Polysemy: EffectRow design

Previous log introduced EffectRow.

It could be tempting to create a type and apply it everywhere in the codebase as follows:

type AppEffects = '[Trace, Embed IO]

displayFile :: Members AppEffects r => FilePath -> Sem r Int
displayFile path = do
  trace $ "Displaying " <> path
  content <- embed $ readFile path
  embed $ putStr content
  return $ length content

intrDisplayFile :: Sem AppEffects a -> IO a
intrDisplayFile =
  runM
    . traceToStdout

While it sounds to be a way to save time, it:

• Obfuscate expression's type
• Reduce reusability
• Can hide potentila implementation issues

We can argue that specifying by hand, only necessary effect can be cumbersome (especially we you add an effect in a big stack call), it's actually a strong hint that you're probably doing something you should be thoughtful about.

See the full the code here.