CONSTANT

README.md

@@ -54,7 +54,7 @@ ok
 55 ok
 ```
 
-Propser also has a basic off-stack memory model using variables and pointers:
+Propser also has a basic off-stack memory model using variables, constants, and pointers:
 ```
 > VARIABLE FOO
 ok
@@ -64,6 +64,10 @@ ok
 ok
 > .
 10 ok
+> 100 CONSTANT HUNDRED
+ok
+> HUNDRED HUNDRED * .
+10000 ok
 ```
 
 And, of course, it knows how to quit:
@@ -75,6 +79,5 @@ bye
 
 ## Future Plans
 
-* Implement `CONSTANT`
 * Implement loading libraries of pre-written functions both from the command-line and at run-time
 * Add much better readline behaviors in the interactive console (up-arrow for history, cursor movement...)

builtins.go

@@ -689,6 +689,36 @@ func (b *Builtins) Variable(c *Context) func(string) error {
 	}
 }
 
+// Constant adds a new word to the dictionary that puts a value on the stack
+func (b *Builtins) Constant(c *Context) func(string) error {
+	return func(next string) error {
+		w := []byte{}
+		for i := 1; i < len(next); i = i + 1 {
+			switch next[i] {
+			case ' ':
+				v, err := c.Stack.Pop()
+				if err != nil {
+					return err
+				}
+				c.Dictionary.AddWord(string(w), Word{
+					Name: string(w),
+					Impl: func(_ string) error {
+						c.Stack.Push(v)
+						return nil
+					},
+				})
+				j, _ := c.RStack.Pop()
+				c.RStack.Push(j + i - 1) // push the end-point onto the stack
+				return nil
+			default:
+				w = append(w, next[i])
+				continue
+			}
+		}
+		return nil
+	}
+}
+
 // Store places a value in memory
 func (b *Builtins) Store(c *Context) func(string) error {
 	return func(_ string) error {

main.go

@@ -82,8 +82,9 @@ func main() {
 	dict.AddWord("2DROP", Word{Name: "2DROP", Source: []string{"DROP", "DROP"}})
 	dict.AddWord("2DUP", Word{Name: "2DUP", Source: []string{"OVER", "OVER"}})
 	dict.AddWord("2OVER", Word{Name: "2OVER", Source: []string{"3", "PICK", "3", "PICK"}})
-	// memory access with variables
+	// memory access with variables and constants
 	dict.AddWord("VARIABLE", Word{Name: "VARIABLE", Impl: b.Variable(&c)})
+	dict.AddWord("CONSTANT", Word{Name: "CONSTANT", Impl: b.Constant(&c)})
 	dict.AddWord("!", Word{Name: "!", Impl: b.Store(&c)})
 	dict.AddWord("@", Word{Name: "@", Impl: b.Fetch(&c)})
 	// debugging