Sat Oct 31 14:09:00 UTC 2015

```I think you misinterpreted that answer. As far as I'm aware, `error`
(except in its type) is no less reasonable than other exceptions, and
you can catch it in IO. So as long as you run a test in IO, you can
test for the call to `error` (as Joachim showed in another reply).

Regards,

Erik

On 31 October 2015 at 11:40, Darren Grant <dedgrant at gmail.com> wrote:
> Unfrotunately the answer to this is not simple:
>
> http://stackoverflow.com/questions/4243117/how-to-catch-and-ignore-a-call-to-the-error-function
>
> 'error' more or less terminates the program in an unreasonable way.
>
> It would be preferable for f2 to result in a type that can contain the error
> result to be parsed.
>
> Cheers,
> Darren
>
> On Oct 31, 2015 21:16, "Roelof Wobben" <r.wobben at home.nl> wrote:
>>
>> Hello,
>>
>> I have made this exercise which can be found at the Craft of Functional
>> Programming  book.
>>
>> -- exercise 32
>>
>> -- Suppose we have to raise 2 to the power n. If n is even, 2*m say, then
>> -- 2n = 22*m = (2m)2
>> -- If n is odd, 2*m+l say, then
>> -- 2n = 22*m+l = (2n)2*2
>> -- Give a recursive function to compute 2n which uses these insights.
>>
>> f2 :: Integer -> Integer
>> f2 n
>>   | n < 0   = error "This will only run for positive numbers"
>>   | n == 0  = 1
>>   | even n = f2 ( n `div` 2) ^ 2
>>   | odd n  = (f2 ( n `div` 2) ^ 2) * 2
>>
>>
>> Now I have to make Hunit tests for it,
>>
>> But is there a way I can test if the error message is shown when a
>> negative number is being used ?
>>
>> Roelof
>>
>> _______________________________________________