Check if a file is empty.

[GregorySchwartz]

isEmpty :: FilePath -> Bool

Replacing "[ -s diff.txt ]" in bash.

[Gabriella439]

So you can do this using a combination of fold, null, and input, like this:

import Prelude hiding (FilePath)
import Turtle

import qualified Control.Foldl.Text

isEmpty :: MonadIO io => FilePath -> io Bool
isEmpty path = fold (input path) Control.Foldl.Text.null

The main reason I suggest combining these three functions is to reduce API proliferation. Otherwise I'd need to create similar utilities for other Folds in the same module for consistency

[PierreR]

Very useful trick, thanks.
I guess to make the compiler happy it should be

isFileEmpty :: MonadIO io => FilePath -> io Bool
isFileEmpty path =
  fold (lineToText <$> input path) Control.Foldl.Text.null

[Gabriella439]

Actually, there may be a simpler solution, which is to use Control.Foldl.null instead of Control.Foldl.Text.null (in other words, to check if the file has 0 lines instead of checking that it has 0 characters)

[PierreR]

Control.Foldl.null does seem to work nicely for me. Nice.