[Haskell-beginners] HUnit - testing for failed pattern match

Xavier Shay xavier-list at rhnh.net
Tue Feb 22 22:34:39 CET 2011

 > Excerpts from Xavier Shay's message of Tue Feb 22 05:59:12 -0500 2011:
 >> Given the following contrived function:
 >>     giveMeThree x
 >>     | x == 3 = True
 >> How do I test with HUnit that 2 is invalid input?
 >>     giveMeThreeTests =
 >>       [ "Likes 3" ~: True ~=? giveMeThree 3
 >>       , "Dislikes 2" ~: ..... -- what?
 >>       ]
 >> I couldn't find anything mentioned in the HUnit user guide. References
 >> to appropriate documentation appreciated.

On 23/02/11 1:31 AM, Edward Z. Yang wrote:
> A partial function will emit an exception, which must be caught from
> IO.  I'm sure HUnit has an appropriate mechanism for handling this,
> but I strongly suggest you redesign your function to be total: instead
> of omitting pattern matches, change it to use Maybe and return Nothing
> in the 'otherwise' case.

In this case, the inputs I am giving to the function are totally invalid 
- they should never occur in a running app. For context, the function 
takes a state, applies a transition, and returns the new state. There 
are certain invalid transitions. In other languages I would throw an 
ArgumentError or similar ... would the Haskell way be to return Nothing 
instead? To me this seems to be complicating my types (Maybe instead of 
a vanilla type) for a benefit that I don't see.

What are the benefits to using Maybe in this case rather than throwing 
an exception?


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