Hi all,
Thanks for the responses. I want to go through and make sure I understand
these.
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First, Henning, won't both of these allocate in proportion to the size of
the map?
Map.foldrWithKey (\k a -> f k a >>) (return ())
Foldable.sequence_ . Map.mapWithKey f
In particular, will the compiler be able to avoid allocating when building
up that large monadic computation in the foldrWithKey?
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Edward said to use foldMapWithKey, which hasn't been released yet, but it
sounds like it will be.
https://github.com/ekmett/containers/commit/40187f32a43689ff02ca2b97465aa4fc...
Even then, I might argue it is somewhat non-obvious to the programmers how
to use this to do a non-allocating "for-each". For example, I am not
totally clear on what will happen with that tree of mappend calls -- will
it allocate thunks? Further, IO is not a monoid, so am I to create an
instance of "Monoid (IO ())" in order to use foldMapWithKey to iterate over
the Map?
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On Tue, Jul 2, 2013 at 10:29 AM, Shachaf Ben-Kiki
http://hackage.haskell.org/packages/archive/lens/3.9.0.2/doc/html/Control-Le...
*
That function looks more overloaded than the traverse in Data.Map that I'm
referring to, e.g. here:
http://www.haskell.org/ghc/docs/latest/html/libraries/containers/Data-Map-St...
I'm afraid I don't understand the proposal then -- is it to use lens
somehow? For the traversal I need to do over a Data.Map.Map, I need to fix
't' to be IO or Par or whatever I'm working with, so that the (k -> a -> t
b) function I'm passing in can do the effects I need.
To be specific I'm proposing providing these variants:
traverseWithKey :: **Applicative t => (k -> a -> t b) -> Map k a -> t
(Map k b)
traverseWithKey_ :: **Applicative t => (k -> a -> t ()) -> Map k a -> t
()
And without exposing the latter natively, I still don't understand how to
trick the former into not allocating, if that's the proposal.
-Ryan
On Tue, Jul 2, 2013 at 2:54 PM, Edward Kmett
On Tue, Jul 2, 2013 at 2:45 PM, Henning Thielemann < lemming@henning-thielemann.de> wrote:
Foldable.sequence_ . Map.mapWithKey f
which looks more elegant.
This has the unfortunate consequence that it builds an entire new map strictly before sequencing it, due to the fact that Data.Map is spine-strict. =(
-Edward