Besides, the aforementioned IOSpec, I would also suggest taking a look at "Purify Code Using Free Monads" [0], which gives a nice introduction to a general approach. If you're faced with library functions (i.e., ones you haven't written yourself) that do IO, it can become a little onerous to mock them all, though. In case the IO you're interested in testing is mostly stdin/stderr/stdout related, you could also look at the 'silently' package [1]. Finally, the 'knob' package [2] allows you to use in-memory file handles, so it helps if your tack ends up being passing Handles to pure functions (so that in the executable, those handles are stdin/stderr/stdout + files etc, but in the test they're just in-memory handles provided by 'knob'). 'System.Environment' has 'withArgs', which provides for the case of testing command-line args.

[0] http://www.haskellforall.com/2012/07/purify-code-using-free-monads.html
[1] https://hackage.haskell.org/package/silently-1.2.4.1
[2] http://hackage.haskell.org/package/knob-0.1.1/docs/Data-Knob.html

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