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