Implementation of "until"

[Dan Burton]

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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