[Haskell-beginners] optimisation of code

[Oleg Nykolyn]

Hi,

Current code re-checks file existence always in same order, so worst case
is - N files and only last of them does not exists.
In that case this code will re-check (N-1) files during each consecutive
retry.
This can be optimized by moving already existing files to the end of file
list(or dropping them from list completely, if files are only added but
never removed).
For this you could re-write `allFilesThere` something like:
allFilesThere fs = liftIO $ do
  existing, non_existing <- partitionM (doesFileExist . fromAbsFile) fs
  return (non_existing++ existing, null non_existing)

Then allFilesThere could start next iteration by checking previously
non-existing files and probably failing much faster.

On Fri, Sep 21, 2018 at 11:25 AM PICCA Frederic-Emmanuel <
frederic-emmanuel.picca at synchrotron-soleil.fr> wrote:

> Hello,
>
> I would like to have your advice in order to optimize this code.
> The purpose is to trigg an action 'a' if a list of files (thousands)
> exists.
> A process copy files from one directory to another.
>
> allFilesThere :: MonadIO m => [Path Abs File] -> m Bool
> allFilesThere fs = liftIO $ allM (doesFileExist . fromAbsFile) fs
>
> trigOnAllFiles :: MonadIO m => m r -> [Path Abs File] -> m r
> trigOnAllFiles a fs = go
>     where
>       go = do
>         r <- allFilesThere fs
>         if r then a else
>             ( do liftIO $ threadDelay 1000000
>                  go)
>
> It works, but it consums a lot's of resources when all the files does not
> exists yet.
> So I would like your advice in order to optimize it :)
>
> thanks for your help.
>
> Frederic
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