Marcin 'Qrczak' Kowalczyk wrote:
[snip]
> I am worried if the following scenario is possible: There are
> two modules. Everything is OK. Now one of them adds an innocent
> instance. It is perfectly correct in its context, but it happens that
> it triggers an ambiguity in the second module. Still neither module
> itself is at fault.
[snip]
> I don't have a concrete case in mind...
[snip]
I do though. I mentioned it before. I don't see how you can get a worse example in
principle than this.
Module 1:
class Integral number where
[ operations on numbers ]
instance Integral number => Show number where
[ extract digits in obvious way ]
Module 2:
class String string where
[ operations on strings ]
class String string => Show string where
[ extract canonical ASCII string representation ]
As a matter of fact GHC would not have a problem with these modules
(unless perhaps someone imports both of them - I don't know if it checks that).
In any case I would rather it didn't have a problem with them. I suspect some of
my code already relies on it not having a problem.
Now suppose J. Random User comes along three years later and adds something that's
an instance of both Integral and String. S/he tries to show this hybrid.
Then we DO have a problem. But not until then. I personally think it's quite
alright to print out a message at this stage along the lines of "I'm sorry, I can't
work out which instance of Show to use, because it could come from Integral (see
Module 1)
or from String (see Module 2)."
Compare the case when Modules 1 & 2 give the same symbol different definitions.
Haskell
doesn't have a problem. It doesn't even have a problem if you import the two modules
in
the same file. It only has a problem if you try and use that symbol in that file.
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