List valid options in error messages

When an option value fails to parse, no custom error message is provided, and a list of valid candidate values is available, include the list as part of the error message. Addresses apple#344.
dduan · Jan 4, 2022 · baebf3e · baebf3e
1 parent e73578d
commit baebf3e
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Showing 2 changed files with 18 additions and 6 deletions.
diff --git a/Sources/ArgumentParser/Usage/UsageGenerator.swift b/Sources/ArgumentParser/Usage/UsageGenerator.swift
@@ -394,8 +394,9 @@ extension ErrorMessageGenerator {
   }
 
   func unableToParseValueMessage(origin: InputOrigin, name: Name?, value: String, key: InputKey, error: Error?) -> String {
-    let valueName = arguments(for: key).first?.valueName
-
+    let argumentValue = arguments(for: key).first
+    let valueName = argumentValue?.valueName
+
     // We want to make the "best effort" in producing a custom error message.
     // We favour `LocalizedError.errorDescription` and fall back to
     // `CustomStringConvertible`. To opt in, return your custom error message
@@ -407,10 +408,20 @@ extension ErrorMessageGenerator {
       case let err?:
         return ": " + String(describing: err)
       default:
+        if let help = argumentValue?.help, !help.allValues.isEmpty {
+            let quotedValues = help.allValues.map { "'\($0)'" }
+            let validList: String
+            if quotedValues.count <= 2 {
+                validList = quotedValues.joined(separator: " and ")
+            } else {
+                validList = quotedValues.dropLast().joined(separator: ", ") + " and \(quotedValues.last!)"
+            }
+          return ". Choose from \(validList)."
+        }
         return ""
       }
     }()
-    
+
     switch (name, valueName) {
     case let (n?, v?):
       return "The value '\(value)' is invalid for '\(n.synopsisString) <\(v)>'\(customErrorMessage)"

diff --git a/Tests/ArgumentParserUnitTests/ErrorMessageTests.swift b/Tests/ArgumentParserUnitTests/ErrorMessageTests.swift
@@ -65,18 +65,19 @@ extension ErrorMessageTests {
 }
 
 fileprivate struct Foo: ParsableArguments {
-  enum Format: String, Equatable, Decodable, ExpressibleByArgument {
+  enum Format: String, Equatable, Decodable, ExpressibleByArgument, CaseIterable {
     case text
     case json
+    case csv
   }
   @Option(name: [.short, .long])
   var format: Format
 }
 
 extension ErrorMessageTests {
   func testWrongEnumValue() {
-    AssertErrorMessage(Foo.self, ["--format", "png"], "The value 'png' is invalid for '--format <format>'")
-    AssertErrorMessage(Foo.self, ["-f", "png"], "The value 'png' is invalid for '-f <format>'")
+    AssertErrorMessage(Foo.self, ["--format", "png"], "The value 'png' is invalid for '--format <format>'. Choose from 'text', 'json' and 'csv'.")
+    AssertErrorMessage(Foo.self, ["-f", "png"], "The value 'png' is invalid for '-f <format>'. Choose from 'text', 'json' and 'csv'.")
   }
 }