Steve's Blog – Finance, Trading and Technology

This past weekend I had reason to use conduit to process the contents of a large number of files as a single stream. Fortunately conduit supplies a Monoid instance. Monoid allows us  to append things together. This example is very short but very useful. This little code block creates many sources and collapses them together into a single source that will be ordered by the file name. Quite cool.

let source = foldl1 (<>) $ map sourceFile $ sort files
runResourceT $ do
  source $$
    someConduit chain ...

Thanks to drb226 for his suggestions.

Update:

So obviously there are other ways of writing this. Tekmo suggested using the >> binad and discard operator which would make our code look like:

let source = foldl1 (<>) $ map sourceFile $ sort files

Sjoerd Visscher suggested using foldMap from Data.Foldable. So that gives us:

let source = foldMap sourceFile $ sort files

Thanks for the feedback.