prosper/eval.go

package main

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

// Context is a set of Dictionary + Stacks + Flags representing a runtime environment
type Context struct {
	Dictionary   Dictionary
	Stack        *Stack // main stack
	RStack       *Stack // "return" stack, for handling loops/backward jumps
	IfStack      *Stack // tracks nested branches
	Flags        Flags
	Words        []string
	Memory       Memory
	StringBuffer string
	Output       io.Writer
}

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

	for i := 0; i < len(line); i = i + 1 {
		switch line[i] {
		case ' ':
			sword := strings.TrimSpace(string(word))
			if len(word) == 0 {
				// empty space, just continue...
				continue
			}
			// Is this a word we know?
			w, _ := c.Dictionary.GetWord(sword)

			// if we're not in immedate mode, and the word isn't immediate, add word to buffer and continue parsing
			if !c.Flags.GetFlag("Immediate") && !w.Immediate {
				c.Words = append(c.Words, sword)
				word = []byte{}
				continue
			}

			ifcheck, _ := c.IfStack.Pick(0)
			if len(c.IfStack.values) == 0 || (len(c.IfStack.values) > 0 && ifcheck == 1) || w.BranchCheck {
				// run word
				c.RStack.Push(i)
				err := c.Exec(w, line[i:])
				if 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:
			word = append(word, line[i])
		}
	}

	return nil
}

// Exec wraps the branched execution of words (either built-in or user-defined)
func (c *Context) Exec(w Word, s string) error {
	if w.Impl != nil {
		// we have an implementation for that word. Run it.
		err := w.Impl(s)
		if err != nil {
			return fmt.Errorf("error during built-in: %w", err)
		}
	} else if len(w.Source) != 0 {
		// user-defined word; let's descend...
		err := c.Eval(strings.Join(w.Source, " ") + " ")
		if err != nil {
			return fmt.Errorf("error during nested evaluation of word %+v: %w", w, err)
		}
	} else if w.Variable > 0 {
		// it's a variable, let's get that address and put it on the stack
		c.Stack.Push(w.Variable)
	} else {
		it, err := strconv.Atoi(w.Name)
		if err == nil {
			// it was a number! put it on the stack.
			c.Stack.Push(it)
		} else {
			return fmt.Errorf("unable to parse word %s", w.Name)
		}
	}
	return nil
}