### Re: suggestion: use lazy pattern matching for Monoid instances of tuples

Petr Pudlák <petr.mvd <at> gmail.com>

2013-08-18 18:26:23 GMT

Sat 17 Aug 2013 10:40:50 PM CEST, Henning Thielemann napsal:
>
> Am 17.08.2013 22:31, schrieb Petr Pudlák:
>>
>> Dear haskellers,
>>
>> currently the instances are defined as
>>
>> |instance (Monoid a,Monoid b) =>Monoid (a,b)where
>> mempty = (mempty, mempty)
>> (a1,b1) `mappend` (a2,b2) = (a1 `mappend` a2, b1 `mappend` b2)|
>
>
> I think that this instance is correct, since it must hold
>
> forall x. mappend mempty x = x
>
> With your instance you get for x=undefined:
>
> mappend mempty undefined = (undefined, undefined)
>
>
> However, the "instance Monoid ()" is too lazy, such that it does not
> satisfy the identity law.
>
I have some doubts in this reasoning. Here `undefined` isn't a value we
can compare, it represents a non-terminating program. How can we compare
two non-terminating programs for equality? I'd say we can only say that
two non-terminating programs are unequal, if they produce a partial
results that are different. For example, `(undefined, 1)` is unequal
`(undefined, 2)`. But how can we observe that `undefined` is distinct
from `(undefined, undefined)`? We can't write a correctly terminating
program that would observe this inequality (of course, unless we use IO
and exception catching).
The only thing this additional laziness can change is to make some
non-terminating programs terminating. This is very different than from
having an instance that fails an equality law with defined values, in
such a case we'd be able to observe some programs behave incorrectly.
I'd say that with `undefined` we can fail other equality laws. For
example: For `MonadPlus` we have `m >>= (\x -> mzero) == mzero`. But it
clearly fails in
```haskell
undefined >>= (\x -> mzero) :: Maybe Int
```
The result is `undefined` instead of `mzero`. (Or does it mean that
Maybe is broken?)
Best regards,
Petr
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