Hi, Am Samstag, den 06.12.2014, 12:08 +0100 schrieb Henning Thielemann:
Consider the following example
let (bigs,smalls) = unzip mix in do f bigs g smalls
'bigs' contains a great amount of data, and thus you prefer that it can be garbage collected as 'f' consumes it. If 'unzip' is actually
(fmap fst mix, fmap snd mix)
then 'mix' (and thus all big data) will be kept in memory until 'g' starts processing.
on a first glance, I agree. But I wouldn’t trust my first glance until I investigated the issue. It also depends on the particular implementation of unzip. You assume that after `f bigs` is done, nothing references mix any more. But `smalls` is going to be a thunk referencing mix, so mix will only be GC’ed _while g is forcing smalls_, not earlier. And if `g` is just a lookup in a tree, and `smalls` is used later as well, then the parts of the tree not traversed will still reference `mix`. It might be possible to implement unzip in a way so that smalls gets fully created while bigs is being forced. But that will require careful code and analysis to verify that it works. And even then a partial evaluation of `bigs` might break this expectation. OTOH, the `(fmap fst mix, fmap snd mix)` might be more efficient if only one component will actually be used and/or if `mix` is going to stay live anyways. Who knows. Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org