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Polysemy: Introduction to interpreters

Gautier DI FOLCO November 27, 2022 [Haskell] #haskell #polysemy #design #effects systems

As seen in the previous log we have an expression as follows:

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

In order to do something useful we need to interpret them in term of IO:

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

It can be read that way:

One of the benefit of effects systems is to be able to change the behavior (via the interpretation) without changing the expression.

As such Trace could be:

See the full the code here.