Better error printing

Sources/_MatchingEngine/Regex/Parse/Diagnostics.swift

@@ -26,6 +26,41 @@ enum ParseError: Error, Hashable {
   case emptyProperty
 }
 
+extension ParseError: CustomStringConvertible {
+  var description: String {
+    switch self {
+    case let .numberOverflow(s):
+      return "number overflow: \(s)"
+    case let .expectedNumDigits(s, i):
+      return "expected \(i) digits in '\(s)'"
+    case let .expectedNumber(s, kind: kind):
+      let radix: String
+      if kind == .decimal {
+        radix = ""
+      } else {
+        radix = " of radix \(kind.radix)"
+      }
+      return "expected a numbers in '\(s)'\(radix)"
+    case let .expected(s):
+      return "expected '\(s)'"
+    case .unexpectedEndOfInput:
+      return "unexpected end of input"
+    case let .misc(s):
+      return s
+    case let .expectedASCII(c):
+      return "expected ASCII for '\(c)'"
+    case .expectedCustomCharacterClassMembers:
+      return "expected custom character class members"
+    case .invalidCharacterClassRangeOperand:
+      return "invalid character class range"
+    case let .invalidPOSIXSetName(n):
+      return "invalid character set name: '\(n)'"
+    case .emptyProperty:
+      return "empty property"
+    }
+  }
+}
+
 // TODO: Fixits, notes, etc.
 
 // TODO: Diagnostics engine, recorder, logger, or similar.

Tests/RegexTests/DiagnosticTests.swift

@@ -86,4 +86,12 @@ extension RegexTests {
     // TODO: Find out way to render value-members of AST, not
     // just children
   }
+
+  func testErrors() {
+    // Note: These don't really "test" anything, but good to
+    // see our output...
+    print("\(ParseError.emptyProperty)")
+    print("\(ParseError.expectedNumber("abc", kind: .decimal))")
+    print("\(ParseError.expectedNumber("abc", kind: .hex))")
+  }
 }