Fix crash with missing parameter

Sources/LeafKit/LeafError.swift

@@ -31,6 +31,8 @@ public struct LeafError: Error {
         /// Attempt to render an AST with cyclical external references
         /// - Provide template name & ordered array of template names that causes the cycle path
         case cyclicalReference(String, [String])
+        /// Parameter missing
+        case missingParameter
 
         // MARK: Wrapped Errors related to Lexing or Parsing
         /// Errors due to malformed template syntax or grammar
@@ -84,6 +86,8 @@ public struct LeafError: Error {
                 return "\(src) - \(key) cyclically referenced in [\(chain.joined(separator: " -> "))]"
             case .lexerError(let e):
                 return "Lexing error - \(e.localizedDescription)"
+            case .missingParameter:
+                return "Expected a parameter but found none"
         }
     }
 

Sources/LeafKit/LeafParser/LeafParser.swift

@@ -183,13 +183,20 @@ internal struct LeafParser {
 
         var group = [ParameterDeclaration]()
         var paramsList = [ParameterDeclaration]()
-
-        func dump() {
-            defer { group = [] }
-            if group.isEmpty { return }
-            group.evaluate()
-            if group.count > 1 { paramsList.append(.expression(group)) }
-            else { paramsList.append(group.first!) }
+
+        func dump() throws {
+          defer { group = [] }
+          if group.isEmpty { return }
+          group.evaluate()
+          if group.count > 1 { paramsList.append(.expression(group)) }
+          else {
+            guard let first = group.first else {
+              // It's better to handle this case as well, even though logically it might never happen
+              // since you're checking if group.isEmpty before.
+              throw LeafError(.missingParameter, file: #file, function: #function, line: #line, column: #column)
+            }
+            paramsList.append(first)
+          }
         }
 
         outer: while let next = peek() {
@@ -200,7 +207,12 @@ internal struct LeafParser {
                     let params = try readParameters()
                     // parameter tags not permitted to have bodies
                     if params.count > 1  { group.append(.expression(params)) }
-                    else { group.append(params.first!) }
+                    else {
+                      guard let firstParam = params.first else {
+                        throw LeafError(.missingParameter)
+                      }
+                      group.append(firstParam)
+                    }
                 case .parameter(let p):
                     pop()
                     switch p {
@@ -214,11 +226,11 @@ internal struct LeafParser {
                     }
                 case .parametersEnd:
                     pop()
-                    dump()
+                    try dump()
                     break outer
                 case .parameterDelimiter:
                     pop()
-                    dump()
+                    try dump()
                 case .whitespace:
                     pop()
                     continue

Tests/LeafKitTests/LeafErrorTests.swift

@@ -45,4 +45,20 @@ final class LeafErrorTests: XCTestCase {
             XCTFail("Wrong error: \(error.localizedDescription)")
         }
     }
+
+    /// Verify that rendering a template with a missing required parameter will throw `LeafError.missingParameter`
+    func testMissingParameterError() {
+      var test = TestFiles()
+      // Assuming "/missingParam.leaf" is a template that requires a parameter we intentionally don't provide
+      test.files["/missingParam.leaf"] = """
+          #(foo.bar.trim())
+          """
+        XCTAssertThrowsError(try TestRenderer(sources: .singleSource(test))
+            .render(path: "missingParam", context: [:])
+            .wait()
+        ) {
+            guard case .missingParameter = ($0 as? LeafError)?.reason else {
+                return XCTFail("Expected LeafError.missingParameter, got \(String(reflecting: $0))")
+            }
+        }
 }