3

Assume we have a file my_file.txt with contents:

foo
bar

and another file my_other_file.txt containing:

baz

I would like to read the contents of these two files using turtle so that I get a Shell of lines which will produce:

foo
bar
baz

In Haskell's turtle library one can read a list of files by using input, for instance:

view $ input "my_file.txt"

We have that 

input :: FilePath -> Shell Line

And Shell has no Monoid instances (which I think makes sense since we cannot associate IO operations), so the only operator I can think of using is (<|>):

view $ foldl (<|>) empty $ map input ["my_file.txt", "my_other_file.txt"]

While this produces the desired effect, I wonder whether there is a library in the turtle eco-system that takes care of this, or whether there is a traverse like operation that can be use on Alternative's.

EDIT: the effect above could be also achieved by using asum:

asum $ input <$> ["my_file.txt", "my_other_file.txt"]
Damian Nadales
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1 Answers1

3

Line has a Monoid instance. If we have a list of Lines, we can mconcat them into a single one:

do
  exampleA <- input "my_file.txt"
  exampleB <- input "my_other_file.txt"
  return $ mconcat [exampleA, exampleB]

Since Shell has an Applicative instance, we can use traverse to use input over a list of files:

traverse input ["my_file.txt","my_other_file.txt"]

We end up with a Shell [Line]. Since Shell is a Functor, we can fmap mconcat (or fold if you don't use a list):

mconcat <$> traverse input ["my_file.txt","my_other_file.txt"]
Zeta
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  • I've tried that but it does not seems to work. `view $ traverse input ["my_file.txt","my_other_file.txt"]` yields `[Line "foo",Line "baz"] [Line "bar",Line "baz"]`, so the overall result when applying `mconcat` will be `Line "foobaz" Line "barbaz"`. Or am I missing something? – Damian Nadales Oct 07 '17 at 23:08