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Some interesting Julia codes

Julia is fast, composable, dynamic, general, reproducible and open source. The codes below were taken from https://julialang.org. I found it very illustrating.

Multiple dispatch

struct Dog end
struct Cat end

dog, cat = Dog(), Cat()
meet(a::Dog, b::Dog) = "The dogs play together"
meet(a::Dog, b::Cat) = "The dog chases the cat"
meet(a::Cat, b::Dog) = "The cat hisses at the dog"
meet(a::Cat, b::Cat) = "The cats ignore each other"

meet(dog, cat)
meet(dog, dog)
meet(cat, cat)

Display customization

struct Nutshell{T}
	contents::T
end
Base.show(io::IO, n::Nutshell) = print(io, "🥜 ", n.contents, " 🥜")
Nutshell("Julia")

Unicode and math

α, β = .5, .3
f(x) = α * x + β
Σ(v) = reduce(+, v)
Σ(f.(1:5))

Comprehensions

[x^2 for x in 1:5]

Broadcasting

A = [1 2; 3 4]
sin.(A) .+ 1
Dict(c => i for (i, c) in enumerate("julia")

Piping and composition

1:5 |> sum |> sqrt
(sqrt ∘ sum)(1:5) # equiv. above
map(exp ∘ abs, [-1, 2, -3])

Destructuring

(a, b, c) = 1:3
a, c # => (1, 3)
head, tail... = ["first", "second", "third"]
head, tail # => ("first", ["second", "third"])

Metaprogramming

ex = :(1 + 2 * 3)
typeof(ex) # => Expr
eval(ex)   # => 7

Easy package install

using DataFrames
# if not installed, prompted for installation, after 'y' answer
df = DataFrame(name = ["Alice", "Bob"], age = [25, 30])

Defaults and keywords args

function greet(name, greeting = "Hello"; punctuation = "!")
	"$greeting, $name$puctuation"
end
greet("Julia")  # "Hello, Julia!"
greet("world", "Hi"; puctuation = ".") # "Hi, world."

Multi-threading

function fib(n)
	n < 2 && return n
	t = Threads.@spawn fib(n-2)
	return fib(n-1) + fetch(t)
end
fib(10)
Threads.nthreads() # => threads assigned to Julia

Build-in REPL modes

# Default Julia mode, backspace from other modes
# Press ] for package mode
# Press ; for shell mode
# Press ? for helps
# Press $ for R mode, if RCall.jl is installed

Code introspection (simplified)

f(x, y) = x + y
@code_warntype f(1, 2)
@code_llvm f(1, 2)
@code_native f(1, 2)
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