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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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
]
)
(* Defines a simple interpreter for a Forth-like stack-based programming
* language.
*)
open Base
(* remove first 2 values off a list and apply a function *)
let pop2 f = function
| a :: b :: l -> f a b l
| _ -> None
(* apply a function on the top 2 values of a list and add to the
* front of the list *)
let pop2a f = pop2 (fun a b l -> Some ((f a b) :: l))
(* apply a function to the contents of an option, if any; function must
* replace option constructor *)
let option_apply f = function
| Some x -> f x
| _ -> None
(* apply a function to the contents of an option, if any *)
let option_map f = option_apply (fun x -> Some (f x))
(* reverse order of first two arguments of a function *)
let flip f = fun x y -> f y x
(* function composition *)
let (|>) f g = fun x -> f(g(x))
(* delayed function application *)
let (@>) f x = f x
let is_none = function
| None -> true
| _ -> false
let int_of_string s = try Some (Int.of_string s) with Failure _ -> None
(* eval_token returns an updated stack given a Map of word bindings, a token
* (either a word, symbol, or number), and a stack
*
* Supports the following words:
* +, -, *, / (integer arithmetic)
* DUP, DROP, SWAP, OVER (stack manipulation)
*
* Supports defining new words using the customary syntax:
* : word-name definition ;.
*
* The only data type supported is signed integers of at least 16 bits size.
*
* 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.
*)
let rec eval_token words = option_apply |> (fun top st ->
try Some ((Int.of_string top) :: st)
with Failure _ -> match Map.find words top with
| Some expr -> eval_input words (Some st) expr
| _ -> (match top with
(* perform arithmetic on top two values of stack,
* or raise stack underflow exception *)
| "+" -> pop2a ( + )
| "-" -> pop2a (flip ( - ))
| "*" -> pop2a ( * )
| "/" -> pop2 (fun a b l -> if Int.equal a 0
then None else Some ((b / a) :: l))
(* duplicate top value of stack or raise stack underflow *)
| "dup" -> (fun st -> option_apply (fun top -> Some (top :: st)) (List.hd st))
(* remove top value of st or raise stack underflow *)
| "drop" -> List.tl
(* swap top two values of stack or raise stack underflow *)
| "swap" -> pop2 (fun a b l -> Some (b :: a :: l))
(* duplicate second value of stack or raise stack underflow *)
| "over" -> pop2 (fun a b l -> Some (b :: a :: b :: l))
| _ -> (fun _ -> None)
) st) |> String.lowercase
(* evaluate a line of input given bindings and initial stack *)
and eval_input words init_stack input =
let tokens = String.split_on_chars input ~on:[' '; '\t'] in
List.fold tokens ~init:init_stack ~f:(flip (eval_token words))
let evaluate inputs =
(option_map List.rev) @> fst @>
List.fold inputs ~init:(Some [], Map.empty (module String))
~f:(fun (stack, words) input ->
match stack with
(* short-circuit *)
| None -> (stack, words)
| _ ->
let suffix = " ;" and prefix = ": " in
let sl = String.length suffix and pl = String.length prefix in
if String.is_suffix input ~suffix:suffix
&& String.is_prefix input ~prefix:prefix
then
let meat = String.drop_suffix
(String.drop_prefix input pl) sl in
let (word, expr) = String.lsplit2_exn meat ~on:' ' in
if String.equal word expr || not (is_none (int_of_string word))
then (None, words)
else (stack, Map.add_exn (Map.remove words word) ~key:word ~data:expr)
else (eval_input words stack input, words)
)
This was a fun way to learn OCaml after coming in with a background in Haskell. The syntax is unusual, but I don't mind such languages. I also had trouble finding the right functional programming idioms and functions, which is why I defined functions like option_map and flip.
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