[Haskell-cafe] Extending the idea of a general Num to other types?
ketil at ii.uib.no
Wed Sep 5 06:47:06 EDT 2007
On Wed, 2007-09-05 at 09:56 +0100, Simon Peyton-Jones wrote:
> Is your suggestion specific to String?
> then I really might have intended to use Complex as a Num type
IME this is much rarer, and I think if a newbie is told that Complex is
not (but needs to be) and instance of Num, it is relatively easy to find
the relevant information (Looking up 'instance' and/or 'class' in the
index of any Haskell text book should do the trick)
> | rephrase to something like "String is not an instance of Num"? For a
> | newbie, it may not be clear that Num is the class and String is the
> | type.
> Good point. Not so easy for multi-parameter type classes! E.g. No instance for (Bar String Int). So we could have
> String is not an instance of class Foo -- single param
> No instance for (Bar String Int) -- multi-param
If you quote things, you can also consider:
'String Int' is not an instance of class 'Bar'.
Downside is that 'String Int' by itself may be confusingly unhaskellish.
More information about the Haskell-Cafe