added *if*

input.scm

@@ -11,6 +11,10 @@
 ; string
 "jordan"
 
+; booleans.  I don't like the look of #t and #f.  They're dumb.
+true
+false
+
 ; ------------------------------------------------------------------------------
 ; basic math
 ; ------------------------------------------------------------------------------
@@ -28,3 +32,7 @@
 x
 
 (quote 1 2 3)
+
+(if 1 2 3)
+(if false 2 3)
+(if true 2 3)

skeam.go

@@ -21,12 +21,15 @@ func (s sexp) String() string {
 type symbol string
 
 var universe = &environment{
+	"true":   true,
+	"false":  false,
 	"+":      proc(addition),
 	"-":      proc(subtraction),
 	"*":      proc(multiplication),
 	"/":      proc(division),
 	"define": special(define),
 	"quote":  special(quote),
+	"if":     special(_if),
 }
 
 // parses the string lexeme into a value that can be eval'd
@@ -178,6 +181,7 @@ func evalall(c chan token, env *environment) {
 		case nil:
 			if v, err := eval(v, env); err != nil {
 				fmt.Println("error:", err)
+				return
 			} else {
 				fmt.Println(v)
 			}

special.go

@@ -22,3 +22,17 @@ func define(env *environment, args ...interface{}) (interface{}, error) {
 func quote(_ *environment, args ...interface{}) (interface{}, error) {
 	return sexp(args), nil
 }
+
+func _if(env *environment, args ...interface{}) (interface{}, error) {
+	if len(args) != 3 {
+		return nil, fmt.Errorf(`received %d arguments in *if*, expected exactly 3`, len(args))
+	}
+	v, err := eval(args[0], env)
+	if err != nil {
+		return nil, err
+	}
+	if b, ok := v.(bool); ok && !b {
+		return eval(args[2], env)
+	}
+	return eval(args[1], env)
+}