implement EMIT and QUIT

builtins.go

@@ -5,6 +5,9 @@ import "fmt"
 // Builtins is a handy holder for our various default words
 type Builtins struct{}
 
+// ErrExit is a special sentinel value to cease computation and quit
+var ErrExit = fmt.Errorf("exit requested")
+
 // Add sums the top two numbers on the stack and pushes the result
 func (b *Builtins) Add(s *Stack) func() error {
 	return func() error {
@@ -169,6 +172,18 @@ func (b *Builtins) Words(d Dictionary) func() error {
 	}
 }
 
+// Emit outputs the UTF-8 rune for the int on the top of the stack
+func (b *Builtins) Emit(s *Stack) func() error {
+	return func() error {
+		i, err := s.Pop()
+		if err != nil {
+			return err
+		}
+		fmt.Print(string(rune(i)) + " ")
+		return nil
+	}
+}
+
 // CR prints a newline to standard out
 func (b *Builtins) CR() func() error {
 	return func() error {
@@ -177,6 +192,13 @@ func (b *Builtins) CR() func() error {
 	}
 }
 
+// Quit exits the repl
+func (b *Builtins) Quit() func() error {
+	return func() error {
+		return ErrExit
+	}
+}
+
 // Debug prints the stack without modifying it
 func (b *Builtins) Debug(s *Stack) func() error {
 	return func() error {

main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"os"
 	"strings"
@@ -24,7 +25,9 @@ func main() {
 	dict.AddWord("ROT", b.Rot(&stack), "")
 	dict.AddWord("WORDS", b.Words(dict), "")
 	dict.AddWord(".S", b.Debug(&stack), "")
+	dict.AddWord("EMIT", b.Emit(&stack), "")
 	dict.AddWord("CR", b.CR(), "")
+	dict.AddWord("QUIT", b.Quit(), "")
 
 	c := Context{
 		Dictionary: dict,
@@ -45,7 +48,10 @@ func main() {
 		}
 		line = strings.TrimSpace(line)
 		err = c.Eval(line + " ") // append a space to make sure we always close out our parse loop
-		if err != nil {
+		if errors.Is(err, ErrExit) {
+			fmt.Printf("bye\n")
+			os.Exit(0)
+		} else if err != nil {
 			fmt.Printf("error in evaluation: %v\n", err)
 		}
 		fmt.Print("ok")