implemented set!

env.go

@@ -24,3 +24,8 @@ func (e environment) get(key symbol) (interface{}, error) {
 func (e environment) set(key symbol, val interface{}) {
 	e[key] = val
 }
+
+func (e environment) defined(key symbol) bool {
+	_, ok := e[key]
+	return ok
+}

input.scm

@@ -12,8 +12,8 @@
 "jordan"
 
 ; booleans.  I don't like the look of #t and #f.  They're dumb.
-true
-false
+#t
+#f
 
 ; ------------------------------------------------------------------------------
 ; basic math
@@ -30,9 +30,10 @@ false
 
 (define x 5)
 x
+(set! x "steve")
+x
 
 (quote 1 2 3)
 
-(if 1 2 3)
-(if false 2 3)
-(if true 2 3)
+(if #f (quote "true-value") (quote "false-value"))
+(if #t (quote "true-value") (quote "false-value"))

skeam.go

@@ -21,8 +21,8 @@ func (s sexp) String() string {
 type symbol string
 
 var universe = &environment{
-	"true":   true,
-	"false":  false,
+	"#t":     true,
+	"#f":     false,
 	"+":      proc(addition),
 	"-":      proc(subtraction),
 	"*":      proc(multiplication),
@@ -30,6 +30,7 @@ var universe = &environment{
 	"define": special(define),
 	"quote":  special(quote),
 	"if":     special(_if),
+	"set!":   special(set),
 }
 
 // parses the string lexeme into a value that can be eval'd
@@ -183,7 +184,9 @@ func evalall(c chan token, env *environment) {
 				fmt.Println("error:", err)
 				return
 			} else {
-				fmt.Println(v)
+				if v != nil {
+					fmt.Println(v)
+				}
 			}
 		default:
 			fmt.Println("error in eval: %v", err)

special.go

@@ -15,7 +15,11 @@ func define(env *environment, args ...interface{}) (interface{}, error) {
 	if !ok {
 		return nil, fmt.Errorf(`first argument to *define* must be symbol, received %v`, reflect.TypeOf(args[0]))
 	}
-	env.set(s, args[1])
+	v, err := eval(args[1], env)
+	if err != nil {
+		return nil, err
+	}
+	env.set(s, v)
 	return nil, nil
 }
 
@@ -36,3 +40,22 @@ func _if(env *environment, args ...interface{}) (interface{}, error) {
 	}
 	return eval(args[1], env)
 }
+
+func set(env *environment, args ...interface{}) (interface{}, error) {
+	if len(args) != 2 {
+		return nil, fmt.Errorf(`received %d arguments in *set!*, expected exactly 2`, len(args))
+	}
+	s, ok := args[0].(symbol)
+	if !ok {
+		return nil, fmt.Errorf(`first argument to *set!* must be symbol, received %v`, reflect.TypeOf(args[0]))
+	}
+	if !env.defined(s) {
+		return nil, fmt.Errorf(`cannot *set!* undefined symbol %v`, s)
+	}
+	v, err := eval(args[1], env)
+	if err != nil {
+		return nil, err
+	}
+	env.set(s, v)
+	return nil, nil
+}