 
        in
        let rec get_ops acc tokens = 
            match tokens with 
            | [] -> acc 
            | hd::tl -> 
                let (op, rest) = 
                    match hd with 
                    | "+" -> Add, tl
                    | "-" -> Sub, tl
                    | "*" -> Mult, tl
                    | "/" -> Div, tl
                    | "dup" -> Dup, tl
                    | "drop" -> Drop, tl
                    | "swap" -> Swap, tl
                    | "over" -> Over, tl 
                    | ":" -> (get_custom_ops tl), []
                    | ";" -> (Noop, []) 
                    | n -> ((if is_number n then Push (Int.of_string n) else Custom_invoke n), tl)
                in get_ops (op::acc) rest 
        and get_custom_ops lst = 
            match lst with 
            | [] -> raise (Failure "Syntax error")
            | hd::tl -> if is_number hd then Bad_op else Custom (hd, List.rev (get_ops [] tl))
        in 
        tokens |> 
        List.map ~f:(fun x -> String.lowercase x) |> 
        get_ops [] |> 
        List.rev
    in
    ops |> 
    List.fold ~f:(fun acc x -> List.append acc (get_operations (String.split ~on:' ' x))) ~init:[] |> 
    List.fold ~f:(fun acc op -> Stack.apply_op acc op) ~init:Stack.empty |>
    Stack.get_list >>= (fun x -> Some (List.rev x))

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?