class Contract

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.

For make_validator and related logic see file lib/contracts/validators.rb For call_with and related logic see file lib/contracts/call_with.rb

Constants

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.

Attributes

args_contracts[R]

kargs_contract[R]

klass[R]

method[R]

ret_contract[R]

Public Class Methods

failure_callback(data, use_pattern_matching: true) click to toggle source

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

# File lib/contracts.rb, line 188
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) click to toggle source

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.

# File lib/contracts.rb, line 118
def self.failure_msg(data)
  indent_amount = 8
  method_name = Contracts::Support.method_name(data[:method])

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

  # Expected
  expected_prefix = "Expected: "
  expected_value = Contracts::Support.indent_string(
    Contracts::Formatters::Expected.new(data[:contract]).contract.pretty_inspect,
    expected_prefix.length,
  ).strip
  expected_line = "#{expected_prefix}#{expected_value},"

  # Actual
  actual_prefix = "Actual: "
  actual_value = Contracts::Support.indent_string(
    data[:arg].pretty_inspect,
    actual_prefix.length,
  ).strip
  actual_line = actual_prefix + actual_value

  # Value guarded in
  value_prefix = "Value guarded in: "
  value_value = "#{data[:class]}::#{method_name}"
  value_line = value_prefix + value_value

  # Contract
  contract_prefix = "With Contract: "
  contract_value = data[:contracts].to_s
  contract_line = contract_prefix + contract_value

  # Position
  position_prefix = "At: "
  position_value = Contracts::Support.method_position(data[:method])
  position_line = position_prefix + position_value

  [
    header,
    Contracts::Support.indent_string(
      [
        expected_line,
        actual_line,
        value_line,
        contract_line,
        position_line,
      ].join("\n"),
      indent_amount,
    ),
  ].join("\n")
end

fetch_failure_callback() click to toggle source

# File lib/contracts.rb, line 216
def self.fetch_failure_callback
  @failure_callback ||= DEFAULT_FAILURE_CALLBACK
end

new(klass, method, *contracts) click to toggle source

Calls superclass method Contracts::Decorator::new

# File lib/contracts.rb, line 58
def initialize(klass, method, *contracts)
  super(klass, method)
  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
  # Extract contract for keyword arguments
  @kargs_contract = args_contracts.find { |c| c.is_a?(Contracts::Builtin::KeywordArgs) }
  args_contracts.delete(kargs_contract) if kargs_contract

  @ret_contract = contracts[-1].values[0]

  @args_validators = args_contracts.map do |contract|
    Contract.make_validator(contract)
  end

  @kargs_validator = kargs_contract ? Contract.make_validator(kargs_contract) : nil

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

  @ret_validator = Contract.make_validator(ret_contract)

  @pattern_match = false

  # == @has_proc_contract
  last_contract = args_contracts.last
  is_a_proc = last_contract.is_a?(Class) && (last_contract <= Proc || last_contract <= Method)
  maybe_a_proc = last_contract.is_a?(Contracts::Maybe) && last_contract.include_proc?

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

  # ====

  @klass, @method = klass, method
end

override_failure_callback(&blk) click to toggle source

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

# File lib/contracts.rb, line 207
def self.override_failure_callback(&blk)
  @failure_callback = blk
end

restore_failure_callback() click to toggle source

Used to restore default failure callback

# File lib/contracts.rb, line 212
def self.restore_failure_callback
  @failure_callback = DEFAULT_FAILURE_CALLBACK
end

valid?(arg, contract) click to toggle source

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.

# File lib/contracts.rb, line 227
def self.valid?(arg, contract)
  make_validator(contract)[arg]
end

Public Instance Methods

[](*args, &blk) click to toggle source

# File lib/contracts.rb, line 231
def [](*args, &blk)
  call(*args, &blk)
end

call(*args, &blk) click to toggle source

# File lib/contracts.rb, line 235
def call(*args, &blk)
  call_with(nil, *args, &blk)
end

failure_exception() click to toggle source

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

# File lib/contracts.rb, line 255
def failure_exception
  if pattern_match?
    PatternMatchingError
  else
    ParamContractError
  end
end

maybe_append_block!(args, blk) click to toggle source

a better way to handle this might be to take this into account before throwing a “mismatched # of args” error. returns true if it appended nil

# File lib/contracts.rb, line 246
def maybe_append_block! args, blk
  return false unless @has_proc_contract && !blk &&
    (@args_contract_index || args.size < args_contracts.size)

  args << nil
  true
end

pattern_match!() click to toggle source

@private Used internally to mark contract as pattern matching contract

# File lib/contracts.rb, line 265
def pattern_match!
  @pattern_match = true
end

pattern_match?() click to toggle source

Used to determine if contract is a pattern matching contract

# File lib/contracts.rb, line 270
def pattern_match?
  @pattern_match == true
end

pretty_contract(contract) click to toggle source

# File lib/contracts.rb, line 105
def pretty_contract contract
  contract.is_a?(Class) ? contract.name : contract.class.name
end

to_s() click to toggle source

# File lib/contracts.rb, line 109
def to_s
  args = args_contracts.map { |c| pretty_contract(c) }.join(", ")
  ret = pretty_contract(ret_contract)
  ("#{args} => #{ret}").gsub("Contracts::Builtin::", "")
end