Introduce the requirement of let for assignment.

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

@@ -4,15 +4,15 @@ import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
 @Serializable
-@SerialName("assignment")
-class Assignment(val symbol: Symbol, val value: Expression) : Expression() {
-  override val type: NodeType = NodeType.Assignment
+@SerialName("letAssignment")
+class LetAssignment(val symbol: Symbol, val value: Expression) : Expression() {
+  override val type: NodeType = NodeType.LetAssignment
 
   override fun <T> visitChildren(visitor: NodeVisitor<T>): List<T> =
     visitor.visitNodes(symbol, value)
 
   override fun equals(other: Any?): Boolean {
-    if (other !is Assignment) return false
+    if (other !is LetAssignment) return false
     return other.symbol == symbol && other.value == value
   }
 

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

@@ -7,7 +7,7 @@ class NodeCoalescer(val handler: (Node) -> Unit) : NodeVisitor<Unit> {
   override fun visitListLiteral(node: ListLiteral): Unit = handle(node)
   override fun visitSymbol(node: Symbol): Unit = handle(node)
   override fun visitFunctionCall(node: FunctionCall): Unit = handle(node)
-  override fun visitDefine(node: Assignment): Unit = handle(node)
+  override fun visitLetAssignment(node: LetAssignment): Unit = handle(node)
   override fun visitSymbolReference(node: SymbolReference): Unit = handle(node)
   override fun visitLambda(node: Lambda): Unit = handle(node)
   override fun visitParentheses(node: Parentheses): Unit = handle(node)

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

@@ -12,7 +12,7 @@ enum class NodeType(val parent: NodeType? = null) {
   ListLiteral(Expression),
   StringLiteral(Expression),
   Parentheses(Expression),
-  Assignment(Expression),
+  LetAssignment(Expression),
   Lambda(Expression),
   PrefixOperation(Expression),
   InfixOperation(Expression),

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

@@ -7,7 +7,7 @@ interface NodeVisitor<T> {
   fun visitListLiteral(node: ListLiteral): T
   fun visitSymbol(node: Symbol): T
   fun visitFunctionCall(node: FunctionCall): T
-  fun visitDefine(node: Assignment): T
+  fun visitLetAssignment(node: LetAssignment): T
   fun visitSymbolReference(node: SymbolReference): T
   fun visitLambda(node: Lambda): T
   fun visitParentheses(node: Parentheses): T
@@ -26,7 +26,7 @@ interface NodeVisitor<T> {
     is BooleanLiteral -> visitBooleanLiteral(node)
     is ListLiteral -> visitListLiteral(node)
     is FunctionCall -> visitFunctionCall(node)
-    is Assignment -> visitDefine(node)
+    is LetAssignment -> visitLetAssignment(node)
     is SymbolReference -> visitSymbolReference(node)
     is Lambda -> visitLambda(node)
     is Parentheses -> visitParentheses(node)