"Functional Programming is writing programs with (pure) functions and (immutable) data"

~ Probably someone from the internet

Functions

Data

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Focus of this talk

ADTs are product types, sum types and compositions of those

Combine values with "AND"

Combine values with "OR"

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* Scala's way of writing ADTs is truly the worst of any FP language -_-'

1. "Exhaustiveness Checking"

Compiler is helping us out:

 => Incredibly helpful for easy and safe refactorings/changes

2. Help with partiality when modelling application state

Can have many inconsistent states!

Using an ADT the compiler will force you to handle partiality/errors:

2.1. Limited amount of possible states

 Possible states: 2 * 2 * 2

 Possible states: 1 + 1 + 1 + (2 * 2)

 => This shows where sum and product in the names come from

 => Great for finite state machines

3. Expressive data types are a design tool and documentation

Can you guess what program this is by just looking at the data model for it?

felixbr.gitlab.io/elm-snake

4. Keeping data separate from implementation details helps with reusability of your domain model

This brings us to...

A lot of things we do with data involves (de)serialization. Writing the code for this by hand is tedious and error-prone.

Typeclasses help to do this without reflection, but normally require to still write the logic for each new type

Serializing to json with circe:

Nobody has time for this...

Circe can derive the typeclass instances for sum and product types:

This is still type-safe and fast (no runtime reflection) and can reduce the boilerplate to zero (for automatic derivation)!

If you want the same for your own code you have to either:

Magnolia example:

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E.g. No typeclass instance for "LocalDate" type