On Wed, 13 Nov 2002, Jan Kort wrote:
Juan Ignacio Garcia Garcia wrote:
*P2> (fromRational ((toRational 4) - ( toRational 5.2 ))) -1.2000000000000002
I can't explain this one, how would fromRational know that it has to create a Double ?
It's the defaulting mechanism that kicks in. The default default is (Integer,Double). Since Integer is not an instance of Fractional, Double is chosen. But this brings up a strange thing in GHCi. Suppose I load the following module into GHCi: \begin{code} module Foo where kalle = (fromRational ((toRational 4) - ( toRational 5.2 ))) default (Rational) \end{code} What happens is the following: Prelude> :l Foo.hs Compiling Foo ( Foo.hs, interpreted ) Ok, modules loaded: Foo. *Foo> kalle (-6) % 5 *Foo> (fromRational ((toRational 4) - ( toRational 5.2 ))) -1.2000000000000002 It seems like GHCi doesn't care about the default directive on the promt. I tried to look in the documentation but I found nothing about defaulting. I believe it is reasonable to expect that the defaulting of the current module also affects the promts (I realise however that this can lead to problems when several modules are in scope..). Other things one might want to have is to be able to see what the current defaulting is and perhaps set it manually from the prompt. At the very least, please document how GHCi handles defaulting. I know it's not the most frequently used feature but it's there and needs to be taken care of. All the best, /Josef