if

src/kiss/Helpers.hx

@@ -0,0 +1,13 @@
+package kiss;
+
+import haxe.macro.Expr;
+import haxe.macro.Context;
+
+class Helpers {
+    public static function withPos(e:ExprDef):Expr {
+        return {
+            pos: Context.currentPos(),
+            expr: e
+        };
+    }
+}

src/kiss/Prelude.hx

@@ -44,4 +44,21 @@ class Prelude {
     public static function areEqual(a, b) {
         return if (a == b) a else Math.NaN;
     }
+
+    public static dynamic function truthy(v:Any) {
+        return switch (Type.typeof(v)) {
+            case TNull: false;
+            case TInt | TFloat: (v : Float) > 0;
+            case TBool: (v : Bool);
+            default:
+                // Empty strings are falsy
+                var str = cast(v, String);
+                if (str != null) {
+                    str.length > 0;
+                } else {
+                    // Any other value is true by default
+                    true;
+                }
+        }
+    }
 }

src/kiss/SpecialForms.hx

@@ -5,6 +5,8 @@ import haxe.macro.Context;
 import kiss.Reader;
 import kiss.Types;
 
+using kiss.Helpers;
+
 // Special forms convert Kiss reader expressions into Haxe macro expressions
 typedef SpecialFormFunction = (args:Array<ReaderExp>, convert:ExprConversion) -> Expr;
 
@@ -22,12 +24,26 @@ class SpecialForms {
             expr: EArray(convert(args[0]), convert(args[1]))
         };
 
+        // TODO first through tenth
+
         map["<"] = foldComparison("_min");
         map["<="] = foldComparison("min");
         map[">"] = foldComparison("_max");
         map[">="] = foldComparison("max");
         map["="] = foldComparison("_eq");
 
+        map["if"] = (args:Array<ReaderExp>, convert:ExprConversion) -> {
+            if (args.length < 2 || args.length > 3) {
+                throw 'if statement has wrong number of arguments: ${args.length}';
+            }
+
+            var condition = macro Prelude.truthy(${convert(args[0])});
+            var thenExp = convert(args[1]);
+            var elseExp = if (args.length > 2) convert(args[2]) else null;
+
+            EIf(condition, thenExp, elseExp).withPos();
+        };
+
         return map;
     }
 

src/test/cases/BasicTestCase.hx

@@ -107,4 +107,15 @@ class BasicTestCase extends Test {
         Assert.equals(true, BasicTestCase.myComp9);
         Assert.equals(false, BasicTestCase.myComp10);
     }
+
+    function testIf() {
+        Assert.equals(true, BasicTestCase.myIf1);
+        Assert.equals(false, BasicTestCase.myIf2);
+        Assert.equals(false, BasicTestCase.myIf3);
+        Assert.equals(false, BasicTestCase.myIf4);
+        Assert.equals(true, BasicTestCase.myIf5);
+        Assert.equals(false, BasicTestCase.myIf6);
+        Assert.equals(true, BasicTestCase.myIf7);
+        Assert.equals(false, BasicTestCase.myIf8);
+    }
 }

src/test/cases/BasicTestCase.kiss

@@ -52,4 +52,13 @@
 (defvar myComp7 (>= 4 3 2 1))
 (defvar myComp8 (>= 4 4 2 1))
 (defvar myComp9 (= 1 1 1 1))
-(defvar myComp10 (= 1 2 1 1))
+(defvar myComp10 (= 1 2 1 1))
+
+(defvar myIf1 (if 1 true false))
+(defvar myIf2 (if 0 true false))
+(defvar myIf3 (if -1 true false))
+(defvar myIf4 (if null true false))
+(defvar myIf5 (if true true false))
+(defvar myIf6 (if false true false))
+(defvar myIf7 (if "string" true false))
+(defvar myIf8 (if "" true false))