Overview
The main types of Turbolift, and their roles
Computation
 Monad, parametrized by set of effects, a.k.a “One Monad To Rule Them All” ^{1}.Signature
 Trait, where we define our Algebra/Service/DSL (as abstract methods).Effect
 Object, through which we can invoke operations of some Algebra/Service/DSL (as concrete methods).Interpreter
 Object that assigns semantics to some Algebra/Service/DSL. It produces aHandler
.Handler
 Object that we can use like generalizedtry ... catch
expression: to delimit scope of effect(s).
Computation Usage
A value of type Computation[+A, U]
describes a… computation,
that requests a set of effects U
, that need to be handled,
before it can return a value of type A
.
A type alias !![A, U]
is defined for convenient infix syntax.
The typelevel set of effects is modelled by intersection types:
Scala type  Meaning as a set of effects  Sample computation type  Same, using infix syntax 

Any 
∅  Computation[Int, Any] 
Int !! Any 
X 
X 
Computation[Int, X] 
Int !! X 
X & Y 
X ∪ Y 
Computation[Int, X & Y] 
Int !! (X & Y) 
Usually, Scala compiler can infer the set of effects requested by the computation.
We can see this in code example on the front page.
The inferred type of program
indicates,
that it requests 3 effects: MyReader
, MyState
and MyError
.
Additionally, !!
is a value alias of Computation
’s companion object:
import turbolift.!!
val myComputation1 = !!.unit
// myComputation1: Computation[Unit, Any] = turbolift.Computation@4c07a028
val myComputation2 = !!.pure(42)
// myComputation2: Computation[Int, Any] = turbolift.Computation@343cd48
For more information, see Computation API.
Effect Usage
To be able to invoke the effect’s operations, we need access to an instance of the effect.
We can create such instance ourselves:
// State inherits from Effect:
import turbolift.effects.State
// Instantiation:
case object MyState extends State[Int]
// Invoking operation:
val computation = MyState.put(42)
// computation: Computation[Unit, MyState] = turbolift.Computation@2c1eafca
For more details, see Defining your own effects & handlers and Effect labelling.
Handler Usage
Application of a handler delimits scope of effect(s). It also transforms type of the computation. In the simplest case, one of effects requested by the computation is eliminated.
val myComputation2 = myComputation1.handleWith(myHandler)
As soon as all effects are eliminated, the computation’s result can be obtained, using run
:
val result = myComputation
.handleWith(myHandler1)
.handleWith(myHandler2)
.handleWith(myHandler3)
.run
…or using unsafeRun
, if the only effect remaining unhandled, is IO
.
In general, a handler of type Handler[F[_], G[_], L, N]
represents a
polymorphic function,
that transforms computations:
∀ A, M. Computation[F[A], M ∪ L] => Computation[G[A], M ∪ N]
// Where `F[_]` is either typelevel identity, or constant function.
Meaning, that application of it, does the following:
 It eLiminates set of effects
L
from incoming computation.  It iNtroduces set of effects
N
into outgoing computation (revealing dependencies of the handler, if there are any).  It passes aMbient set of effects
M
unaffected, from incoming to outgoing computation.  It applies type constructor
G[_]
toA
.  It constraints type returned by the incoming computation to be equal
F[A]
.
In the example below, myHandler
eliminates single MyChoice
effect, introduces no effects,
accepts any type of computation (identity), and wraps the result type in Vector[_]
.
import turbolift.Handler
import turbolift.effects.Choice
case object MyChoice extends Choice
type MyChoice = MyChoice.type
val myHandler: Handler[[X] =>> X, Vector, MyChoice, Any] = MyChoice.handler
Handlers can be transformed or composed in various ways.
For example, this sequences 3 independent handlers:
val myHandler123 = myHandler1 &&&! myHandler2 &&&! myHandler3
For more operations, see Handler API.
Signature & Interpreter Usage
Those 2 types are used only during Defining your own effects & handlers.

Slogan coined by Eric Torreborre. ↩