Picture and Drawing

Each Doodle backend defines, by convention, two types called Picture and Drawing. These are the main types you'll interact with.

The quick overview is:

• Picture is a backend independent description of a picture, that accepts backend specific algebra implementations to produce a backend specific Drawing
• Drawing is an effect that, when run, draws a picture on a specific backend.

The generic Picture has type signature

trait Picture[-Alg <: Algebra, A]

where

• Alg is the type of the algebras the Picture requires;
• A is the type of the result produced when the Picture is transformed into a Drawing, and that Drawing is run.

Each backend specializes the full Picture type to fix Alg to the algebras the backend supports. Most of the time you'll only target a particular backend. In this case you can drop the Alg parameter and use a simpler Picture[A] type.

A Picture is a first class value (a value that can be passed to a method or returned from a method). A Picture is conceptually a function from an Algebra to a Drawing[A]. Picture is not implemented as a function, in the Scala sense, because the input parameter (the algebra) is an implicit parameter. In Scala 3 this is a context function. As Doodle supports both Scala 2 and Scala 3 we can't use this language feature and have a custom type instead.

The Drawing type is the effect type that the backend transforms a Picture into. When run, a Drawing[A] will draw a picture and produce a value of type A. It is needed much less often than Picture. If you see the Drawing type you're probably either implementing your own backend or working directly with algebras instead of using the conveniences that Doodle provides.