Introduce the requirement of let for assignment.

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),