Great Error Message

IHP error messages should be clear, actionable, and practical. A great error message tells the user what went wrong and what the user needs to do to fix the problem. It should direct the user to the solution most likely needed.

In case the error messages are produced by GHC, we should use troubleshooting helpers in the StatusServer to show actionable steps for a solution. In case a Haskell exception is thrown at runtime, we can use the ErrorController to explain the error message in the IHP context.

Examples

Here are some examples of improved error messages.

Unclear error message:

AutoRoute: Failed parsing UUID

Improved:

AutoRoute: Failed parsing argument as UUID. You most likely are trying to use AutoRoute with a non-UUID attribute. See https://ihp.digitallyinduced.com/Guide/routing.html#parameter-types for details on how to configure this.

GHC Error Messages

When GHC Haskell error messages are not helpful or confusing it’s best to write down a better alternative and submit it to the GHC issue tracker.

Examples of this:

• Improve error messages when dealing with type level lists
• Improve error message on wrong indentation of let expression inside do block
• Improve error messages of implicit parameter constraints