Implement negation operator.

examples/syntax.pork

@@ -19,7 +19,7 @@ main = { in
   falseValue = false
 
   invert = { value in
-    if value then false else true
+    !value
   }
 
   [

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

@@ -13,7 +13,8 @@ enum class NodeType(val parent: NodeType? = null, vararg traits: NodeTypeTrait)
   Parentheses(Expression),
   Define(Expression),
   Lambda(Expression),
-  InfixOperation(Expression),
+  PrefixOperation(Expression, Operation),
+  InfixOperation(Expression, Operation),
   SymbolReference(Expression),
   FunctionCall(Expression),
   If(Expression);

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

@@ -2,5 +2,6 @@ package gay.pizza.pork.ast
 
 enum class NodeTypeTrait {
   Intermediate,
+  Operation,
   Literal
 }

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

@@ -0,0 +1,5 @@
+package gay.pizza.pork.ast
+
+class PrefixOperation(val op: PrefixOperator, val expression: Expression) : Expression() {
+  override val type: NodeType = NodeType.PrefixOperation
+}

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

@@ -0,0 +1,5 @@
+package gay.pizza.pork.ast
+
+enum class PrefixOperator(val token: String) {
+  Negate("!")
+}

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

@@ -111,6 +111,11 @@ class Printer(private val buffer: StringBuilder) : Visitor<Unit> {
     append(")")
   }
 
+  override fun visitPrefixOperation(node: PrefixOperation) {
+    append(node.op.token)
+    visit(node.expression)
+  }
+
   override fun visitInfixOperation(node: InfixOperation) {
     visit(node.left)
     append(" ")

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

@@ -13,6 +13,7 @@ interface Visitor<T> {
   fun visitListLiteral(node: ListLiteral): T
 
   fun visitParentheses(node: Parentheses): T
+  fun visitPrefixOperation(node: PrefixOperation): T
   fun visitInfixOperation(node: InfixOperation): T
 
   fun visitProgram(node: Program): T
@@ -23,6 +24,7 @@ interface Visitor<T> {
     is ListLiteral -> visitListLiteral(node)
     is Parentheses -> visitParentheses(node)
     is InfixOperation -> visitInfixOperation(node)
+    is PrefixOperation -> visitPrefixOperation(node)
     is Define -> visitDefine(node)
     is Lambda -> visitLambda(node)
     is FunctionCall -> visitFunctionCall(node)

src/main/kotlin/gay/pizza/pork/compiler/KotlinCompiler.kt

@@ -57,6 +57,9 @@ class KotlinCompiler : Visitor<String> {
   override fun visitParentheses(node: Parentheses): String =
     "(${visit(node.expression)})"
 
+  override fun visitPrefixOperation(node: PrefixOperation): String =
+    "${node.op.token}${visit(node.expression)}"
+
   override fun visitInfixOperation(node: InfixOperation): String =
     "${visit(node.left)} ${node.op.token} ${visit(node.right)}"
 

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

@@ -60,6 +60,18 @@ class PorkEvaluator(root: Scope) : Visitor<Any> {
 
   override fun visitParentheses(node: Parentheses): Any = visit(node.expression)
 
+  override fun visitPrefixOperation(node: PrefixOperation): Any {
+    val value = visit(node.expression)
+    return when (node.op) {
+      PrefixOperator.Negate -> {
+        if (value !is Boolean) {
+          throw RuntimeException("Cannot negate a value which is not a boolean.")
+        }
+        !value
+      }
+    }
+  }
+
   override fun visitInfixOperation(node: InfixOperation): Any {
     val left = visit(node.left)
     val right = visit(node.right)

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

@@ -96,6 +96,11 @@ class PorkParser(val source: PeekableSource<Token>) {
         return BooleanLiteral(false)
       }
 
+      TokenType.Negation -> {
+        expect(TokenType.Negation)
+        return PrefixOperation(PrefixOperator.Negate, readExpression())
+      }
+
       TokenType.If -> {
         return readIf()
       }

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

@@ -15,6 +15,7 @@ enum class TokenType(val char: Char? = null, val keyword: String? = null, val pr
   RightBracket(char = ']'),
   LeftParentheses(char = '('),
   RightParentheses(char = ')'),
+  Negation(char = '!'),
   Comma(char = ','),
   False(keyword = "false"),
   True(keyword = "true"),