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.
For installation and learning resources, refer to the exercism help page.
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 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
(executable
(name test)
(libraries base str oUnit core)
(preprocess (pps ppx_jane)))
(alias
(name runtest)
(deps (:x test.exe))
(action (run %{x})))
I really miss Haskell do-notation for monads :(
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