reporting error with its parent context

Sources/Errors.swift

@@ -19,15 +19,56 @@ public class TemplateDoesNotExist: Error, CustomStringConvertible {
 }
 
 public struct TemplateSyntaxError : Error, Equatable, CustomStringConvertible {
-  public let description:String
-  var lexeme: Lexeme?
+  public let reason: String
+  public private(set) var description: String
+
+  public internal(set) var template: Template?
+  public internal(set) var parentError: Error?
+
+  var lexeme: Lexeme? {
+    didSet {
+      description = TemplateSyntaxError.description(reason: reason, lexeme: lexeme, template: template)
+    }
+  }
   
-  public init(_ description:String) {
-    self.description = description
+  static func description(reason: String, lexeme: Lexeme?, template: Template?) -> String {
+    if let template = template, let range = lexeme?.range {
+      let templateName = template.name.map({ "\($0):" }) ?? ""
+      let tokenContent = template.templateString.substring(with: range)
+      let line = template.templateString.rangeLine(range)
+      let highlight = "\(String(Array(repeating: " ", count: line.offset)))^\(String(Array(repeating: "~", count: max(tokenContent.length - 1, 0))))"
+      
+      return "\(templateName)\(line.number):\(line.offset): error: \(reason)\n"
+        + "\(line.content)\n"
+        + "\(highlight)\n"
+    } else {
+      return reason
+    }
+  }
+
+  init(reason: String, lexeme: Lexeme? = nil, template: Template? = nil, parentError: Error? = nil) {
+    self.reason = reason
+    self.parentError = parentError
+    self.template = template
+    self.lexeme = lexeme
+    self.description = TemplateSyntaxError.description(reason: reason, lexeme: lexeme, template: template)
+  }
+  
+  public init(_ description: String) {
+    self.init(reason: description)
   }
   
   public static func ==(lhs:TemplateSyntaxError, rhs:TemplateSyntaxError) -> Bool {
-    return lhs.description == rhs.description
+    guard lhs.description == rhs.description else { return false }
+
+    switch (lhs.parentError, rhs.parentError) {
+    case let (lhsParent? as TemplateSyntaxError?, rhsParent? as TemplateSyntaxError?):
+      return lhsParent == rhsParent
+    case let (lhsParent?, rhsParent?):
+      return String(describing: lhsParent) == String(describing: rhsParent)
+    default:
+      return lhs.parentError == nil && rhs.parentError == nil
+    }
   }
   
 }
@@ -47,7 +88,7 @@ public class ErrorReporterContext {
 public protocol ErrorReporter: class {
   var context: ErrorReporterContext! { get set }
   func reportError(_ error: Error) -> Error
-  func contextAwareError(_ error: Error, at range: Range<String.Index>?, context: ErrorReporterContext) -> Error?
+  func renderError(_ error: Error) -> String
 }
 
 open class SimpleErrorReporter: ErrorReporter {
@@ -55,21 +96,28 @@ open class SimpleErrorReporter: ErrorReporter {
   
   open func reportError(_ error: Error) -> Error {
     guard let context = context else { return error }
-    return contextAwareError(error, at: (error as? TemplateSyntaxError)?.lexeme?.range, context: context) ?? error
+
+    return TemplateSyntaxError(reason: (error as? TemplateSyntaxError)?.reason ?? "\(error)",
+                               lexeme: (error as? TemplateSyntaxError)?.lexeme,
+                               template: context.template,
+                               parentError: (error as? TemplateSyntaxError)?.parentError
+    )
   }
   
-  // TODO: add stack trace using parent context
-  open func contextAwareError(_ error: Error, at range: Range<String.Index>?, context: ErrorReporterContext) -> Error? {
-    guard let range = range, range != .unknown else { return nil }
-    let templateName = context.template.name.map({ "\($0):" }) ?? ""
-    let tokenContent = context.template.templateString.substring(with: range)
-    let line = context.template.templateString.rangeLine(range)
-    let highlight = "\(String(Array(repeating: " ", count: line.offset)))^\(String(Array(repeating: "~", count: max(tokenContent.length - 1, 0))))"
-    let description = "\(templateName)\(line.number):\(line.offset): error: \(error)\n\(line.content)\n\(highlight)"
-    let error = TemplateSyntaxError(description)
-    return error
+  open func renderError(_ error: Error) -> String {
+    guard let templateError = error as? TemplateSyntaxError else { return error.localizedDescription }
+    
+    var descriptions = [templateError.description]
+    
+    var currentError: TemplateSyntaxError? = templateError
+    while let parentError = currentError?.parentError {
+      descriptions.append(renderError(parentError))
+      currentError = parentError as? TemplateSyntaxError
+    }
+    
+    return descriptions.reversed().joined(separator: "\n")
   }
-
+  
 }
 
 extension Range where Bound == String.Index {