A powerful JSON::Serializable like argument parser
0.1.3 Latest release released
16 1 1
Chris Watson



A powerful argument parser which uses a class or struct to define the arguments and their types.


  1. Add the dependency to your shard.yml:

        github: watzon/arg_parser
  2. Run shards install


ArgParser works very similarly to JSON::Serializable and works on both classes and structs. To use it, simply include ArgParser in your class or struct, and define the arguments you want to parse as instance variables.

struct MyArgs
  include ArgParser

  getter name : String
  getter age : Int32
  getter active : Bool

ArgParser parses arguments such as those that come from ARGV, though in reality all that really matters is that it's given an array of strings. ArgParser defines an initializer for your type which takes the array of strings, and parses it into your type.

args =["--name", "John Doe", "--age", "20", "--active"])   # => "John Doe"
args.age    # => 20 # => true

Positional arguments are supported as well. To keep things in your struct clean, all instance variables added by ArgParser itself are prefixed with an underscore.

args =["\"Hello world\"", --name", "John Doe", "--age", "20", "--active"])
args._positional_args # => ["Hello world"]

Supported Types

By default ArgParser supports the following types:

  • String
  • Int
  • UInt
  • Float
  • BigInt
  • BigFloat
  • BigDecimal
  • BigRational
  • Bool
  • URI
  • UUID

Any type which implements from_arg can be used as an argument type. For types which don't implement from_arg, you can define a converter which implements from_arg as a proxy for that type.


Converers are simply modules which have a self.from_arg method which takes a value string, and returns the converted value. For Example:

module MyConverter
  def self.from_arg(value : String)
    # do something with value

Converters can be used through the ArgParser::Field annotation.

struct MyArgs
  include ArgParser

  @[ArgParser::Field(converter: MyConverter)]
  getter name : SomeType


Aliases are simply other names for an argument. For example, if you want to use -n as an alias for --name, you can do so with the ArgParser::Field annotation.

struct MyArgs
  include ArgParser

  @[ArgParser::Field(alias: "-n")]
  getter name : String

Currently only a single alias is supported.

Default Values

Default values can be specified in the same way you would normally specify them in Crystal. For example, if you want to set a default value for name:

struct MyArgs
  include ArgParser

  getter name : String = "John Doe"


Validators allow you to validate user input. For example, if you want to make sure that the user's input matches a pattern, you can do so with a validator.

struct MyArgs
  include ArgParser

  getter name : String

On invalid input, the method on_validation_error is called. By default, this method raises an ArgParser::ValidationError, but you can override it to do whatever you want.

struct MyArgs
  include ArgParser

  getter name : String

  def on_validation_error(field : Symbol, value, errors : Array(String))
    # allow it, but print a warning
    puts "Invalid value for #{field}: #{value}"

All validation errors are also added to the _validation_errors hash. This can be useful if you want to do something with the errors after parsing.

args =["--name", "John Doe", "--age", "foo", "--active"])
args._validation_errors # => {"age" => ["must be an integer"]}

Modifying the Behavior of ArgParser

ArgParser is designed to be configurable so it can handle a wide variety of use cases. As such, it includes several overridable methods which can be used to modify its behavior. These are:

  • on_validation_error - called when a validation error occurs; by default calls add_validation_error and then raises ArgParser::ValidationError
  • on_unknown_attribute - called when an unknown attribute is encountered; by default raises ArgParser::UnknownAttributeError
  • on_missing_attribute - called when a required attribute is missing; by default raises ArgParser::MissingAttributeError
  • on_conversion_error - called when a value isn't able to be converted to the specified type; by default raises ArgParser::ConversionError

In addition, the way keys are parsed can be modified by overriding the parse_key method. By default, it simply removes one or two dashes from the beginning of the key. For example, --name becomes name, and -n becomes n.


  1. Fork it (
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request


  github: watzon/arg_parser
  version: ~> 0.1.3
License MIT
Crystal 1.7.1


Dependencies 0

Development Dependencies 1

  • spectator master
    {'branch' => 'master', 'gitlab' => 'arctic-fox/spectator'}

Dependents 0

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