How to write a function that performs a check action with one effect es1, and then returns a stream with other effects es2?

[jerbaroo]

Here is a code example, starting with definition of effects used:

data FileSystemE :: Effect where
  FileExists :: FilePath -> FileSystemE m Bool
  ReadFile   :: FilePath -> FileSystemE m (Stream m Word8)

fileExists :: FileSystemE :> es => FilePath -> Eff es Bool
fileExists = Eff.send . FileExists

readFile :: FileSystemE :> es => FilePath -> Eff es (Stream (Eff es) Word8)
readFile = Eff.send . ReadFile

Secondly here is a usage example. As you can see it's a bit messy:

newtype DoesNotExistError = DoesNotExistError FilePath deriving Show

-- | First check if file exists, then read byte stream from the file.
readFileCheck
  :: (Error DoesNotExistError :> es1, FileSystemE :> es1, FileSystemE :> es2)
  => FilePath
  -> Eff es1 (Eff es2 (Stream (Eff es2) Word8))
readFileCheck path = fileExists path >>= \case
  False -> throwError $ FileNotExistsError path
  True  -> pure $ readFile path

[arybczak]

You can try to modify the constructor like that:

ReadFile   :: Error DoesNotExistError :> es => FilePath -> FileSystemE (Eff es) (Stream (Eff es) Word8)

[jerbaroo]

The following ensures an effect FileSystemE supports reading a stream of bytes from a file, with an additional Error effect inside the stream.

newtype ReadFileError = ReadFileIOException IOException deriving Show

data FileSystemE :: Effect where
  ReadFile :: FilePath -> FileSystemE (Eff es) (Stream (Eff (Error ReadFileError : es)) Word8)
  
readFile :: (FileSystemE :> es) => FilePath -> Eff es (Stream (Eff (Error ReadFileError : es)) Word8)
readFile = Eff.send . ReadFile

runFileSystemIO = Eff.interpret \env -> \case
  (ReadFile path) -> Eff.localSeqLift env \lift -> pure
    $ onStreamException (throwError . ReadFileIOException)
    $ Stream.morphInner (Eff.raise . lift) $ Streamly.File.read path