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Polysemy: EffectRow

Gautier DI FOLCO November 30, 2022 [Haskell] #haskell #polysemy #design #effects systems

As seen in the previous log we ended up with:

import Polysemy
import Polysemy.Trace

displayFile :: FilePath -> Sem '[Trace, Embed IO] Int
displayFile path = do
  trace $ "Displaying " <> path
  content <- embed $ readFile path
  embed $ putStr content
  return $ length content

intrDisplayFile :: Sem '[Trace, Embed IO] a -> IO a
intrDisplayFile =
  runM
    . traceToStdout

As you can see '[Trace, Embed IO] is repeated twice:

It's actually an EffectRow which represents a type-level list of Effect.

This duplication looks nice but would be impractical.

For example, switching order in the expression:

displayFile :: FilePath -> Sem '[Embed IO, Trace] Int

Would create an error:

src/Intro.hs:24:36-63: error:
    • Couldn't match type: Embed IO
                     with: Trace
      Expected: Sem '[Trace, Embed IO] Int
        Actual: Sem '[Embed IO, Trace] Int
    • In the first argument of ‘intrDisplayFile’, namely
        ‘(displayFile "/tmp/hello.txt")’
      In the first argument of ‘(>>=)’, namely
        ‘intrDisplayFile (displayFile "/tmp/hello.txt")’
      In the expression:
        intrDisplayFile (displayFile "/tmp/hello.txt") >>= print
   |
24 | mainDisplayFile = intrDisplayFile (displayFile "/tmp/hello.txt") >>= print
   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We would have the same issue calling it from another function:

displayFile' :: FilePath -> Sem '[Embed IO, Trace] Int
displayFile' = displayFile
src/Intro.hs:24:16-26: error:
    • Couldn't match type: Trace
                     with: Embed IO
      Expected: FilePath -> Sem '[Embed IO, Trace] Int
        Actual: FilePath -> Sem '[Trace, Embed IO] Int
    • In the expression: displayFile
      In an equation for ‘displayFile'’: displayFile' = displayFile
   |
24 | displayFile' = displayFile
   | 

Actually '[Trace, Embed IO] is a concrete EffectRow.

There is also a constrained EffectRow, which is defined like this:

displayFile :: Members '[Trace, Embed IO] r => FilePath -> Sem r Int
displayFile path = do
  trace $ "Displaying " <> path
  content <- embed $ readFile path
  embed $ putStr content
  return $ length content

we can also define them one by one:

displayFile' :: (Member Trace r, Member (Embed IO) r) => FilePath -> Sem r Int
displayFile' = displayFile

in these example, function definitonsare unchanged.

However the EffectRow is now polymorphic and has constraints attached to it.

Now we are able to call any function with other EffectRow (as long as it fits the constraints).

While concrete EffectRow is mandatory at interpretation time, during expression building it's limiting.

See the full the code here.