Better exception for non-exhaustive patterns in case

Question

Is there a way to get GHCi to produce better exception messages when it finds at runtime that a call has produced value that does not match the function's pattern matching?

It currently gives the line numbers of the function which produced the non-exhaustive pattern match which though helpful at times does require a round of debugging which at times I feel is doing the same set of things over and over. So before I tried to put together a solution I wanted to see if something else exists.

An exception message that in addition to giving the line numbers shows what kind of call it attempted to make?

Is this even possible?

Answer 1

Try turning on warnings in ghci. This enables the compile time warnings you can get with ghc by passing -W, for example. You can do this several ways:

ghci -fwarn-incomplete-patterns

Or Neil Mitchell describes how he sets this up in his .ghci. Here is the relevant excerpt:

:set -fwarn-incomplete-patterns

You can manually enter this at ghci as well, but it would be a pain to do so each time you start it. Entered this way, it only works for statements entered at the prompt, not for loading files with :l. Instead you can put this comment at the top of the file you want to warn about incomplete patterns:

{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}