Scala’s Implicits

Pablo Francisco Pérez Hidalgo (@pfcoperez)github.com/pfcoperez

Scala’s Implicits Implicit Values

Pablo Francisco Pérez Hidalgo (@pfcoperez)github.com/pfcoperez

Scala 101

Programming 101: Formal vs Actual parameters

Scala 101: Lists of parameters

Scala 101: Methods are not functions

… but they are kind of cousins.

Know your implicit things

The compiler’s toy box: Implicit parameters Just for methods

The compiler’s toy box: Implicit parameters Just for methods

The compiler’s toy box: Implicit parameters Just for methods

The compiler’s toy box: Implicit conversions

Conversion:Domain = R

-10.0, 1.3, 9.2, ….

Conversion, I am your SOURCE TYPE

And, I am your TARGET TYPE

Implicit conversions: A silly example

Facts

Actual implicit parameters are values.

Formal implicit parameters are all those in the implicit parameter list.

Only methods can have - a single - implicit parameter list.

Functions are values.Guinea Pigs are pets, not food.

Implicit conversions are specially tagged functions, that is values.

Methods are not functions but can be assigned to function values thus becoming functions so they can become implicit conversions.

Know your implicit context

The implicit context

Compound Noun (singular)The place from where the magic unicorns come...

The implicit contexterror: ambiguous implicit values: both value c of type => C and value d of type => C

WHY?! WHAT?!Where did that implicit value came from???!!!

I DONT KNOW MORTY! BUT… RUN! RUN! RUN!

The implicit context is, once tamed, is... MOSTLY HARMLESSImplicit Context (n)The implicit context at a given point of your source code is the set of implicit values available for the compiler to perform type conversions or complete function calls when that point of code is providing an unexpected type object or missing some parameter in a function call.

That is… a mere backpack full of values for the compiler to use.

Identifying the limits of your implicit context

Identifying the limits of your implicit context: Scope

Identifying the limits of your implicit context: Implicit parameters

Identifying the limits of your implicit context: Target & goal companion objects

Conversion Source Type Target Type

A B

B A

B A

Patterns, let’s take it easy

Representing context

Representing context

Basic exercise

Fighting type erasure:The compiler & implicit values to the rescue

Adventure time

Problem

Old-school solution

Traditional OOP

Solution

Type classes to therescue

Maintainable += Animal

Maintainable += Equipment

Type classes to therescue

That wasn’t just a hack...

A magnetic way of solving the same problem...

A magnetic way of solving the same problem...