External cheat sheet

A useful external cheat sheet can be found at The Fast Track to julia 1.0. Below are my own useful notes.

Type hierarchy

To see types above and below

supertype()
subtypetree() # self-defined

To see who exported the object:

parentmodule()
parentmodule(var"@which") # for macros

Macros

Macro Parent module Description
@kwdef TODO TODO
@forward TODO TODO
@show TODO TODO
@time TODO TODO
@test TODO TODO
@testset TODO TODO
@which InteractiveUtils TODO
@enum TODO TODO
@btime TODO TODO
@code_warntype InteractiveUtils Warn of potential type instability.
@inferred Test Check if returned type differs from inferred return type.

Type stability

Useful functions for declaring the right types

zero
zeros
one
ones
similar(A)
zeros(axes(A))
eltype

Operator Left operand Right operand Returns Example Type–value relationships :: value (or name) type value / assertion x::Float64 — asserts or annotates isa value type Bool 1.0 isa Float64 → true typeof — value type typeof(1.0) → Float64 Type–type relationships <: type type Bool Float64 <: AbstractFloat → true supertype — type type supertype(Float64) → AbstractFloat subtypes — type Vector{Type} subtypes(AbstractFloat) → [BigFloat, Float16, Float32, Float64]

Are there any other operators that would be good to include in there?

A few come to mind that fit naturally in the same conceptual space:

Type–value relationships:

convert(T, x) — explicitly convert a value to type T. Related to :: but doesn't just assert, it actually transforms.
typeassert(x, T) — the function equivalent of x::T at runtime, throws TypeError if it fails.

Type–type relationships:

>: — the reverse of <:, i.e. "is supertype of". AbstractFloat >: Float64 → true. Less commonly seen but exists.
typeintersect(T, S) — returns the intersection of two types, useful in metaprogramming.
promote_type(T, S) — given two types, returns the type Julia would promote them to in arithmetic. e.g. promote_type(Float32, Float64) → Float64. Very relevant for numerical work.

Introspection:

isabstracttype(T) — returns Bool
isconcretetype(T) — returns Bool. Important distinction in Julia since you can only instantiate concrete types.
fieldnames(T) — returns the field names of a struct type
fieldtypes(T) — returns the types of each field

Design principles

Workflow

module Oscillations

say_hello() = println(“Hello!”)

Your other definitions here

end # module

using .Oscillations osc = Oscillations

then in the file: include(“Oscillations.jl”)