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Polysemy: EffectRow design

Gautier DI FOLCO December 04, 2022 [Haskell] #haskell #polysemy #design #effects systems

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 =
    . traceToStdout

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

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.