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)

evaluator/src/main/kotlin/gay/pizza/pork/evaluator/EvaluationVisitor.kt

@@ -17,7 +17,7 @@ class EvaluationVisitor(root: Scope) : NodeVisitor<Any> {
     return currentScope.call(node.symbol.id, Arguments(arguments))
   }
 
-  override fun visitDefine(node: Assignment): Any {
+  override fun visitLetAssignment(node: LetAssignment): Any {
     val value = visit(node.value)
     currentScope.define(node.symbol.id, value)
     return value

examples/fib.pork

@@ -8,6 +8,6 @@ func fib(n) {
 }
 
 export func main() {
-  result = fib(20)
+  let result = fib(20)
   println(result)
 }

examples/syntax.pork

@@ -1,41 +1,41 @@
 export func main() {
-  three = 3
-  two = 2
+  let three = 3
+  let two = 2
 
-  calculateSimple = { in
+  let calculateSimple = { in
     (50 + three) * two
   }
 
-  calculateComplex = { in
+  let calculateComplex = { in
     three + two + 50
   }
 
-  multiply = { a, b in
+  let multiply = { a, b in
     a * b
   }
 
   // calculates the result
-  calculateSimpleResult = calculateSimple()
-  calculateComplexResult = calculateComplex()
-  multiplyResult = multiply(50, 50)
+  let calculateSimpleResult = calculateSimple()
+  let calculateComplexResult = calculateComplex()
+  let multiplyResult = multiply(50, 50)
 
-  list = [10, 20, 30]
-  trueValue = true
-  falseValue = false
+  let list = [10, 20, 30]
+  let trueValue = true
+  let falseValue = false
 
-  invert = { value in
+  let invert = { value in
     !value
   }
 
-  notEqual = { a, b in
+  let notEqual = { a, b in
     a != b
   }
 
-  equal = { a, b in
+  let equal = { a, b in
     a == b
   }
 
-  results = [
+  let results = [
     calculateSimpleResult,
     calculateComplexResult,
     multiplyResult,

parser/src/main/kotlin/gay/pizza/pork/parser/Parser.kt

@@ -31,6 +31,14 @@ class Parser(source: PeekableSource<Token>, val attribution: NodeAttribution) {
     ListLiteral(items)
   }
 
+  private fun readLetAssignment(): LetAssignment = within {
+    expect(TokenType.Let)
+    val symbol = readSymbolRaw()
+    expect(TokenType.Equals)
+    val value = readExpression()
+    LetAssignment(symbol, value)
+  }
+
   private fun readSymbolRaw(): Symbol = within {
     expect(TokenType.Symbol) { Symbol(it.text) }
   }
@@ -43,8 +51,6 @@ class Parser(source: PeekableSource<Token>, val attribution: NodeAttribution) {
       }
       expect(TokenType.RightParentheses)
       FunctionCall(symbol, arguments)
-    } else if (next(TokenType.Equals)) {
-      Assignment(symbol, readExpression())
     } else {
       SymbolReference(symbol)
     }
@@ -114,6 +120,10 @@ class Parser(source: PeekableSource<Token>, val attribution: NodeAttribution) {
         readListLiteral()
       }
 
+      TokenType.Let -> {
+        readLetAssignment()
+      }
+
       TokenType.Symbol -> {
         readSymbolCases()
       }

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

@@ -71,7 +71,8 @@ class Printer(buffer: StringBuilder) : NodeVisitor<Unit> {
     append(")")
   }
 
-  override fun visitDefine(node: Assignment) {
+  override fun visitLetAssignment(node: LetAssignment) {
+    append("let ")
     visit(node.symbol)
     append(" = ")
     visit(node.value)

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

@@ -31,6 +31,7 @@ enum class TokenType(vararg properties: TokenTypeProperty) {
   Import(Keyword("import"), KeywordFamily),
   Export(Keyword("export"), KeywordFamily),
   Func(Keyword("func"), KeywordFamily),
+  Let(Keyword("let"), KeywordFamily),
   Whitespace(CharConsumer { it == ' ' || it == '\r' || it == '\n' || it == '\t' }),
   BlockComment(CommentFamily),
   LineComment(CommentFamily),