A software engineer website

Polysemy: Final vs Embed

Gautier DI FOLCO February 01, 2023 [Haskell] #haskell #polysemy #design #effects systems

In a previous log, we introduce Final as an alternative to Embed.

If we look at the intents:

Each of them come with an interpreter allowing to interpret an effect into other effects of the same type.

Let's start with Embed.

runEmbedded ::
  forall m1 m2 r a.
  Member (Embed m2) r =>
  (forall x. m1 x -> m2 x) ->
  InterpreterFor (Embed m1) r

This is pretty straightforward: for two embedded Monad, we have to provide a function to convert the first one into the second one.

Now, we can have a look at Final' equivalent:

finalToFinal ::
  forall m1 m2 r a.
  Member (Final m2) r =>
  (forall x. m1 x -> m2 x) ->
  (forall x. m2 x -> m1 x) ->
  InterpreterFor Sem (Final m1) r

A bit more complex: we have to provide two functions to go back and forth between the two Monads.

It's actually due to a property highlight in Final's documentation:

Beware: Final actions are interpreted as actions of the final monad, [..] This means that any interpreter built using Final will not respect local/global state semantics based on the order of interpreters run.

See the full the code here and here.