forth

Simple implementation for very few limited subset of forth in haskell

feel free to add more features by contributing.

Structure

Program parses and evaluates all expressions using the following structure:

type Program = [Expr]
type Stack = [Int]
type Row = (String, Program)
type Table = [Row]

data Evaluator = Evaluator {
  program :: Program,
  stack :: Stack,
  table :: Table
} deriving Show

A full compiled code will populate Program and Table with expressions:

1. Program: All expressions compiled, a list of expressions
2. Table: Store compiled words, a list of tuple string expressions

Expressions

data Expr =
  Lit Int
  | Word String -- word definition
  | Add
  | Sub
  | Mul
  | Div
  | Dup
  | Swap
  | Over
  | Drop
  | Dot
  | Emit
  | Equal
  | Cr
  | Lf
  deriving Show

Evaluator

To evaluate a full compiled Program and Table use:

eval :: Evaluator -> IO Evaluator

eval get a single Evalutator data and recursively runs all expressions using Program and Table by pushing/poping values from top of the Stack, and produces side effects for commands like Dot and Emit.

GHCI Example

This following program outputs 'forth' in the standard output:

ghci> :l forth.hs
ghci> let source = ": f 102 ; : o 111 ; : r 114 ; : t 116 ; : h 104 ; : out f emit o emit r emit t emit h emit ; out 10 emit"
ghci> let Right (prog, table) = parseTokens source []
ghci> eval (Evaluator prog [] table)
forth
Evaluator {program = [], stack = [], table = [("out",[Emit,Word "h",Emit,Word "t",Emit,Word "r",Emit,Word "o",Emit,Word "f"]),("h",[Lit 104]),("t",[Lit 116]),("r",[Lit 114]),("o",[Lit 111]),("f",[Lit 102])]}

Forth Repl

Call main to start an interactive repl:

ghci> :l forth.hs
ghci> main
: three 3 ;
  ok
: four 4 ;
  ok
three four * .
12  ok

Running 4fh code

Inside GHCI load forth.hs and run hello.4fh to print 'hello, world':

ghci> :l forth.ts
ghci> run4fh "hello.4fh"
hello, world