On Fri, Mar 27, 2009 at 07:44:05PM +0000, Ivan Moore wrote:
Many thanks for the fantastic answers.
I have a question related to your answer
The reason (which is a bit confusing) is that it typechecks just fine---if there *were* a type which is an instance of both Integral and Floating (and I guess round needs RealFrac as well), n could have that type. There isn't such a type in the standard libraries, but in theory you could make up your own type which is an instance of both.
If there were such a type, could "10" have that type and then would my problem have not existed? (in which case, why doesn't it!?) (an answer of - "ask again when you've used the language a bit more" would be perfectly fine if it requires a lot more understanding of the language to understand the answer than a newbie like me has - I'm just curious)
If there were such a type, and if it were one of the types to which ghci was allowed to default, then you would not have had a problem. But if you look at the methods of the Integral and Floating classes, you will see that such a type would likely be silly. You can't really have a type which is simultaneously Integral (i.e. whole numbers, with no fractional part) and Floating (i.e. floating point numbers which can be divided, square rooted, etc.). To have a type be an instance of both would require a radical reinterpretation of what these type classes mean. -Brent