BIO Hierarchy

BIO is a set of typeclasses and algebras for programming in tagless final style using bifunctor effect types with variance.

Key syntactic features:

  1. Ergonomic F summoner that is a single point of entry to all methods in the hierarchy
  2. Import-less syntax. Syntax is automatically available whenever any typeclass from the hierarchy is imported, e.g. immediately after IDE auto-import.

These syntactic features allow you to write in a low ceremony, IDE-friendly and newcomer-friendly style:

import{F, Monad2, Primitives2}

def adder[F[+_, +_]: Monad2: Primitives2](i: Int): F[Nothing, Int] =
   .flatMap(ref => ref.update(_ + i) *> ref.get)

Key semantic features:

  1. Typed error handling with bifunctor effect types
  2. Automatic conversions to equivalent cats.effect instances using import
  3. No ambiguous implicit errors. It’s legal to have both Monad2 and Applicative2 as constraints, despite the fact that Monad2 provides an Applicative2.
  4. Primitive concurrent data structures: Ref, Promise, Semaphore

To use it, add fundamentals-bio library:

libraryDependencies += "io.7mind.izumi" %% "fundamentals-bio" % "1.2.5"

If you’re using Scala 2.12 you must enable -Ypartial-unification and either -Xsource:2.13 or -Xsource:3 for this library to work correctly:

// REQUIRED options for Scala 2.12
scalacOptions += "-Ypartial-unification"
scalacOptions += "-Xsource:2.13" // either this
// scalacOptions += "-Xsource:3" // or this

Most likely you’ll also need to add Kind Projector plugin:

addCompilerPlugin("org.typelevel" % "kind-projector" % "0.13.2" cross CrossVersion.full)


The following graphic shows the current BIO hierarchy.



Auxiliary algebras:



Raw inheritance hierarchy:



Syntax, Implicit Punning

All implicit syntax in BIO is available automatically without wildcard imports with the help of so-called “implicit punning”, as in the following example:


def loop[F[+_, +_]: Monad2]: F[Nothing, Nothing] = {
  val unitEffect: F[Nothing, Unit] = Monad2[F].unit
  unitEffect.flatMap(_ => loop)

Note: a .flatMap method is available on the unitEffect value of an abstract type parameter F, even though we did not import any syntax implicits using a wildcard import.

The flatMap method was added by the implicit punning on the Monad2 name. In short, implicit punning just means that instead of creating a companion object for a type with the same name as the type, we create “companion” implicit conversions with the same name. So that whenever you import the type, you are also always importing the syntax-providing implicit conversions.

This happens to be a great fit for Tagless Final Style, since nearly all TF code will import the names of the used typeclasses.

Implicit Punning for typeclass syntax relieves the programmer from having to manually import syntax implicits in every file in their codebase.

cats-effect compatibilty

Import for shim compatibilty with cats-effect. You can run http4s and all the other cats-effect libraries with just BIO typeclasses, without requiring any cats-effect typeclasses.

Data Types

Ref2, Promise2 and Semaphore2 provide basic concurrent mutable state primitives. They require a Primitives2 capability to create. With Primitiives2[F] in implicit scope, use F.mkRef/F.mkPromise/F.mkSemaphore respectively. (See also example at top of the page)

Free monad, as well as FreeError and FreePanic provide building blocks for DSLs when combined with a DSL describing functor.

Morphism1/2/3 provide unboxed natural transformations for functors with 1,2,3-arity respectively, with Isomorphism1/2/3 modeling two-way transformations.

Misc Capabilities

Clock1/2/3 gives access to current time.

Entropy1/2/3 models random numbers.

UnsafeRun2 allows executing effects (it is required for conversion to cats’ ConcurrentEffect which also allows unsafe running)


distage-example is a full example application written in Tagless Final Style using BIO typeclasses.

You may also find a video walkthrough of using BIO on Youtube by DevInsideYou — Tagless Final with BIO

Removal of trifunctor hierarchy

Since version 1.2.0 the trifunctor hierarchy has been removed, due to the fact that since ZIO version 2.0 they cannot be implemented for ZIO and also because of lack of use.

See details in