My first Julia script

Weaving code in Hugo posts with Julia

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1100 words (about 6 minutes to read)

Programming / #JuliaLang #LiterateProgramming #LiterateBlogging #Site

Drawn with [Luxor.jl](
Drawn with Luxor.jl

Merry Christmas! I wrote a little Julia code.

The inspiration

Been getting frustrated with Python’s type hinting system. I usually start with loose and informal code, but eventually I specify types. And when I do, I want the language to check my work. I dislike relying on an external tool like MyPy that runs separately.

I’ve also been looking at Pandas a lot recently for work stuff. Okay, Pandas looks interesting to my non-data science brain. I mainly use it to filter Excel files for database updates. But I can’t help noticing how often the Julia programming language comes up in those posts about Data Science in Python.

So I check out Julia. It intrigues me. The type system and concurrency tools look nice, of course. But what’s this? Math code that looks more like math?

julia> f(x) = 2x^2 - 5
f (generic function with 1 method)

julia> [f(x) for x in [0, 1, 2, 3]]
4-element Array{Float64,1}:

By way of contrast, this is Python’s equivalent of those two lines of Julia code.

from math import sqrt

def f(x):
	return sqrt(2 * x ** 2) - 5

[f(x) for x in [0, 1, 2, 3]]

It’s similar enough that I don’t feel massively disoriented. But the math stuff is just a little bit friendlier.

Time to run through the “is this language worth my time” checklist.

So yeah. I can poke around a little more.

I love literate programming. One of the first things I did was look to see if someone in the Julia world did too. And they do!

There’s Literate.jl, which processes Markdown and code in Julia scripts. Weave.jl is more my style, processing Julia code in Markdown files. I can write my post and weave it into an ordinary-looking Markdown file. Hugo won’t have to know the difference.

The setup

Julia treats environment and package management as core functionality. Everything I need is in Pkg. Not to pick too much on Python — it really is a great language — but its environment management options are infamously byzantine.

To set up a package for my existing site, I drop into the REPL’s pkg mode.

julia> ]

Here I can initialize my project and add dependencies.

(v1.5) pkg> initialize .
(rgb-hugo-legacy)> add Weave

Now I have Project.toml and Manifest.toml files describing my Hugo site’s new Julia needs. I can start writing this post.

Writing with Weave

Write the stuff you want to write, using Julia-flavored Markdown. Any code block fenceposted with triple backticks and labeled as “julia” gets evaluated by Weave.

Set different chunk options for the block if you want to tweak the code’s treatment.

```julia; term = true
f(x) = √2x^2 - 5
[f(x) for x in [0, 1, 2, 3]]

Weave does its thing, and produces something interesting depending on what ouput options you use.

julia> f(x) = √2x^2 - 5
f (generic function with 1 method)

julia> [f(x) for x in [0, 1, 2, 3]]
4-element Array{Float64,1}:

The script

AKA the point of this blog post. It looks in my content folder for recently modified .jmd files. Anything found gets handed off to weave, which does the hard work. Heck, weave even has a hugo option so I can generate Markdown specifically formatted to satisfy Hugo.

using Logging

using Weave

const content_folder = "content"
const weave_extensions = [".jmd"]

function main()
    weave_files = []
    @debug "content is in $content_folder"

    for (root, dirs, files) in walkdir(content_folder)
        for file in files
            ext = splitext(file)[2]

            if ext in weave_extensions
                weave_file = joinpath(root, file)
                @debug "Found weave file" weave_file
                target_file = joinpath(dirname(weave_file), "")

                if isfile(target_file) && mtime(weave_file) >= mtime(target_file)
                    push!(weave_files, weave_file)

    @info "Weave-able files found:" weave_files

    for weave_file in weave_files
        @info "Weaving" weave_file
        weave(weave_file; out_path=:doc, doctype="hugo")
    @info "Done?"


This is probably not idiomatic Julia. Maybe it’ll get there when I learn what idiomatic Julia even looks like.

Obviously there’s no error handlng of any kind. That can come later.

A few things I noticed:

  • functions like walkdir end up making the flow look a bit like Python
  • I kept making my code more complicated than it needed to be, when both Julia and Weave were ready with reasonable defaults.
  • especially in regard to types. Everything works fine without specifying details. I can find out what happens when I add details later.
❯ just weave
julia --project=. scripts/weave-content.jl
┌ Info: Files that need weaving:
│   weave_files =
│    1-element Array{Any,1}:
└     "content/post/2020/12/my-first-julia-script/index.jmd"
┌ Info: Weaving
└   weave_file = "content/post/2020/12/my-first-julia-script/index.jmd"
┌ Info: Weaving chunk 1 from line 45
└   progress = 0.0
┌ Info: Weaving chunk 2 from line 163
└   progress = 0.3333333333333333
┌ Info: Weaving chunk 3 from line 249
└   progress = 0.6666666666666666
┌ Info: Weaved all chunks
└   progress = 1
[ Info: Weaved to /home/random/Sites/rgb-hugo-legacy/content/post/2020/12/my-first-julia-script/
[ Info: Done?

Okay. That’s great. I mean — all that so I could do a little math, but whatever.

That’s it?

Hey. Maybe we could do something cool. Make a cover image for this post with Luxor.

Let’s try it out. I’ll borrow heavily from the Luxor manual since I don’t really know what I’m doing,

using Colors
using Luxor

function draw(x, y)
    foregroundcolors = Colors.diverging_palette(rand(0:360), rand(0:360), 200, s = 0.99, b=0.8)
    translate(x-100, y-150)

    for i in 1:500
        circle(rand(-50:350), rand(0:300), 15, :fillstroke)

    translate(x-125, y+150)
    spiral(100, -1, period=20π, :stroke)

currentwidth = 850
currentheight = 500
Drawing(currentwidth, currentheight, "cover.png")
draw(0, 0)

Much fancier!


First off, PNG format works better than SVG when you’re drawing 500 random circles.

❯ exa -l content/post/2020/12/my-first-julia-script/cover*
.rw-r--r--  99k random 25 Dec 12:42 cover.png
.rw-r--r-- 4.8M random 25 Dec 12:37 cover.svg

Second, I added an eval = false chunk option after the image was good enough. No point regenerating the cover every time I fix a typo.

That’s enough writing about writing with Julia. I have a couple other drafts I want to revisit now.

Besides, it’s Christmas! Christmas 2020. Which means my only regret is forgetting to order Christmas-themed face masks.