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Effect Systems in Haskell - Part II
Sanchayan Maity
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Agenda

  • Cover two papers on Effect Systems.
    • Generalized Evidence Passing for Effect Handlers1
    • Effect Handlers in Haskell, Evidently2
  • Some sections today's discussion isn't going to cover
    • Efficiency/Performance of the library or effect system itself
    • How to use effect systems
    • Comparison of effect system libraries or how to choose one

Recap, what's it all about

  • Separate syntax from semantics
  • Interpret your abstract syntax tree in various ways
  • Not losing performance while having both

Recap, why effect systems

  • Monads to model effects but monads don't compose3

  • transformers/mtl has limitations

    • Monad transformer stacks are rigid
    • Doesn't allow handling something like Reader Int (Reader String) due to functional dependencies
    class Monad m => MonadReader r m | m -> r
    • More than a few effects in stack become unwieldy
    • n-square instances problem

Evidence passing based effect system libraries

  • EvEff
  • MpEff
  • speff
  • cleff based on ReaderT IO
  • effectful based on ReaderT IO
  • others?

Some myths4

  • Many extensible effects libraries are implemented with free(r) monads (True)
  • Therefore extensible effects = free(r) monads (False)
  • Free(r) monads require certain mathematical concepts to grasp (True)
  • Free(r) monads don't have very good performance (True, to some extent)
  • Therefore extensible effects are slow, ivory-towerish toys (False)

Effects can be implemented in various way

  • free monads
  • ReaderT IO
  • CPS
  • delimited continuations

What's the gist

  • How do you pass the handler for the effect?

Generalized control flow

  • Languages that expose a yield primitive actually have a way to access delimited continuations! Central result of the paper by James-Sabry 56.

WTH are delimited continuations

  • Delimited Continuations for Everyone7

How does one define an effect

data Reader a e ans = Reader { ask :: !(Op () a e ans)
                             }

data State a e ans = State { get :: !(Op () a e ans)
                           , put :: !(Op a () e ans)
                           }

Multi-prompt delimited control

data Ctl e a = Pure { result :: !a }
             | forall h b e' ans.
               Control {
      -- prompt marker to yield to (in type context `::ans`)
                  marker :: Marker h e' ans,
      -- the final action, just needs the resumption (:: b -> Eff e' ans) to be evaluated.
                op     :: !((b -> Eff e' ans) -> Eff e' ans),
      -- the (partially) build up resumption;
      -- (b -> Eff e a) :~: (b -> Eff e' ans)` by the time
      -- we reach the prompt
                  cont   :: !(b -> Eff e a) }

data Context e where
  CCons :: !(Marker h e' ans) -> !(h e' ans) -> !(ContextT e e')
          -> !(Context e) -> Context (h :* e)
  CNil  :: Context ()

newtype Eff e a = Eff { unEff :: Context e -> Ctl e a }

Key ideas

  • Multi Prompt
  • Evidence Passing
  • Tail Resumptive Operations
  • Bubbling Yields
  • Short cut resumptions
  • Monadic Translation
  • Bind-inlining and Join-Point Sharing

Dig in

  • Dig in to the paper!

Resources

Questions?