The Eff monad, one monad to rule them all

Eric Torreborre

The Eff monad

One monad to rule them all!

It must be evident how multifarious and how mutually complicated are the considerations which the working of such an engine involve. There are frequently several distinct sets of effects going on simultaneously; all in a manner independent of each other, and yet to a greater or less degree exercising a mutual influence.

Ada Lovelace,

1842

You could have invented monads!

Exceptions?

Exceptions?

State?

State?

Genius!

Can we have both state and error?

First idea: stack them!

1996!

Which order?

DOES NOT WORK!

THIS ONE WORKS!

DIFFERENCE?

DIFFERENCE?

DIFFERENCE?

ORDER IS

IMPORTANT!

GENERALISE

STACKING HAS A COST!

SIMILAR EFFECTS?

IN PRACTICE

Specialized stacks

Specialized functions

WHAT IF?

We need to use both Hadoop and S3?

WHAT ELSE?

WHAT ELSE?

Type-level

effects

Eff monadValue-level

effects

ContinuationsInterprete

rs

Member typeclas

s

Effect

Type-level effects

Value-level effects

Value-level effect

Continuation

s

Option Interpreter

State Interpreter

Continuation1

Continuation2

Option Interpreter Continuation3 +

State Interpreter

OPTION RELOADED

KLEISLI

OPTION AS EFFECT

EFF

ERROR AS EFFECT

NEVER RE-IMPLEMENT flatMap

EVER AGAIN!

EFFECTS UNITE!

Type-level effects

Value-level effects

Value-level effects

Member typeclass

LIBRARY

scalaz.Future

STACKOVERFLOW:-(

STACK SAFE

TRANSFORM EFFECTS

SO, SILVER BULLET?

SORRYNO

Implicits

ImplicitsUse it in prod'

RenamingsCheck laws

Missing functionsBenchmarking, Scalajs...

CREDITS

LINKSMonads for functional programmingMonad transformers and modular interpreters

Extensible effectsReflection without remorseFreer monad, more extensible effects

THANKS!

Matchers

Matchers

Matchers

Matchers

MORE TROUBLE

List(5, 7, 1)

TOO BIG?

ONE MORE ERROR LAYER!