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fredrikekre / Runic.jl

A code formatter for Julia with rules set in stone.
MIT License
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formatter julia julia-language

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Runic.jl

A code formatter with rules set in stone.

Test codecov code style: runic

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

Runic can be installed with Julia's package manager:

julia -e 'using Pkg; Pkg.add("Runic")'

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("Runic")'

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))' --"
$ 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 no path is given, or if path is `-`, input is read from stdin.

       -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` to be installed.

       --help
           Print this message.

       -i, --inplace
           Format files in place.

       -o <file>, --output=<file>
           File to write formatted output to. If no output is given, or if the file
           is `-`, output is written to stdout.

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

Most code editors have code formatting capabilities and many can be configured to use Runic. Example configuration for some editors are given in the following sections.

[!IMPORTANT] Note that these configurations depend on third party plugins. They works as advertised but use it at your own risk.

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 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()"

VS Code

Runic can be used as a formatter in VS Code using the extension Custom Local Formatters.

After installing the extension you can configure Runic as a local formatter by adding the following entry to your settings.json:

"customLocalFormatters.formatters": [
    {
      "command": "julia --project=@runic -e 'using Runic; exit(Runic.main(ARGS))'",
      "languages": ["julia"]
    }
]

Using the "Format Document" VS Code command will now format the file using Runic. Note that the first time you execute the command you will be prompted to select a formatter since the Julia language extension also comes with a formatter.

Emacs

Runic can be used as a formatter in Emacs using apheleia. Refer to the apheleia repository for installation and setup instruction.

Runic is not (yet) available directly in apheleia so the following configuration needs to be added to your .emacs. This assumes that Runic is installed in the @runic shared project as suggested in the Installation section above.

(push `(runic . ("julia" "--project=@runic" "-e" "using Runic; exit(Runic.main(ARGS))" "--")) apheleia-formatters)
(push '(julia-mode . runic) apheleia-mode-alist)

Adopting Runic formatting

Here is a checklist for adopting Runic formatting in a project:

Ignore formatting commits in git blame

When setting up Runic formatting for a repository for the first time (or when upgrading to a new version of Runic) the formatting commit will likely result in a large diff with mostly non functional changes such as e.g. whitespace. Since the diff is large it is likely that it will show up and interfere when using git-blame. To ignore commits during git-blame you can i) add them to a file .git-blame-ignore-revs and ii) tell git to use this file as ignore file by running

git config blame.ignoreRevsFile .git-blame-ignore-revs

See the git-blame documentation for details.

For example, such a file may look like this:

# Adding Runic formatting
<commit hash of formatting commit>

# Upgrading Runic from 1.0 to 2.0
<commit hash of formatting commit>

Badge

If you want to show that your project is formatted with Runic you can add the following badge in the repository README:

[![code style: runic](https://img.shields.io/badge/code_style-%E1%9A%B1%E1%9A%A2%E1%9A%BE%E1%9B%81%E1%9A%B2-black)](https://github.com/fredrikekre/Runic.jl)

The badge looks like this: code style: runic

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:

git ls-files -z -- '*.jl' | xargs -0 --no-run-if-empty julia --project=@runic -m Runic --check --diff

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

You can use fredrikekre/runic-action to run 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: '1'
      # - uses: julia-actions/cache@v2
      - uses: fredrikekre/runic-action@v1
        with:
          version: '1'

See fredrikekre/runic-action for details.

[!IMPORTANT] Please be aware of Runic's version policy when configuring the version. Pinning to a major release (as above with version: '1') may cause occasional CI failures whenever there is a new minor release of Runic that happens to impact your code base. When this happens you simply have to i) re-run Runic on the new version, ii) commit the result, and iii) add the commit to the ignore list. This is still recommended since minor releases should be relatively rare, and if you use Runic you presumably want these minor bugfixes to be applied to your code base. The alternative is to pin to a minor version and manually upgrade to new minor versions.

Git hooks

Runic can be used together with pre-commit using fredrikekre/runic-pre-commit. After installing pre-commit you can add the following to your .pre-commit-config.yaml to run Runic before each commit:

repos:
  - repo: https://github.com/fredrikekre/runic-pre-commit
    rev: v1.0.0
    hooks:
      - id: runic

See fredrikekre/runic-pre-commit for details.

If you don't want to use pre-commit you can also use a plain git hook. 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
git diff-index -z --name-only --diff-filter=AM master | \
    grep -z '\.jl$' | \
    xargs -0 --no-run-if-empty julia --project=@runic -m Runic --check --diff

Version policy

Runic adheres to Semantic Versioning. Semantic versioning is easy to apply and understand when it comes to the API (e.g. the CLI and public methods of the Runic library) but it is less clear how to apply it to changes in the formatted output. Runic makes the following policy:

Formatting specification

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

Toggle formatting

It is possible to toggle formatting around expressions where you want to disable Runic's formatting. This can be useful in cases where manual formatting increase the readability of the code. For example, manually aligned array literals may look worse when formatted by Runic.

The source comments # runic: off and # runic: on will toggle the formatting off and on, respectively. The comments must be on their own line, they must be on the same level in the syntax tree, and they must come in pairs. An exception to the pairing rule is made at top level where a # runic: off comment will disable formatting for the remainder of the file. This is so that a full file can be excluded from formatting without having to add a # runic: on comment at the end of the file.

[!NOTE] Note that it is enough that a comment contain the substring # runic: off or # runic: on so that they can be combined with other "pragmas" such as e.g. Literate.jl line filters like #src.

[!NOTE] For compatibility with JuliaFormatter the comments #! format: off and #! format: on are also recognized by Runic.

For example, the following code will toggle off the formatting for the array literal A:

function foo()
    a = rand(2)
    # runic: off
    A = [
        -1.00   1.41
         3.14  -4.05
    ]
    # runic: on
    return A * a
end

Line width limit

No. Use your Enter key or refactor your code.

Newlines in blocks

The body of blocklike expressions (e.g. if, for, while, function, struct, etc.) always start and end with a newline. Examples:

-if c x end
+if c
+    x
+end

-function f(x) x^2 end
+function f(x)
+    x^2
+end

An exception is made for empty blocks so that e.g.

struct A end

is allowed.

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

Explicit return

Explicit return statements are ensured in function and macro definitions by adding return in front of the last expression, with some exceptions listed below.

Note that adding return changes the expression in a way that is visible to macros. Therefore it is, in general, not valid to add return to a function defined inside a macro since it isn't possible to know what the macro will expand to. For this reason this formatting rule is disabled for functions defined inside macros with the exception of some known and safe ones from Base (e.g. @inline, @generated, ...).

For the same reason mentioned above, if the last expression in a function is a macro call it isn't valid to step in and add return inside. Instead the return will be added in front of the macro call like any other expression (unless there is already a return inside of the macro as described above).

Examples:

 function f(n)
-    sum(rand(n))
+    return sum(rand(n))
 end

 macro m(args...)
-    :(generate_expr(args...))
+    return :(generate_expr(args...))
 end

Potential changes

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

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. Trailing commas are enforced for array/tuple literals (where adding another item is common) but optional for function/macro calls/definitions.

-(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 ,. Trailing commas are enforced for array/tuple literals (where adding another item is common) but optional for function/macro calls/definitions. 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

Trailing semicolons

Trailing semicolons are removed in the body of blocklike expressions. Examples

 function f(x)
-    y = x^2;
-    z = y^2; # z = x^4
-    return z;
+    y = x^2
+    z = y^2  # z = x^4
+    return z
 end

Trailing semicolons at top level and module level are kept since they are sometimes used there for output supression (e.g. Documenter examples or scripts that are copy-pasted/included in the REPL).

Literal floating point numbers

Floating point literals are normalized so that they:

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

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

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}

Whitespace miscellaneous

Trailing spaces

Trailing spaces are removed in code and comments (but not inside of multiline strings where doing so would change the meaning of the code). Examples:

-1 + 1 
+1 + 1

-x = 2 # x is two 
+x = 2 # x is two

Tabs

Tabs are replaced with spaces. Example:

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

Vertical spacing

Extra vertical spacing is trimmed 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

Any newlines at the start of a file are removed and if the file ends with more than one newline the extra ones are removed.