Parametron

This simple library implements DSL for validating and type casting input parameters for method.

Add this line to your application's Gemfile:

gem 'parametron', '>= 0.3'

And then execute:

$ bundle

Or install it as:

$ gem install parametron

    class VictimStrict
        include Parametron # <- should be at the top of class/module

        params_for(:fetch, strict: true) do
          required :city,   validator: /\w+/
          required :year,   validator: /\d{4}/
          optional :title,  validator: ->(str){ str != "Moscow" }, cast: ->(str){ str.upase }
          optional :number, validator: /\d+/, default: 42
          optional :gears,  default: ->(obj){ choose_by(obj) }
          optional :weel_d, as: :weel_diameter
          optional :load,   cast: Float
        end

        def fetch params
          # .. do something useful with params
        end
      end

The main aim of this small gem is to implement base AOP (aspect oriented programming) to separate validation, conversion and setting defaults for methods from real businnes logic which should be the only body of that method without validators, convertors, default setters overhead.

In order to get this functionality you only need to include Parametron module into top of your class and after that describe desired incoming parameters for method.

    class Example
        include Parametron # <- should be at the top of class/module

        params_for(:fetch) do
            ...
        end
    end

Class method params_for accepts one or two arguments: first is symbolized name of method of interest and second one (optional) is hash of two options:

• strict which is defaultly set to false
• reject which is defaultly set to false too

strict when set to true means that when you call your method and try to send unknown (not described) key - it raise error Parametron::ExcessParameter reject when set to true reject undescribed keys from method arguments

    class ExampleStrict
        include Parametron # <- should be at the top of class/module

        params_for(:fetch, strict: true) do
            optional :one
        end

        def fetch par
        end
    end

    ex = ExampleStrict.new.fetch({unexpected: 'argument'})
    # => raises Parametron::ExcessParameter

    class ExampleReject
        include Parametron # <- should be at the top of class/module

        params_for(:fetch, reject: true) do
            optional :one
        end

        def fetch par
          par
        end
    end

    ex = ExampleReject.new.fetch({one: '1', unexpected: 'argument'})
    p ex
    # => {one: '1'}

Method params_for should provide description block where you describe required and optional parameters for given method:

    params_for(:fetch) do
      required :city,   validator: /\w+/
      required :year,   validator: /\d{4}/
      optional :title,  validator: ->(str){ str != "Moscow" }, cast: ->(str){ str.upase }
      optional :number, validator: /\d+/, default: 42
      optional :gears,  default: ->(obj){ choose_by(obj) }
      optional :weel_d, as: :weel_diameter
      optional :load,   cast: Float
    end

When parameter is required it must present in argument hash. Instead, parameter described as optional might present but don't must.

All of that parameter descriptors can have such refinements:

• validator with Regexp or proc predicat to be sure that input parameter is valid
• cast which could be one of: Integer, Float, proc to convert input parameter
• default to set input parameter to its default value in case if it not presents
• as to rename input parameter name to new one

When cast options is a lambda/proc you could have current input hash and (if need) original input hash. It is depends of arity of lambda you provide. If your cast lambda arity is 1 then it receive current value to cast; when its arity is 2 then it receive |current_value, current_casted_parameters| and when its arity is 3 it will get |current_value, current_casted_parameters, original_params| This could be handy when you want to have access to other parameters from input hash already casted.

Normally parametron raise exceptions when mandatory required parameter missed, when cast expect something other than it gets and so on. You could change this behavior if you need adding exception handler:

    params_for(:fetch) do
      required :city,   validator: /\w+/
      required :year,   validator: /\d{4}/
      on_exception ->(ex) {
        # return something other then exception `ex`
        return false
      }
    end

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 new Pull Request