Code style linter for Crystal

(a single-celled animal that catches food and moves about by extending fingerlike projections of protoplasm)


Ameba is a static code analysis tool for the Crystal language. It enforces a consistent Crystal code style, also catches code smells and wrong code constructions.

See also Roadmap.


Run ameba binary within your project directory to catch code issues:

$ ameba
Inspecting 107 files


[W] Lint/UnusedArgument: Unused argument `location`. If it's necessary, use `_` as an argument name to indicate that it won't be used.
> source.issues.each do |issue, location|

[W] Lint/UselessAssign: Useless assignment to variable `s`
> return s += issues.size

src/ameba/formatter/ [Correctable]
[C] Style/RedundantReturn: Redundant `return` detected
> return s += issues.size

Finished in 389.45 milliseconds
107 inspected, 3 failures

Watch a tutorial

🎬 Watch the LuckyCast showing how to use Ameba


Rules that are marked as [Correctable] in the output can be automatically corrected using --fix flag:

$ ameba --fix

Explain issues

Ameba allows you to dig deeper into an issue, by showing you details about the issue and the reasoning by it being reported.

To be convenient, you can just copy-paste the PATH:line:column string from the report and paste behind the ameba command to check it out.

$ ameba crystal/command/           # show explanation for the issue
$ ameba --explain crystal/command/ # same thing

Run in parallel

Some quick benchmark results measured while running Ameba on Crystal repo:

$ CRYSTAL_WORKERS=1 ameba #=> 29.11 seconds
$ CRYSTAL_WORKERS=2 ameba #=> 19.49 seconds
$ CRYSTAL_WORKERS=4 ameba #=> 13.48 seconds
$ CRYSTAL_WORKERS=8 ameba #=> 10.14 seconds


As a project dependency:

Add this to your application's shard.yml:

    github: crystal-ameba/ameba
    version: ~> 1.4.0

Build bin/ameba binary within your project directory while running shards install.


$ brew tap crystal-ameba/ameba
$ brew install ameba


Build the image:

$ docker build -t .

To use the resulting image on a local source folder, mount the current (or target) directory into /src:

$ docker run -v $(pwd):/src

Also available on GitHub:

From sources

$ git clone && cd ameba
$ make install


Default configuration file is .ameba.yml. It allows to configure rule properties, disable specific rules and exclude sources from the rules.

Generate new file by running ameba --gen-config.


List of sources to run Ameba on can be configured globally via:

In this example we define default globs and exclude src/compiler folder:

  - "**/*.cr"
  - "!lib"

  - src/compiler

Specific sources can be excluded at rule level:

  - src/server/
  - src/server/


One or more rules, or a one or more group of rules can be included or excluded via command line arguments:

$ ameba --only   Lint/Syntax # runs only Lint/Syntax rule
$ ameba --only   Style,Lint  # runs only rules from Style and Lint groups
$ ameba --except Lint/Syntax # runs all rules except Lint/Syntax
$ ameba --except Style,Lint  # runs all rules except rules in Style and Lint groups

Or through the configuration file:

  Enabled: false

Inline disabling

One or more rules or one or more group of rules can be disabled using inline directives:

# ameba:disable Style/LargeNumbers
time = Time.epoch(1483859302)

time = Time.epoch(1483859302) # ameba:disable Style/LargeNumbers, Lint/UselessAssign
time = Time.epoch(1483859302) # ameba:disable Style, Lint

Editors & integrations

Credits & inspirations