Never touched Haskell before, but found that path separators were needed to get this working (on WSL with Ubuntu if it matters)

For example

    -  capsPath <- getFullPath (T.unpack (coerce dir <> coerce videoName <> ".en.vtt"))
    +  capsPath <- getFullPath (T.unpack (coerce dir <> "/" <> coerce videoName <> ".en.vtt"))

Needed to apply the same change in a few places.

Also found

    --sub-langs en

doesn't work if the videos has for example en-CA as the subtitle language.

And finally, youtube-dlc seems to inconveniently discard the VTT file, breaking glancer, unless you specify `-k` to keep the file.