Implementation of "until"

[David Feuer]

This is not more accurate. until forces its third argument iff p is strict,
but it's always strict in its third argument, because

until p f _|_ = _|_

for all p and f.

There are two potential reasons to prefer a version that forces its third
argument immediately:

1. Exceptions are usually better than infinite, unproductive loops.
2. It seems conceivable that having more precise strictess information will
help GHC. I don't know if it ever will in this case or not.

On Sun, Nov 9, 2014 at 2:47 PM, Dan Burton <danburton.email at gmail.com>
wrote:

> It's more accurate to say that "until" is *as lazy as* its predicate
> argument.
>
> -- infinite loop, slow space leak
> until ((== 10) . (!! 3)) [1, 2, 3, 4] (\xs -> xs ++ xs)
> -- first-iteration errors out due to strictness of predicate
> until ((== 10) . (!! 3)) [1,2,3,undefined] (\xs -> xs ++ xs)
>
> Your proposed changes introduce arbitrary strictness which I don't see any
> particular benefit to. Is there a specific example you had in mind where
> the behavior is better?
>
> -- Dan Burton
>
> On Sun, Nov 9, 2014 at 11:14 AM, David Feuer <david.feuer at gmail.com>
> wrote:
>
>> I got to looking at the `until` function (in GHC.Base):
>>
>> -- | @'until' p f@ yields the result of applying @f@ until @p@ holds.
>> until                   :: (a -> Bool) -> (a -> a) -> a -> a
>> until p f = go
>>   where
>>     go x | p x          = x
>>          | otherwise    = go (f x)
>>
>> This function doesn't immediately *appear* to be strict in its third
>> argument (`x`), but it actually is:
>>
>> 1. If `p` is lazy, then either it's `const True`, so `until p f x = x`,
>> or it's `const False`, so `until p f x = _|_`. In the first case, it's
>> certainly strict in `x`, and in the second case, it goes into an infinite
>> loop, so it's trivially strict in `x`.
>>
>> 2. If `p` is not lazy, then `until` is obviously strict in `x`.
>>
>> I wondered, therefore, whether there might be some benefit, for
>> strictness analysis, to redefining `until`, either like this:
>>
>> until p f = \ !y -> go y
>>   where
>>     go x | p x          = x
>>          | otherwise    = go (f x)
>>
>> or (probably not) like this:
>>
>> until p f = go
>>   where
>>     go !x | p x          = x
>>           | otherwise    = go (f x)
>>
>> The only *semantic* change here is that it can, potentially, replace an
>> infinite loop with an exception. GHC definitely analyses the strictness
>> differently; what I can't tell is whether this will actually help it
>> produce better code in some cases. Specifically, the current `until`
>> implementation gets
>>
>>  Str=DmdType <C(S),C(U)><L,C(U)><L,U>,
>>
>> whereas the modified ones get
>>
>>  Str=DmdType <C(S),C(U)><L,C(U)><S,1*U>,
>>
>> I don't actually know how to read these, but I can see they're different
>> in the third argument. The two alternatives are also analyzed differently,
>> with the `go` function getting different strictness info. I would
>> speculate, however, that the one that modifies `go` could lead to
>> double-forcing in the loop in some cases, which would presumably be bad.
>>
>> David
>>
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>
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