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Fix fromListWithDef divergence #42

Merged
merged 2 commits into from
Apr 12, 2024
Merged

Fix fromListWithDef divergence #42

merged 2 commits into from
Apr 12, 2024

Commits on Apr 12, 2024

  1. Add test for fromListWithDef with infinite input

    @pgujjula
    pgujjula committed Apr 12, 2024
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  2. #39 Fix fromListWithDef divergence 

    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.
    @pgujjula
    pgujjula committed Apr 12, 2024
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