On Wed, 2005-05-04 at 17:18 -0700, Greg Buchholz wrote:
Here's a little quirk I ran into recently. While making a little vector data type in class Num (code below), I didn't implement an instance of "fromInteger" (thinking I didn't need it). Well as you can probably guess, it turns out I did need it, and subsequently got a run time exception. Which surprised me a little, since it seems like it could have been caught at compile time. (Although I did ignore a warning). This has probably been answered a thousand times, but could someone point me in the direction of documentation explaining why it compiled?
Thanks,
Greg Buchholz
Hi Greg, Perhaps this section of the report might help:
From Section "4.3.2 Instance Declarations" in the Haskell Report:
http://www.haskell.org/onlinereport/decls.html#instance-decls "If no binding is given for some class method then the corresponding default class method in the class declaration is used (if present); if such a default does not exist then the class method of this instance is bound to undefined and no compile-time error results." Cheers, Bernie.