Fix fromListWithDef divergence
#42
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I'm not quite sure about this approach. The existing implementation is better for finite lists, because Could it be possible to rewrite |
Sounds good, it will take me a little while to figure out. I'll try to finish it by the end of the day. |
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Actually I think I can optimize it a little more. I'll fix it shortly, my apologies. |
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@Bodigrim Ok, it's ready now! |
src/Data/Chimera/Internal.hs
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| Left l -> | ||
| if l == kk | ||
| then (G.replicate kk a, []) | ||
| else (G.fromListN kk (xs ++ replicate l a), []) |
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I'd probably write it as (if l == kk then ... else ..., []), but up to you.
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Ok, I rewrote it that way
fromListWithDef diverges when passed an infinite list, for the following
reasons:
Reason 1:
* fromListWithDef calls Data.Primitive.Array.fromListN (bits + 1).
fromListN requires that the length of the list that is passed to it
is exactly bits + 1. To ensure this, it forces the spine of the list
that is passed to it.
* The list that is passed to fromListN is generated by go0, which
calls go. When xs is infinite, go k xs will be infinite. Therefore,
fromListN will throw an error since go0 yields a list longer than
bits + 1.
Even if we take the first bits + 1 elements of go0 before passing to
fromListN, there is another problem.
Reason 2:
* As fromListN forces the spine, it will compute go 0 xs0, go 1 xs1,
..., go 63 xs63, for some xs0, xs1, ..., xs63.
* This in turn requires computing measureOff (2^0) xs0,
measureOff (2^1) x1, ..., measureOff (2^63) x63, which is
essentially divergent.
To avoid these problems, we
* Add a check in go to break recursion and ensure that the list that
is passed to fromListN has length (bits + 1).
* Rearrange how we compute go, so that measureOff is not forced while
forcing the spine of go.
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I added a test for infinite inputs to |
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Thanks a ton! |
fromListWithDefdiverges when passed an infinite list, for the following reasons:Reason 1:
fromListWithDefcallsData.Primitive.Array.fromListN (bits + 1).fromListNrequires that the length of the list that is passed to it is exactlybits + 1. To ensure this, it forces the spine of the list that is passed to it.fromListNis generated bygo0, which callsgo. Whenxsis infinite,go k xswill be infinite. Therefore,fromListNwill throw an error sincego0yields a list longer thanbits + 1.Even if we take the first
bits + 1elements ofgo0before passing tofromListN, there is another problem.Reason 2:
As
fromListNforces the spine, it will computego 0 xs0,go 1 xs1, ...,go 63 xs63, for somexs0,xs1, ...,xs63.This in turn requires computing
measureOff (2^0) xs0,measureOff (2^1) x1, ...,measureOff (2^63) x63, which is essentially divergent.To avoid these problems, we implementfromListWithDefin terms offromInfinite. Note that the implementation offromInfiniteis similar to the old implementation offromListWithDef, except that it doesn't callmeasureOff, which is the source of the divergence.Updated solution: To avoid these problems, we
Add a check in
goto break recursion and ensure that the list that is passed tofromListNhas lengthbits + 1.Rearrange how we compute
go, so thatmeasureOffis not forced while forcing the spine ofgo.This PR depends on PR #41, which should be merged first.
Fixes issue #39