[Hat] Thoughts, Int vs Integer

[Thomas Davie]

On 24 Jun 2006, at 20:41, Neil Mitchell wrote:

> Hi,
>
> Often when compiling with Hat I run into the Int vs Integer issue, in
> fact this is pretty much the biggest problem I have getting hat traces
> generated. I was considering if there was any possible change that
> could be performed by Hat to remove this restriction.
>
> The best I came up with makes semantic changes to the program, but
> these changes are "very minimal" at a guess :)
>
> The idea basically revolves around "type Int = Integer" - replace all
> occurances of Int with Integer at the type level, make all functions
> that operate on Int now operate on Integer instead, and all numeric
> literals can now become Integer, since there is no Int.
>
> There are two semantic changes from this which I have thought of:
>
> 1) big_number * 2 will never overflow, which will give a different
> semantics. Personally this sounds like a good idea, and having a
> hat-overflow tool to detect when an Integer goes above 2^31 would
> probably be a useful addition. I don't consider this change to be all
> that bad, since its compatible with the Haskell98 report (I think).
>
> 2) More seriously, type classes will lead to different code being
> invoked. For example while 0 == 0 might have invoked Int equality now
> it will invoke Integer equality. My guess is that if there is a
> program where the type class for Int is not a subset of the behaviour
> of that for Integer, its very weird and confusing and probably buggy.
>
> The changes to the generator that would need to be made are (as far as
> I can tell) as follows:
>
> * Replace all occurances of Int with Integer
> * Make all numeric literals 0 :: Integer
> * On encountering a typeclass if there existing an instance for one of
> Int or Integer, then use that one. If instances exists for both Int
> and Integer then throw away the Int version. For a prototype,
> allow-undecideable-instances & allow-overlapping-instances should
> handle this automatically.
> * Any primitive operations that are performed on Int and not Integer
> would need to be worked around, probably on a case by case basis, but
> this should be a small number.
>
> Comments or thoughts?

The basic problem with this is that the bug may well be that an Int  
is overflowing... You don't want the bug to not manifest it's self in  
the self tracing version.

Bob