gregwebs/FileLocation.hs — pull request #4 by travitch (2012-01-15 19:29:06+00:00)
Title: Variant of err allowing runtime-generated messages

Not sure if you are interested, but this change adds err', which allows messages to be generated
at runtime instead of just compile-time strings.

Usage:

main = do
args <- getArgs
$err' (show args)

I use this frequently in cases branches that should be impossible, but should generate a
descriptive error message (including the offending value) in case I am wrong.