Closures

First-Class Functions

• "First-class functions" are a thing. A thing is "first-class" if all the sensible operations of the language do indeed apply to it

• In Rust, functions are first class

  • They have their own types

  • They can be passed as parameters, returned as results, stored in arrays, used as struct and enum fields, etc

  • They can be created in any block scope

  • References are available (indeed, a function essentially is a static reference)

  https://play.rust-lang.org/?gist=92076106592f0e34b7e7d7d7f25ea207

Closures

• The problem with pure functions is that anything they compute has to be based on arguments

• A closure is a function that is allowed to access its "environment": names that are statically in scope at the point of its declaration

• Closures in Rust are mostly first-class, and can access parameters and locals like anything else

• Syntax has wacky pipes and stuff; types are optional and inferred

Closures, Environments, Types

• "Pure" closures are just functions

  https://play.rust-lang.org/?gist=bc4d677696ac6afa887a25c425e7114d

• Closures that capture the environment are each their own type, but all obey some corresponding Fn trait

  https://play.rust-lang.org/?gist=da8d32b5212edf47427787743e271b63

• However, be careful about generics vs trait objects

Closures As Environment Structs

• Think of a closure as a code pointer and some anonymous struct holding the captured vars or refs to them and its implementation

• Three kinds of implementation of the anonymous struct:

  • &self: Closure is of trait Fn

  • &mut self: Closure is of trait FnMut

  • self: Closure is of trait FnOnce

• Also, environment values themselves can be:

  • By reference (normal case)

  • Owned (move closure)

Generics vs Trait Objects

• Each closure has a different size, so can't just handle them willy-nilly

• Each closure is of a different type, so generics don't work so well with them

• This leaves the joy of trait objects

  https://play.rust-lang.org/?gist=be0c56a6315761ae8c1eac46401ce0d1

Final Notes

• Values implementing Copy confuse everybody

• This stuff is hard; get some experience with it right now