-- Global variables that hold the input:
subj = "5 DUP * ."      -- what we are interpreting (example)
pos  = 1                -- where are are (1 = "at the beginning")

-- Low-level functions to read things from "pos" and advance "pos".
-- Note: the "pat" argument in "parsebypattern" is a pattern with
-- one "real" capture and then an empty capture.
parsebypattern = function (pat)
    local capture, newpos = string.match(subj, pat, pos)
    if newpos then pos = newpos; return capture end
  end
parsespaces     = function () return parsebypattern("^([ \t]*)()") end
parseword       = function () return parsebypattern("^([^ \t\n]+)()") end
parsenewline    = function () return parsebypattern("^(\n)()") end
parserestofline = function () return parsebypattern("^([^\n]*)()") end
parsewordornewline = function () return parseword() or parsenewline() end

-- A "word" is a sequence of one or more non-whitespace characters.
-- The outer interpreter reads one word at a time and executes it.
-- Note that `getwordornewline() or ""' returns a word, or a newline, or "".
getword          = function () parsespaces(); return parseword() end
getwordornewline = function () parsespaces(); return parsewordornewline() end

-- The dictionary.
-- Entries whose values are functions are primitives.
_F = {}
_F["%L"] = function () assert(load(parserestofline(), '=L', nil, _ENV))() end

-- The "processor". It can be in any of several "modes".
-- Its initial behavior is to run modes[mode]() - i.e.,
-- modes.interpret() - until `mode' becomes "stop".
mode  = "interpret"
modes = {}
run = function () while mode ~= "stop" do modes[mode]() end end

-- Initially the processor knows only this mode, "interpret"...
-- Note that "word" is a global variable.
interpretprimitive = function ()
    if type(_F[word]) == "function" then _F[word](); return true end
  end
interpretnonprimitive = function () return false end   -- stub
interpretnumber       = function () return false end   -- stub
p_s_i = function () end  -- print state, for "interpret" (stub)
modes.interpret = function ()
    word = getwordornewline() or ""
    p_s_i()
    local _ = interpretprimitive() or
              interpretnonprimitive() or
              interpretnumber() or
              error("Can't interpret: "..word)
  end

  -- ( Program 3a: the core of miniforth )
  
  -- Our first program in MiniForth.
-- It defines a meaning for newlines (they are no-ops; go ahead),
-- for "" ("at end-of-text, change the mode to 'stop'"),
-- and for "[L" - read everything until a "L]", and interpret
-- what is between the "[L" and the "L] as Lua code, with eval.
-- Then it creates a data stack ("DS") and four words - "5", "DUP",
-- "*", "." - that operate on it.
--
subj = [=[
%L _F["\n"] = function () end
%L _F[""]   = function () mode = "stop" end
%L _F["[L"] = function () eval(parsebypattern("^(.-)%sL]()")) end
%L DS = { n = 0 }
%L push = function (stack, x) stack.n = stack.n + 1; stack[stack.n] = x end
%L pop  = function (stack) local x = stack[stack.n]; stack[stack.n] = nil; stack.n = stack.n - 1; return x end
%L _F["5"]   = function () push(DS, 5) end
%L _F["DUP"] = function () push(DS, DS[DS.n]) end
%L _F["*"]   = function () push(DS, pop(DS) * pop(DS)) end
%L _F["."]   = function () io.write(" "..pop(DS)) end
5 DUP * .
]=]

-- Now run it. There's no visible output.
pos = 1
mode = "interpret"
run()

-- At this point the dictionary (_F) has eight words.

   -- ( Program 3b: a first program in miniforth )