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.
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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
A Makefile is provided with a default target to compile your solution and run the tests. At the command line, type:
make
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.
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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 state_type =
{ stack : int list
; commands : (string * string) list
; has_error : bool }
let init_state = {stack = []; commands = []; has_error = false}
let is_number string =
String.fold string ~init:true ~f:(fun result char ->
if not (Char.is_digit char) then false else result )
;;
let is_command_definition string = Char.( = ) string.[0] ':'
let add_command commands string =
let tokens = String.split string ~on:' ' |> List.to_array in
let command_name = tokens.(1) in
if is_number command_name
then None
else
let command_body =
String.concat_array
~sep:" "
(Array.sub tokens ~pos:2 ~len:(Array.length tokens - 3))
in
Some ((command_name, command_body) :: commands)
;;
let math stack op = match stack with a :: b :: tl -> Some (op b a :: tl) | _ -> None
let divide stack = match stack with 0 :: _tl -> None | _ -> math stack ( / )
let dup stack = match stack with hd :: tl -> Some (hd :: hd :: tl) | _ -> None
let drop stack = match stack with _hd :: tl -> Some tl | _ -> None
let swap stack = match stack with a :: b :: tl -> Some (b :: a :: tl) | _ -> None
let over stack = match stack with a :: b :: tl -> Some (b :: a :: b :: tl) | _ -> None
let rec apply_token state token =
match List.Assoc.find state.commands token ~equal:String.( = ) with
| Some command ->
let new_state = evaluate_single state command in
if new_state.has_error then None else Some new_state.stack
| _ ->
(match String.lowercase token with
| "+" -> math state.stack ( + )
| "-" -> math state.stack ( - )
| "*" -> math state.stack ( * )
| "/" -> divide state.stack
| "dup" -> dup state.stack
| "drop" -> drop state.stack
| "swap" -> swap state.stack
| "over" -> over state.stack
| number when is_number number -> Some (Int.of_string number :: state.stack)
| _ -> None)
and evaluate_single state string =
if is_command_definition string
then
match add_command state.commands string with
| None -> {state with has_error = true}
| Some new_commands -> {state with commands = new_commands}
else
String.split string ~on:' '
|> List.fold_until
~init:state
~finish:(fun state -> state)
~f:(fun state token ->
match apply_token state token with
| None -> Stop {state with has_error = true}
| Some new_stack -> Continue {state with stack = new_stack} )
;;
let evaluate strings =
List.fold_until
strings
~init:init_state
~finish:(fun state -> Some (List.rev state.stack))
~f:(fun state string ->
let new_state = evaluate_single state string in
if new_state.has_error then Stop None else Continue new_state )
;;
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