Neil, Does the following sum up the situation? The class Num has subclasses containing various numeric types and the literal 1 is a value for one or more of those types. Hence the Haskell compiler says the instance 1) is OK. But at run time, without the quantified (1:Int), the 1 could of more than one type, which causes a problem. Thanks for the quick and informative response, Pat Neil Brown wrote:
On 01/07/10 12:37, Patrick Browne wrote:
Why do some cases such as 1) fail to run even if they are the only instantiation.
-- 1) Compiles but does not run instance LocatedAt Int String where spatialLocation(1)="home"
That instance is fine. I presume the problem is that you are trying to run this function using "spatialLocation 1" as the function call. The problem with that is that the "1" part has type Num a => a, i.e. it can be any Num type. Even though you only have one instance, that's not used to constrain the type for "1". The call "spatialLocation (1::Int)" works correctly. Looking at your other examples, all the ones that don't work have a numeric type for the parameter, so I suspect it is the same issue throughout.
Thanks,
Neil.
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