Question

I’ve been looking for answer for this question and found this blog post. It says that fail has advantages over error because it isn’t constrained to the IO monad.

Should i use fail instead of error all over my code? Does fail raise Control.Exception.catchable exceptions in IO?

EDIT: I’ve found an update for the link above.

Answer 1

Here’s my advice:

• If you’re in IO code already, use proper exceptions via throwIO from Control.Exception.

• If your code uses a monad stack already, add errors to your monad stack if it doesn’t support it already, and use those.

• If you are in non-monadic code, write total functions. I.e., avoid error and incomplete patterns if you can. Using fail here would only force your code to be unnecessarily monadic. If you need exceptional results, use a proper data type (such as Maybe or Either or a custom datatype).