Darren Grant dedgrant at gmail.com
Sat Oct 31 10:40:45 UTC 2015

```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
>
> _______________________________________________