Proposal: plug the space leak in transpose

[David Feuer]

There's no use of head or tail functions. Two list comprehensions
equivalent to two applications of mapMaybe, rather than one of mapMaybe and
a second of unzip. The whole situation is rather sad; a generalization of
the selector thunk trick could in principle plug the leak without hurting
performance, but that will require various GHC changes.

On Mon, Aug 24, 2020, 1:25 AM Henning Thielemann <
lemming at henning-thielemann.de> wrote:

>
> On Sun, 23 Aug 2020, David Feuer wrote:
>
> > Data.List.transpose, unfortunately, can potentially leak space. The
> > problem is that it walks a list of lists twice: once to get the heads
> > and once to get the tails. Depending on the way the result is consumed,
> > it's possible that heads or tails that are never used will be retained
> > by the garbage collector. I have a fix[*] that probably makes the
> > function slower in typical cases, but that plugs the leak. What do y'all
> > say?
>
> Your way sounds more correct than using 'head' and 'tail'. I have no
> numbers, though.
>
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