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to Forth in the OCaml Track

Published at Jan 06 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
        ]
  )

forth.ml

open Core
open Core.Option.Monad_infix

module SM = Map.Make(String)

let binary f = function | a::b::xs -> Some ((f b a)::xs) | _ -> None
let f_div    = function | 0::_     -> None               | x -> binary (/) x
let f_dup    = function | a::xs    -> Some (a::a::xs)    | _ -> None
let f_drop   = function | _::xs    -> Some xs            | _ -> None
let f_swap   = function | a::b::xs -> Some (b::a::xs)    | _ -> None
let f_over   = function | a::b::xs -> Some (b::a::b::xs) | _ -> None

let default_words = [
    ("+", binary ( + )) ; ("-", binary (-)) ;
    ("*", binary ( * )) ; ("/", f_div)      ;
    ("DUP" , f_dup )    ; ("DROP", f_drop) ;
    ("SWAP", f_swap)    ; ("OVER", f_over)
  ]

let numregexp = Str.regexp "[0-9]+"

let evaluate progs =
  let rec run sw stack defs =
    let is_term x = x=";" in
    let is_number x = Str.string_match numregexp x 0 in
    match sw with
    | [] -> stack (* end of program *)
    | ":"::d::sw' ->
       if is_number d then None
       else
         let (newdef, sw'') = List.split_while ~f:(fun x -> not (is_term x)) sw' in
         Option.filter (List.hd sw'') ~f:is_term >>=
           (fun _ ->
             let newf s = run newdef (Some s) defs in
             run (List.tl_exn sw'') stack (SM.set defs ~key:d ~data:newf))
    | w::sw' ->
       stack >>= (fun stack' ->
        if is_number w then
          run sw' (Some ((int_of_string w)::stack')) defs
        else
          SM.find defs w >>= (fun f -> run sw' (f stack') defs))
  in
  let separators = [ ' ' ; '\t' ; '\n' ; '\r' ] in
  let swords = List.map progs ~f:(String.split_on_chars ~on:separators) |>
                 List.concat |> List.map ~f:String.uppercase in
  let defs = SM.of_alist_exn default_words in
  run swords (Some []) defs >>| List.rev

dune

(executable
 (name test)
 (libraries base str oUnit core)
 (preprocess (pps ppx_jane)))
 

(alias
  (name    runtest)
  (deps    (:x test.exe))
  (action  (run %{x})))

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vzaliva's Reflection

I really miss Haskell do-notation for monads :(