Purescript - Read a file from disc with the browser API

File.purs

import Control.Monad.Aff (Aff, Canceler(..), makeAff)
import Control.Monad.Eff.Class (liftEff)
import Control.Monad.Eff.Exception (error)
import DOM (DOM)
import DOM.Event.EventTarget (addEventListener, eventListener, removeEventListener)
import DOM.File.FileReader as FR
import DOM.File.Types (Blob, fileReaderToEventTarget)
import DOM.HTML.Event.EventTypes as Events
import Data.Foreign (readString)

-- | Reads the contents of a DOM API `File` from disk, with the assumption that
-- | the contents is plain text.
readAsText ∷ ∀ e. Blob → Aff String
readAsText blob = makeAff \cb → do
  reader ← liftEff FR.fileReader
  let
    et = fileReaderToEventTarget reader
    listener = eventListener \_ → do
      removeEventListener Events.load listener false et
      r ← runExcept ∘ readString <$> FR.result reader
      cb $ lmap (const (error "A problem occurred while reading a file as text")) r
  addEventListener Events.load listener false et
  FR.readAsText blob reader
pure (Canceler \_ → liftEff (removeEventListener Events.load listener false et))