Runic.jl

A code formatter with rules set in stone.

Runic is a formatter for the Julia programming language built on top of JuliaSyntax.jl.

Similarly to gofmt, Runic have no configuration. The formatting rules are set in stone (although not yet complete). This approach is something that is appreciated by most Go programmers, see for example the following quote:

Gofmt's style is no one's favorite, yet gofmt is everyone's favorite.

Table of contents

• Installation
• Usage
  • CLI
  • Editor integration
• Checking formatting
• Formatting specification

Installation

Runic can be installed with Julia's package manager:

julia -e 'using Pkg; Pkg.add(url = "https://github.com/fredrikekre/Runic.jl")'

For CLI usage and editor integration (see Usage) it is recommended to install Runic in a separate project such as e.g. the shared project @runic:

julia --project=@runic -e 'using Pkg; Pkg.add(url = "https://github.com/fredrikekre/Runic.jl")'

Usage

CLI

The main interface to Runic is the command line interface (CLI) through the main function invoked with the -m flag. See the output of julia -m Runic --help below for usage details.

The following snippet can be added to your shell startup file so that the CLI can be invoked a bit more ergonomically. This assumes Runic is installed in the @runic shared project as suggested in the Installation section above. Adjust the --project flag if you installed Runic elsewhere.

alias runic="julia --project=@runic -m Runic"

Note

The -m command line flag is only available in Julia 1.12 and later. In earlier versions you have to invoke the main function explicitly, for example:

julia -e 'using Runic; exit(Runic.main(ARGS))' -- <args>

For this incantation the following shell alias can be used:

alias runic="julia --project=@runic -e 'using Runic; exit(Runic.main(ARGS))' --"

$ julia -m Runic --help
NAME
       Runic.main - format Julia source code

SYNOPSIS
       julia -m Runic [<options>] <path>...

DESCRIPTION
       `Runic.main` (typically invoked as `julia -m Runic`) formats Julia source
       code using the Runic.jl formatter.

OPTIONS
       <path>...
           Input path(s) (files and/or directories) to process. For directories,
           all files (recursively) with the '*.jl' suffix are used as input files.
           If path is `-` input is read from stdin and output written to stdout.

       -c, --check
           Do not write output and exit with a non-zero code if the input is not
           formatted correctly.

       -d, --diff
           Print the diff between the input and formatted output to stderr.
           Requires `git` or `diff` to be installed.

       --fail-fast
           Exit immediately after the first error. Only applicable when formatting
           multiple files in the same invocation.

       --help
           Print this message.

       -i, --inplace
           Edit files in place. This option is required when passing multiple input
           paths.

       -o, --output <file>
           Output file to write formatted code to. If the specified file is `-`
           output is written to stdout. This option can not be used together with
           multiple input paths.

In addition to the CLI there is also the two function Runic.format_file and Runic.format_string. See their respective docstrings for details.

Editor integration

Neovim

Runic can be used as a formatter in Neovim using conform.nvim. Refer to the conform.nvim repository for installation and setup instructions.

Runic is not (yet) available directly in conform.nvim so the following configuration needs to be passed to the setup function. This assumes Runic is installed in the @runic shared project as suggested in the Installation section above. Adjust the --project flag if you installed Runic elsewhere.

require("conform").setup({
    formatters = {
        runic = {
            command = "julia",
            args = {"--project=@runic", "-e", "using Runic; exit(Runic.main(ARGS))", "--", "-"},
        },
    },
    formatters_by_ft = {
        julia = {"runic"},
    },
    default_format_opts = {
        -- Increase the timeout in case Runic needs to precompile
        -- (e.g. after upgrading Julia and/or Runic).
        timeout_ms = 10000,
    },
})

Note that conform (and thus Runic) can be used as formatexpr for the gq command. This is enabled by adding the following to your configuration:

vim.o.formatexpr = "v:lua.require('conform').formatexpr()"

Checking formatting

Runic has a check-mode that verifies whether files are correctly formatted or not. This mode is enabled with the --check flag. In check mode Runic will exit with a non-zero code if any of the input files are incorrectly formatted. As an example, the following invocation can be used:

julia -m Runic --check --diff $(git ls-files -- '*.jl')

This will run Runic's check mode (--check) on all .jl files in the repository and print the diff (--diff) if the files are not formatted correctly. If any file is incorrectly formatted the exit code will be non-zero.

Github Actions

Here is a complete workflow file for running Runic on Github Actions:

name: Runic formatting
on:
  push:
    branches:
      - 'master'
      - 'release-'
    tags:
      - '*'
  pull_request:
jobs:
  runic:
    name: Runic
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: julia-actions/setup-julia@v2
        with:
          version: "nightly" # Only nightly have the -m flag currently
      - uses: julia-actions/cache@v2
      - name: Install Runic
        run: |
          julia --color=yes -e 'using Pkg; Pkg.add(url = "https://github.com/fredrikekre/Runic.jl")'
      - name: Run Runic
        run: |
          julia --color=yes -m Runic --check --diff $(git ls-files -- '*.jl')

Git Hooks

Runic can be run in a Git pre-commit hook to automatically check formatting before committing. Here is an example hook (.git/hooks/pre-commit):

#!/usr/bin/env bash

# Redirect output to stderr.
exec 1>&2

# Run Runic on added and modified files
mapfile -t files < <(git diff-index --name-only --diff-filter=AM master | grep '\.jl$')
julia --project=@runic -m Runic --check --diff "${files[@]}"

Formatting specification

This is a list of things that Runic currently is doing:

• Line width limit
• Indentation
• Spaces around operators, assignment, etc
• Spaces around keywords
• Multiline listlike expressions
• Spacing in listlike expressions
• Literal floating point numbers
• Literal hex and oct numbers
• Parentheses around operator calls in colon
• in instead of ∈ and =
• Braces around right hand side of where
• Trailing whitespace

Line width limit

No. Use your Enter key or refactor your code.

Indentation

Consistently four spaces for each indentation level.

Standard code blocks (function, for, while, ...) all increase the indentation level by one until the closing end. Examples:

 function f()
-  for i in 1:2
-    # loop
-  end
-  while rand() < 0.5
-    # loop
-  end
+    for i in 1:2
+        # loop
+    end
+    while rand() < 0.5
+        # loop
+    end
 end

Listlike expressions like e.g. tuples, function calls, array literals, etc. also increase the indentation level by one until the closing token. This only has an effect if the list span multiple lines. Examples:

 x = (
-  a, b, c, d,
-  e, f, g, h,
+    a, b, c, d,
+    e, f, g, h,
 )

 foo(
-  a, b, c, d,
-  e, f, g, h,
+    a, b, c, d,
+    e, f, g, h,
 )

 [
-  a, b, c, d,
-  e, f, g, h,
+    a, b, c, d,
+    e, f, g, h,
 ]

The examples above both result in "hard" indentation levels. Other expressions that span multiple lines result in "soft" indentation levels. The difference between the two is that soft indentation levels don't nest (this is really only applicable to multiline operator call chains).

 using Foo:
-  foo, bar
+    foo, bar

 x = a + b +
-  c
+    c

 x = a ? b :
-  c
+    c

Without soft indentation levels operators chains can result in ugly (but logically correct) indentation levels. For example, the following code:

x = a + b *
        c +
    d

would be "correct". Such a chain looks better the way it is currently formatted:

x = a + b *
    c +
    d

Vertical spacing

Runic removes empty vertical spacing so that there are at maximum two empty lines between expressions. Examples:

-function f()
-     x = 1
-
-
-
-    return x
-end
+function f()
+     x = 1
+
+
+    return x
+end

Spaces around operators, assignment, etc

Runic formats spaces around infix operators, assignments, comparison chains, and type comparisons (binary <: and >:), and some other operator-like things. If the space is missing it will be inserted, if there are multiple spaces it will be reduced to one. Examples:

-1+2*3
-1  +  2  *  3
+1 + 2 * 3
+1 + 2 * 3

-x=1
-x=+1
-x+=1
-x.+=1
+x = 1
+x = +1
+x += 1
+x .+= 1
-1<2>3
-1  <  2  >  3
+1 < 2 > 3
+1 < 2 > 3

-T<:Integer
-T  >:  Integer
+T <: Integer
+T >: Integer

-x->x
-a  ?  b  :  c
+x -> x
+a ? b : c

Note that since Runic's rules are applied consistently, no matter the context or surrounding code, the "spaces around assignment" rule also means that there will be spaces in keyword arguments in function definitions and calls. Examples:

-foo(; a=1) = a
-foo(a=1)
+foo(; a = 1) = a
+foo(a = 1)

Exceptions to the rule above are :, ^, ::, and unary <: and >:. These are formatted without spaces around them. Examples:

-a : b
+a:b

-a ^ 5
+a^5

-a :: Int
+a::Int

-<: Integer
->:  Integer
+<:Integer
+>:Integer

Potential changes

• Perhaps the rule for some of these should be "at least one space" instead. This could help with alignment issues. Discussed in issue #12.

Spaces around keywords

Consistently use single space around keywords. Examples:

-struct  Foo
+struct Foo

-mutable  struct  Bar
+mutable struct Bar

-function  foo(x::T)  where  {T}
+function foo(x::T) where {T}

Multiline listlike expressions

Listlike expressions (tuples, function calls/definitions, array literals, etc.) that already span multiple lines are formatted to consistently have a leading and a trailing newline, as well as a trailing comma where applicable. Examples:

-(a,
-    b)
+(
+    a,
+    b,
+)

-foo(a,
-    b)
+foo(
+    a,
+    b,
+)

-[1 2
- 3 4]
+[
+    1 2
+    3 4
+]

Note that currently there is no line-length limit employed so expressions that only take up a single line, even if they are long, are not formatted like the above. Thus, only expressions where the original author have "committed" to mulitples lines are affected by this rule.

Spacing in listlike expressions

Listlike expressions (tuples, function calls/definitions, array literals, etc.) use a consistent rule of no space before , and a single space after ,. In single line expressions there is no trailing ,. In multiline expressions there is a trailing ,. Leading/trailing spaces are removed. Examples:

-f(a,b)
-(a,b)
-[a,  b]
+f(a, b)
+(a, b)
+[a, b]


-(a,b,)
+(a, b)
 (
     a,
-    b
+    b,
 )

-( a, b )
+(a, b)

Potential changes

• Perhaps the rule for some of these should be "at least one space" instead. This could help with alignment issues. Discussed in issue #12.

Literal floating point numbers

Floating point literals are normalized so that they:

• always have a decimal point
• always have a digit before and after the decimal point
• never have leading zeros in the integral and exponent part
• never have trailing zeros in the fractional part
• always use e instead of E for the exponent

Examples:

-1.
-.1
+1.0
+0.1

-01.2
-1.0e01
-0.10
+1.2
+1.0e1
+0.1

-1.2E5
+1.2e5

Potential changes

• Always add the implicit + for the exponent part, i.e. 1.0e+1 instead of 1.0e1. Discussed in issue #13.
• Allow multiple trailing zeros in the fractional part, i.e. don't change 1.00 to 1.0. Such trailing zeros are sometimes used to align numbers in literal array expressions. Discussed in issue #14.

Literal hex and oct numbers

Hex literals are padded with zeros to better highlight the resulting type of the literal: UInt8 to 2 characters, UInt16 to 4 characters, UInt32 to 8 characters etc. Examples:

-0x1
-0x123
-0x12345
+0x01
+0x0123
+0x00012345

Similarly, oct literals are also padded:

-0o644
-0o644644
+0o000644
+0o00000644644

Parentheses around operator calls in colon

Add parentheses around operator calls in colon expressions to better highlight the low precedence of :. Examples:

-1 + 2:3 * 4
-1 + 2:3
-1:3 * 4
+(1 + 2):(3 * 4)
+(1 + 2):3
+1:(3 * 4)

in instead of ∈ and =

The keyword in is used consistently instead of ∈ and = in for loops. Examples:

-for i = 1:2
+for i in 1:2

-for i ∈ 1:2
+for i in 1:2

Note that ∈ not replaced when used as an operator outside of loop contexts in order to be symmetric with ∉ which doesn't have a direct ASCII equivalent. See #17 for more details.

Braces around right hand side of where

Braces are consistently used around the right hand side of where expressions. Examples:

-T where T
-T where T <: S where S <: Any
+T where {T}
+T where {T <: S} where {S <: Any}

Trailing whitespace

Trailing spaces are removed. Example:

-1 + 1 
+1 + 1