Class: Contract

Inherits:
Contracts::Decorator show all
Defined in:
lib/contracts.rb

Overview

This is the main Contract class. When you write a new contract, you’ll write it as:

Contract [contract names] => return_value

This class also provides useful callbacks and a validation method.

Constant Summary collapse

DEFAULT_FAILURE_CALLBACK =

Default implementation of failure_callback. Provided as a block to be able to monkey patch #failure_callback only temporary and then switch it back. First important usage - for specs.

Proc.new do |data|
  raise data[:contracts].failure_exception.new(failure_msg(data), data)
end

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Methods inherited from Contracts::Decorator

inherited

Constructor Details

#initialize(klass, method, *contracts) ⇒ Contract

decorator_name :contract



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# File 'lib/contracts.rb', line 96

def initialize(klass, method, *contracts)
  if contracts[-1].is_a? Hash
    # internally we just convert that return value syntax back to an array
    @args_contracts = contracts[0, contracts.size - 1] + contracts[-1].keys
    @ret_contract = contracts[-1].values[0]
    @args_validators = @args_contracts.map do |contract|
      Contract.make_validator(contract)
    end
    @ret_validator = Contract.make_validator(@ret_contract)
  else
    fail "It looks like your contract for #{method} doesn't have a return value. A contract should be written as `Contract arg1, arg2 => return_value`."
  end
  @klass, @method= klass, method
  @has_func_contracts = args_contracts.index do |contract|
    contract.is_a? Contracts::Func
  end
end

Instance Attribute Details

#args_contracts ⇒ Object (readonly)

Returns the value of attribute args_contracts.



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# File 'lib/contracts.rb', line 94

def args_contracts
  @args_contracts
end

#klass ⇒ Object (readonly)

Returns the value of attribute klass.



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# File 'lib/contracts.rb', line 94

def klass
  @klass
end

#method ⇒ Object (readonly)

Returns the value of attribute method.



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# File 'lib/contracts.rb', line 94

def method
  @method
end

#ret_contract ⇒ Object (readonly)

Returns the value of attribute ret_contract.



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# File 'lib/contracts.rb', line 94

def ret_contract
  @ret_contract
end

Class Method Details

.failure_callback(data, use_pattern_matching = true) ⇒ Object

Callback for when a contract fails. By default it raises an error and prints detailed info about the contract that failed. You can also monkeypatch this callback to do whatever you want…log the error, send you an email, print an error message, etc.

Example of monkeypatching:

def Contract.failure_callback(data)
  puts "You had an error!"
  puts failure_msg(data)
  exit
end



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# File 'lib/contracts.rb', line 164

def self.failure_callback(data, use_pattern_matching=true)
  if data[:contracts].pattern_match? && use_pattern_matching
    return DEFAULT_FAILURE_CALLBACK.call(data)
  end

  fetch_failure_callback.call(data)
end

.failure_msg(data) ⇒ Object

Given a hash, prints out a failure message. This function is used by the default #failure_callback method and uses the hash passed into the failure_callback method.



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# File 'lib/contracts.rb', line 127

def self.failure_msg(data)
 expected = if data[:contract].to_s == "" || data[:contract].is_a?(Hash)
              data[:contract].inspect
            else
              data[:contract].to_s
            end

 position = Support.method_position(data[:method])
 method_name = Support.method_name(data[:method])

 header = if data[:return_value]
   "Contract violation for return value:"
 else
   "Contract violation for argument #{data[:arg_pos]} of #{data[:total_args]}:"
 end

%{#{header}
  Expected: #{expected},
  Actual: #{data[:arg].inspect}
  Value guarded in: #{data[:class]}::#{method_name}
  With Contract: #{data[:contracts]}
  At: #{position} }
end

.fetch_failure_callback ⇒ Object 



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# File 'lib/contracts.rb', line 192

def self.fetch_failure_callback
  @failure_callback ||= DEFAULT_FAILURE_CALLBACK
end

.make_validator(contract) ⇒ Object

This is a little weird. For each contract we pre-make a proc to validate it so we don’t have to go through this decision tree every time. Seems silly but it saves us a bunch of time (4.3sec vs 5.2sec)



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# File 'lib/contracts.rb', line 211

def self.make_validator(contract)
  # if is faster than case!
  klass = contract.class
  if klass == Proc
    # e.g. lambda {true}
    contract
  elsif klass == Array
    # e.g. [Num, String]
    # TODO account for these errors too
    lambda { |arg|
      return false unless arg.is_a?(Array) && arg.length == contract.length
      arg.zip(contract).all? do |_arg, _contract|
        Contract.valid?(_arg, _contract)
      end
    }
  elsif klass == Hash
    # e.g. { :a => Num, :b => String }
    lambda { |arg|
      return false unless arg.is_a?(Hash)
      contract.keys.all? do |k|
        Contract.valid?(arg[k], contract[k])
      end
    }
  elsif klass == Contracts::Args
    lambda { |arg|
      Contract.valid?(arg, contract.contract)
    }
  elsif klass == Contracts::Func
    lambda { |arg|
      arg.is_a?(Method) || arg.is_a?(Proc)
    }
  else
    # classes and everything else
    # e.g. Fixnum, Num
    if contract.respond_to? :valid?
      lambda { |arg| contract.valid?(arg) }
    elsif klass == Class
      lambda { |arg| arg.is_a?(contract) }
    else
      lambda { |arg| contract == arg }
    end
  end
end

.override_failure_callback(&blk) ⇒ Object

Used to override failure_callback without monkeypatching.

Takes: block parameter, that should accept one argument - data.

Example usage:

Contract.override_failure_callback do |data|
  puts "You had an error"
  puts failure_msg(data)
  exit
end



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# File 'lib/contracts.rb', line 183

def self.override_failure_callback(&blk)
  @failure_callback = blk
end

.restore_failure_callback ⇒ Object

Used to restore default failure callback



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# File 'lib/contracts.rb', line 188

def self.restore_failure_callback
  @failure_callback = DEFAULT_FAILURE_CALLBACK
end

.valid?(arg, contract) ⇒ Boolean

Used to verify if an argument satisfies a contract.

Takes: an argument and a contract.

Returns: a tuple: [Boolean, metadata]. The boolean indicates whether the contract was valid or not. If it wasn’t, metadata contains some useful information about the failure.

Returns:

  • (Boolean) 


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# File 'lib/contracts.rb', line 203

def self.valid?(arg, contract)
  make_validator(contract)[arg]
end

Instance Method Details

#[](*args, &blk) ⇒ Object 



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# File 'lib/contracts.rb', line 255

def [](*args, &blk)
  call(*args, &blk)
end

#call(*args, &blk) ⇒ Object 



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# File 'lib/contracts.rb', line 259

def call(*args, &blk)
  call_with(nil, *args, &blk)
end

#call_with(this, *args, &blk) ⇒ Object 



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# File 'lib/contracts.rb', line 263

def call_with(this, *args, &blk)

  _args = blk ? args + [blk] : args

  # check contracts on arguments
  # fun fact! This is significantly faster than .zip (3.7 secs vs 4.7 secs). Why??
  last_index = @args_validators.size - 1
  # times is faster than (0..args.size).each
  _args.size.times do |i|
    # this is done to account for extra args (for *args)
    j = i < last_index ? i : last_index
    #unless true #@args_contracts[i].valid?(args[i])
    unless @args_validators[j][_args[i]]
      call_function = Contract.failure_callback({:arg => _args[i], :contract => @args_contracts[j], :class => @klass, :method => @method, :contracts => self, :arg_pos => i+1, :total_args => _args.size})
      return unless call_function
    end
  end

  if @has_func_contracts
    # contracts on methods
    @args_contracts.each_with_index do |contract, i|
      if contract.is_a? Contracts::Func
      args[i] = Contract.new(@klass, args[i], *contract.contracts)
      end
    end
  end

  result = if @method.respond_to? :bind
    # instance method
    @method.bind(this).call(*args, &blk)
  else
    # class method
    @method.call(*args, &blk)
  end
  unless @ret_validator[result]
    Contract.failure_callback({:arg => result, :contract => @ret_contract, :class => @klass, :method => @method, :contracts => self, :return_value => true})
  end

  this.verify_invariants!(@method) if this.respond_to?(:verify_invariants!)

  result
end

#failure_exception ⇒ Object

Used to determine type of failure exception this contract should raise in case of failure



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# File 'lib/contracts.rb', line 307

def failure_exception
  if @pattern_match
    PatternMatchingError
  else
    ContractError
  end
end

#pattern_match! ⇒ Object

Used internally to mark contract as pattern matching contract



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# File 'lib/contracts.rb', line 317

def pattern_match!
  @pattern_match = true
end

#pattern_match? ⇒ Boolean

Used to determine if contract is a pattern matching contract

Returns:

  • (Boolean) 


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# File 'lib/contracts.rb', line 322

def pattern_match?
  @pattern_match
end

#pretty_contract(c) ⇒ Object 



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# File 'lib/contracts.rb', line 114

def pretty_contract c
  c.is_a?(Class) ? c.name : c.class.name
end

#to_s ⇒ Object 



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# File 'lib/contracts.rb', line 118

def to_s
  args = @args_contracts.map { |c| pretty_contract(c) }.join(", ")
  ret = pretty_contract(@ret_contract)
  ("#{args} => #{ret}").gsub("Contracts::", "")
end