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 do |data|
  if data[:return_value]
    # this failed on the return contract
    fail ReturnContractError.new(failure_msg(data), data)
  else
    # this failed for a param contract
    fail data[:contracts].failure_exception.new(failure_msg(data), data)
  end
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

Returns a new instance of Contract.


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# File 'lib/contracts.rb', line 86

def initialize(klass, method, *contracts)
  unless contracts.last.is_a?(Hash)
    unless contracts.one?
      fail %{
        It looks like your contract for #{method.name} doesn't have a return
        value. A contract should be written as `Contract arg1, arg2 =>
        return_value`.
      }.strip
    end
    contracts = [nil => contracts[-1]]
  end

  # 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

  @args_contract_index = args_contracts.index do |contract|
    contract.is_a? Contracts::Args
  end

  @ret_validator = Contract.make_validator(ret_contract)

  # == @has_proc_contract
  last_contract = args_contracts.last
  is_a_proc = last_contract.is_a?(Class) && (last_contract <= Proc || last_contract <= Method)

  @has_proc_contract = is_a_proc || last_contract.is_a?(Contracts::Func)
  # ====

  # == @has_options_contract
  last_contract = args_contracts.last
  penultimate_contract = args_contracts[-2]
  @has_options_contract = if @has_proc_contract
                            penultimate_contract.is_a?(Hash)
                          else
                            last_contract.is_a?(Hash)
                          end
  # ===

  @klass, @method = klass, method
end

Instance Attribute Details

#args_contractsObject (readonly)

Returns the value of attribute args_contracts.


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# File 'lib/contracts.rb', line 85

def args_contracts
  @args_contracts
end

#klassObject (readonly)

Returns the value of attribute klass.


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# File 'lib/contracts.rb', line 85

def klass
  @klass
end

#methodObject (readonly)

Returns the value of attribute method.


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# File 'lib/contracts.rb', line 85

def method
  @method
end

#ret_contractObject (readonly)

Returns the value of attribute ret_contract.


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# File 'lib/contracts.rb', line 85

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 178

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 146

def self.failure_msg(data)
  expected = Contracts::Formatters::Expected.new(data[:contract]).contract
  position = Contracts::Support.method_position(data[:method])
  method_name = Contracts::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_callbackObject


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# File 'lib/contracts.rb', line 206

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 225

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 do |arg|
      return false unless arg.is_a?(Array) && arg.length == contract.length
      arg.zip(contract).all? do |_arg, _contract|
        Contract.valid?(_arg, _contract)
      end
    end
  elsif klass == Hash
    # e.g. { :a => Num, :b => String }
    lambda do |arg|
      return false unless arg.is_a?(Hash)
      contract.keys.all? do |k|
        Contract.valid?(arg[k], contract[k])
      end
    end
  elsif klass == Contracts::Args
    lambda do |arg|
      Contract.valid?(arg, contract.contract)
    end
  elsif klass == Contracts::Func
    lambda do |arg|
      arg.is_a?(Method) || arg.is_a?(Proc)
    end
  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 197

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

.restore_failure_callbackObject

Used to restore default failure callback


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# File 'lib/contracts.rb', line 202

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 217

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 269

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

#call(*args, &blk) ⇒ Object


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# File 'lib/contracts.rb', line 273

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 299

def call_with(this, *args, &blk)
  args << blk if blk

  # Explicitly append blk=nil if nil != Proc contract violation anticipated
  maybe_append_block!(args, blk)

  # Explicitly append options={} if Hash contract is present
  maybe_append_options!(args, blk)

  # Loop forward validating the arguments up to the splat (if there is one)
  (@args_contract_index || args.size).times do |i|
    contract = args_contracts[i]
    arg = args[i]
    validator = @args_validators[i]

    unless validator && validator[arg]
      return unless Contract.failure_callback(:arg => arg,
                                              :contract => contract,
                                              :class => klass,
                                              :method => method,
                                              :contracts => self,
                                              :arg_pos => i+1,
                                              :total_args => args.size,
                                              :return_value => false)
    end

    if contract.is_a?(Contracts::Func)
      args[i] = Contract.new(klass, arg, *contract.contracts)
    end
  end

  # If there is a splat loop backwards to the lower index of the splat
  # Once we hit the splat in this direction set its upper index
  # Keep validating but use this upper index to get the splat validator.
  if @args_contract_index
    splat_upper_index = @args_contract_index
    (args.size - @args_contract_index).times do |i|
      arg = args[args.size - 1 - i]

      if args_contracts[args_contracts.size - 1 - i].is_a?(Contracts::Args)
        splat_upper_index = i
      end

      # Each arg after the spat is found must use the splat validator
      j = i < splat_upper_index ? i : splat_upper_index
      contract = args_contracts[args_contracts.size - 1 - j]
      validator = @args_validators[args_contracts.size - 1 - j]

      unless validator && validator[arg]
        return unless Contract.failure_callback(:arg => arg,
                                                :contract => contract,
                                                :class => klass,
                                                :method => method,
                                                :contracts => self,
                                                :arg_pos => i-1,
                                                :total_args => args.size,
                                                :return_value => false)
      end

      if contract.is_a?(Contracts::Func)
        args[args.size - 1 - i] = Contract.new(klass, arg, *contract.contracts)
      end
    end
  end

  # If we put the block into args for validating, restore the args
  args.slice!(-1) if blk
  result = if method.respond_to?(:call)
             # proc, block, lambda, etc
             method.call(*args, &blk)
           else
             # original method name referrence
             method.send_to(this, *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_exceptionObject

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


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# File 'lib/contracts.rb', line 389

def failure_exception
  if @pattern_match
    PatternMatchingError
  else
    ParamContractError
  end
end

#maybe_append_block!(args, blk) ⇒ Object

a better way to handle this might be to take this into account before throwing a “mismatched # of args” error.


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# File 'lib/contracts.rb', line 283

def maybe_append_block! args, blk
  return unless @has_proc_contract && !blk &&
      (@args_contract_index || args.size < args_contracts.size)
  args << nil
end

#maybe_append_options!(args, blk) ⇒ Object

Same thing for when we have named params but didn't pass any in.


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# File 'lib/contracts.rb', line 290

def maybe_append_options! args, blk
  return unless @has_options_contract
  if @has_proc_contract && args_contracts[-2].is_a?(Hash) && !args[-2].is_a?(Hash)
    args.insert(-2, {})
  elsif args_contracts[-1].is_a?(Hash) && !args[-1].is_a?(Hash)
    args << {}
  end
end

#pattern_match!Object

Used internally to mark contract as pattern matching contract


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# File 'lib/contracts.rb', line 399

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 404

def pattern_match?
  @pattern_match
end

#pretty_contract(c) ⇒ Object


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# File 'lib/contracts.rb', line 133

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

#to_sObject


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# File 'lib/contracts.rb', line 137

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