Support if without else.

src/main/kotlin/gay/pizza/pork/ast/If.kt

@@ -3,7 +3,7 @@ package gay.pizza.pork.ast
 class If(
   val condition: Expression,
   val thenExpression: Expression,
-  val elseExpression: Expression
+  val elseExpression: Expression? = null
 ) : Expression {
   override val type: NodeType = NodeType.If
 }

src/main/kotlin/gay/pizza/pork/ast/Printer.kt

@@ -44,8 +44,10 @@ class Printer(private val buffer: StringBuilder) : Visitor<Unit> {
     visit(node.condition)
     append(" then ")
     visit(node.thenExpression)
-    append(" else ")
-    visit(node.elseExpression)
+    if (node.elseExpression != null) {
+      append(" else ")
+      visit(node.elseExpression)
+    }
   }
 
   override fun visitSymbol(node: Symbol) {

src/main/kotlin/gay/pizza/pork/eval/None.kt

@@ -0,0 +1,3 @@
+package gay.pizza.pork.eval
+
+data object None

src/main/kotlin/gay/pizza/pork/eval/PorkEvaluator.kt

@@ -24,12 +24,16 @@ class PorkEvaluator(root: Scope) : Visitor<Any> {
     return if (condition == true) {
       visit(node.thenExpression)
     } else {
-      visit(node.elseExpression)
+      if (node.elseExpression != null) {
+        visit(node.elseExpression)
+      } else {
+        None
+      }
     }
   }
 
   override fun visitSymbol(node: Symbol): Any {
-    return Unit
+    return None
   }
 
   override fun visitLambda(node: Lambda): CallableFunction {
@@ -43,7 +47,7 @@ class PorkEvaluator(root: Scope) : Visitor<Any> {
         for (expression in node.expressions) {
           value = visit(expression)
         }
-        value ?: Unit
+        value ?: None
       } finally {
         currentScope = currentScope.leave()
       }
@@ -88,6 +92,6 @@ class PorkEvaluator(root: Scope) : Visitor<Any> {
     for (expression in node.expressions) {
       value = visit(expression)
     }
-    return value ?: Unit
+    return value ?: None
   }
 }

src/main/kotlin/gay/pizza/pork/parse/PorkParser.kt

@@ -18,8 +18,11 @@ class PorkParser(val source: PeekableSource<Token>) {
     val condition = readExpression()
     expect(TokenType.Then)
     val thenExpression = readExpression()
-    expect(TokenType.Else)
-    val elseExpression = readExpression()
+    var elseExpression: Expression? = null
+    if (peekType(TokenType.Else)) {
+      expect(TokenType.Else)
+      elseExpression = readExpression()
+    }
     return If(condition, thenExpression, elseExpression)
   }