prosper/eval.go

package main

import (
	"fmt"
	"strconv"
	"strings"
)

// Context is a set of Dictionary + Stacks representing a runtime environment
type Context struct {
	Dictionary Dictionary
	Stack      *Stack
	RStack     *Stack
	Immediate  bool
	Words      []string
}

// Eval evaulates a given line, recursively descending into given words as needed
func (c *Context) Eval(line string) error {
	// state
	var word []byte
	var comment bool

	for i := 0; i < len(line); i = i + 1 {
		switch line[i] {
		case '(', ')': // comments
			if len(word) == 0 {
				if line[i] == '(' {
					comment = true
					continue
				}
				comment = false
				continue
			} else {
				word = append(word, line[i])
			}
		case ' ':
			sword := strings.TrimSpace(string(word))
			if !c.Immediate { // continue building our subroutine if we're not in immediate mode...
				if len(word) != 0 { // don't add empty words to our list
					// Was that a word we know?
					w, _ := c.Dictionary.GetWord(sword)
					// check if it's an IMMEDIATE mode toggle word
					c.Immediate = w.Immediate
					if !c.Immediate {
						c.Words = append(c.Words, sword)
						word = []byte{}
						continue
					}
				}
			}
			if len(word) == 0 || comment {
				// empty space, just continue...
				continue
			}

			int, err := strconv.Atoi(sword)
			if err == nil {
				// it was a number! put it on the stack.
				c.Stack.Push(int)
				word = []byte{}
				continue
			}

			// it wasn't a number. Is it a word we know?
			w, err := c.Dictionary.GetWord(sword)
			if err != nil {
				return fmt.Errorf("could not parse %s; %v", w.Name, err)
			}
			// run word
			c.RStack.Push(i)
			if err = c.Exec(w); err != nil {
				return err
			}
			i, err = c.RStack.Pop()
			if err != nil {
				return fmt.Errorf("error while popping from return stack: %v", err)
			}
			word = []byte{}
		default:
			if !comment {
				word = append(word, line[i])
			}
		}
	}

	return nil
}

func (c *Context) Exec(w Word) error {
	if w.Impl != nil {
		// we have an implementation for that word. Run it.
		err := w.Impl()
		if err != nil {
			return err
		}
	} else if len(w.Source) != 0 {
		// user-defined word; let's descend...
		err := c.Eval(strings.Join(w.Source, " ") + " ")
		if err != nil {
			return err
		}
	}
	return nil
}