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Polysemy: Interpretation and effects injection

Gautier DI FOLCO December 14, 2022 [Haskell] #haskell #polysemy #design #effects systems

As seen previously we can have interpreters which relies on other interpreters.

While you could simply add the required effects to the list, you may want to hide it (eg. to have dynamique interpreters).

In order to do it, you have many raise* functions.

Let's have a look at View:

data View v m a where
  See :: View v m v

viewToState :: Member (State s) r => (s -> Sem r v) -> InterpreterFor (View v) r

View gives a way to cache expensive computations dynamically.

Let's imagine a simple app:

app :: Members '[Embed IO, View String] r => Sem r ()
app = do
  name0 <- see
  embed $ putStrLn $ "Your name is " <> name0
  name1 <- see
  embed $ putStrLn $ "Hello " <> name1

we can leverage raiseUnder which injects an effect as follows:

intrApp :: Sem '[View String, Embed IO] a -> IO a
intrApp =
    . evalState ()
    . viewToState (\() -> embed $ putStrLn "What's your name?" >> getLine)
    . raiseUnder @(State ())

Here raiseUnder will change effects to '[View String, State (), Embed IO]

See how it runs:

What's your name?
Your name is Gautier
Hello Gautier

Note: actually viewToState is more general and invalidate the cache once the state changes

See the full the code here.