Ord methods are surprisingly strict

[Zemyla]

I figured the biggest wins on lazy <= et al would come from ADTs with two
constructors with at least one nullary. Bool, Maybe, and [] all qualify.

And generally, with operations that can potentially be lazy in either
argument, the convention is that it's in the first argument.

Also, there is at least one compelling reason for the comparison operators
to be lazy. Take the infinite search monad:

newtype Search r a = Search { find :: (a -> r) -> a }

It has an Alt instance for finding the better of two results with a given
valuation function:

instance Ord r => Alt (Search r) where
  Search ma <!> Search mb = Search $ \p -> let
    a = ma p
    b = mb p
    in if p a >= p b then a else b

For Search Bool, which uses a predicate as its valuation function, this
means that, if p a is True, it doesn't need to evaluate b or p b at all.


On Thu, May 7, 2020, 14:00 David Feuer <david.feuer at gmail.com> wrote:

> Even aside from the memory leak potential and the performance degradation
> from allocation, adding extra tests will degrade performance in typical
> cases for many types. Comparing against minBound/maxBound isn't always
> free. For a simple Int comparison, that's an extra compare and conditional
> jump every time. For such a fundamental operation, that's a big ouch!
>
> On Thu, May 7, 2020, 2:53 PM Christopher Allen <cma at bitemyapp.com> wrote:
>
>> If one of my applications develops a memory leak because of this
>> potential change, where should I invoice you for the time wasted tracking
>> down & fixing it? My short-term contract rate is $250/hour but I could be
>> argued down to $200/hour.
>>
>> On May 7, 2020, at 1:00 PM, Sven Panne <svenpanne at gmail.com> wrote:
>>
>> Am Do., 7. Mai 2020 um 15:26 Uhr schrieb David Feuer <
>> david.feuer at gmail.com>:
>>
>>> I believe this is all about strictness analysis. If these were lazy,
>>> then users would have to be very careful to force the lazy arguments when
>>> they don't need that laziness to avoid building unnecessary thunks.
>>>
>>
>> This argument holds basically for every potentially lazy function, and I
>> fail to see why comparisons should be special, so this is not really
>> convincing. :-) We have easy ways to make things more strict, but not the
>> other way around.
>>
>> In any case, it has historically been the case that the reference
>> implementations in the Haskell language/library report define the
>> strictness of the defined functions, too. Otherwise things could be very
>> surprising, in both ways (too lazy, too strict). Furthermore, the report is
>> *very* explicit about the derived instances:
>> https://www.haskell.org/onlinereport/haskell2010/haskellch11.html#x18-18300011.1 And
>> (), Bool, ... are defined via deriving:
>> https://www.haskell.org/onlinereport/haskell2010/haskellch9.html#x16-1710009
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