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Class Data provides a convenient way to define simple classes for value-alike objects.

The simplest example of usage:

Measure = Data.define(:amount, :unit)

# Positional arguments constructor is provided
distance = Measure.new(100, 'km')
#=> #<data Measure amount=100, unit="km">

# Keyword arguments constructor is provided
weight = Measure.new(amount: 50, unit: 'kg')
#=> #<data Measure amount=50, unit="kg">

# Alternative form to construct an object:
speed = Measure[10, 'mPh']
#=> #<data Measure amount=10, unit="mPh">

# Works with keyword arguments, too:
area = Measure[amount: 1.5, unit: 'm^2']
#=> #<data Measure amount=1.5, unit="m^2">

# Argument accessors are provided:
distance.amount #=> 100
distance.unit #=> "km"

Constructed object also has a reasonable definitions of == operator, to_h hash conversion, and deconstruct / deconstruct_keys to be used in pattern matching.

::define method accepts an optional block and evaluates it in the context of the newly defined class. That allows to define additional methods:

Measure = Data.define(:amount, :unit) do
  def <=>(other)
    return unless other.is_a?(self.class) && other.unit == unit
    amount <=> other.amount
  end

  include Comparable
end

Measure[3, 'm'] < Measure[5, 'm'] #=> true
Measure[3, 'm'] < Measure[5, 'kg']
# comparison of Measure with Measure failed (ArgumentError)

Data provides no member writers, or enumerators: it is meant to be a storage for immutable atomic values. But note that if some of data members is of a mutable class, Data does no additional immutability enforcement:

Event = Data.define(:time, :weekdays)
event = Event.new('18:00', %w[Tue Wed Fri])
#=> #<data Event time="18:00", weekdays=["Tue", "Wed", "Fri"]>

# There is no #time= or #weekdays= accessors, but changes are
# still possible:
event.weekdays << 'Sat'
event
#=> #<data Event time="18:00", weekdays=["Tue", "Wed", "Fri", "Sat"]>

See also Struct, which is a similar concept, but has more container-alike API, allowing to change contents of the object and enumerate it.

Methods
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Class Public methods

define(*symbols) → class

Defines a new Data class.

measure = Data.define(:amount, :unit)
#=> #<Class:0x00007f70c6868498>
measure.new(1, 'km')
#=> #<data amount=1, unit="km">

# It you store the new class in the constant, it will
# affect #inspect and will be more natural to use:
Measure = Data.define(:amount, :unit)
#=> Measure
Measure.new(1, 'km')
#=> #<data Measure amount=1, unit="km">

Note that member-less Data is acceptable and might be a useful technique for defining several homogenous data classes, like

class HTTPFetcher
  Response = Data.define(:body)
  NotFound = Data.define
  # ... implementation
end

Now, different kinds of responses from HTTPFetcher would have consistent representation:

#<data HTTPFetcher::Response body="<html...">
#<data HTTPFetcher::NotFound>

And are convenient to use in pattern matching:

case fetcher.get(url)
in HTTPFetcher::Response(body)
  # process body variable
in HTTPFetcher::NotFound
  # handle not found case
end

Source: show 

static VALUE
rb_data_s_def(int argc, VALUE *argv, VALUE klass)
{
    VALUE rest;
    long i;
    VALUE data_class;

    rest = rb_ident_hash_new();
    RBASIC_CLEAR_CLASS(rest);
    for (i=0; i<argc; i++) {
        VALUE mem = rb_to_symbol(argv[i]);
        if (rb_is_attrset_sym(mem)) {
            rb_raise(rb_eArgError, "invalid data member: %"PRIsVALUE, mem);
        }
        if (RTEST(rb_hash_has_key(rest, mem))) {
            rb_raise(rb_eArgError, "duplicate member: %"PRIsVALUE, mem);
        }
        rb_hash_aset(rest, mem, Qtrue);
    }
    rest = rb_hash_keys(rest);
    RBASIC_CLEAR_CLASS(rest);
    OBJ_FREEZE_RAW(rest);
    data_class = anonymous_struct(klass);
    setup_data(data_class, rest);
    if (rb_block_given_p()) {
        rb_mod_module_eval(0, 0, data_class);
    }

    return data_class;
}

DataClass::members → array_of_symbols

Returns an array of member names of the data class:

Measure = Data.define(:amount, :unit)
Measure.members # => [:amount, :unit]

Source: show 

#define rb_data_s_members_m rb_struct_s_members_m

new(*args) → instance
new(**kwargs) → instance
::[](*args) → instance
::[](**kwargs) → instance

Constructors for classes defined with ::define accept both positional and keyword arguments.

Measure = Data.define(:amount, :unit)

Measure.new(1, 'km')
#=> #<data Measure amount=1, unit="km">
Measure.new(amount: 1, unit: 'km')
#=> #<data Measure amount=1, unit="km">

# Alternative shorter initialization with []
Measure[1, 'km']
#=> #<data Measure amount=1, unit="km">
Measure[amount: 1, unit: 'km']
#=> #<data Measure amount=1, unit="km">

All arguments are mandatory (unlike Struct), and converted to keyword arguments:

Measure.new(amount: 1)
# in `initialize': missing keyword: :unit (ArgumentError)

Measure.new(1)
# in `initialize': missing keyword: :unit (ArgumentError)

Note that Measure#initialize always receives keyword arguments, and that mandatory arguments are checked in initialize, not in new. This can be important for redefining initialize in order to convert arguments or provide defaults:

Measure = Data.define(:amount, :unit) do
  NONE = Data.define

  def initialize(amount:, unit: NONE.new)
    super(amount: Float(amount), unit:)
  end
end

Measure.new('10', 'km') # => #<data Measure amount=10.0, unit="km">
Measure.new(10_000)     # => #<data Measure amount=10000.0, unit=#<data NONE>>

Source: show 

static VALUE
rb_data_initialize_m(int argc, const VALUE *argv, VALUE self)
{
    VALUE klass = rb_obj_class(self);
    rb_struct_modify(self);
    VALUE members = struct_ivar_get(klass, id_members);
    size_t num_members = RARRAY_LEN(members);

    if (argc == 0) {
        if (num_members > 0) {
            rb_exc_raise(rb_keyword_error_new("missing", members));
        }
        return Qnil;
    }
    if (argc > 1 || !RB_TYPE_P(argv[0], T_HASH)) {
        rb_error_arity(argc, 0, 0);
    }

    if (RHASH_SIZE(argv[0]) < num_members) {
        VALUE missing = rb_ary_diff(members, rb_hash_keys(argv[0]));
        rb_exc_raise(rb_keyword_error_new("missing", missing));
    }

    struct struct_hash_set_arg arg;
    rb_mem_clear((VALUE *)RSTRUCT_CONST_PTR(self), num_members);
    arg.self = self;
    arg.unknown_keywords = Qnil;
    rb_hash_foreach(argv[0], struct_hash_set_i, (VALUE)&arg);
    // Freeze early before potentially raising, so that we don't leave an
    // unfrozen copy on the heap, which could get exposed via ObjectSpace.
    OBJ_FREEZE_RAW(self);
    if (arg.unknown_keywords != Qnil) {
        rb_exc_raise(rb_keyword_error_new("unknown", arg.unknown_keywords));
    }
    return Qnil;
}

Instance Public methods

self == other → true or false

Returns true if other is the same class as self, and all members are equal.

Examples:

Measure = Data.define(:amount, :unit)

Measure[1, 'km'] == Measure[1, 'km'] #=> true
Measure[1, 'km'] == Measure[2, 'km'] #=> false
Measure[1, 'km'] == Measure[1, 'm']  #=> false

Measurement = Data.define(:amount, :unit)
# Even though Measurement and Measure have the same "shape"
# their instances are never equal
Measure[1, 'km'] == Measurement[1, 'km'] #=> false

Source: show 

#define rb_data_equal rb_struct_equal

deconstruct → array

Returns the values in self as an array, to use in pattern matching:

Measure = Data.define(:amount, :unit)

distance = Measure[10, 'km']
distance.deconstruct #=> [10, "km"]

# usage
case distance
in n, 'km' # calls #deconstruct underneath
  puts "It is #{n} kilometers away"
else
  puts "Don't know how to handle it"
end
# prints "It is 10 kilometers away"

Or, with checking the class, too:

case distance
in Measure(n, 'km')
  puts "It is #{n} kilometers away"
# ...
end

Source: show 

#define rb_data_deconstruct rb_struct_to_a

deconstruct_keys(array_of_names_or_nil) → hash

Returns a hash of the name/value pairs, to use in pattern matching.

Measure = Data.define(:amount, :unit)

distance = Measure[10, 'km']
distance.deconstruct_keys(nil) #=> {:amount=>10, :unit=>"km"}
distance.deconstruct_keys([:amount]) #=> {:amount=>10}

# usage
case distance
in amount:, unit: 'km' # calls #deconstruct_keys underneath
  puts "It is #{amount} kilometers away"
else
  puts "Don't know how to handle it"
end
# prints "It is 10 kilometers away"

Or, with checking the class, too:

case distance
in Measure(amount:, unit: 'km')
  puts "It is #{amount} kilometers away"
# ...
end

Source: show 

#define rb_data_deconstruct_keys rb_struct_deconstruct_keys

self.eql?(other) → true or false

Equality check that is used when two items of data are keys of a Hash.

The subtle difference with == is that members are also compared with their eql? method, which might be important in some cases:

Measure = Data.define(:amount, :unit)

Measure[1, 'km'] == Measure[1.0, 'km'] #=> true, they are equal as values
# ...but...
Measure[1, 'km'].eql? Measure[1.0, 'km'] #=> false, they represent different hash keys

See also Object#eql? for further explanations of the method usage.

Source: show 

#define rb_data_eql rb_struct_eql

hash → integer

Redefines Object#hash (used to distinguish objects as Hash keys) so that data objects of the same class with same content would have the same hash value, and represented the same Hash key.

Measure = Data.define(:amount, :unit)

Measure[1, 'km'].hash == Measure[1, 'km'].hash #=> true
Measure[1, 'km'].hash == Measure[10, 'km'].hash #=> false
Measure[1, 'km'].hash == Measure[1, 'm'].hash #=> false
Measure[1, 'km'].hash == Measure[1.0, 'km'].hash #=> false

# Structurally similar data class, but shouldn't be considered
# the same hash key
Measurement = Data.define(:amount, :unit)

Measure[1, 'km'].hash == Measurement[1, 'km'].hash #=> false

Source: show 

#define rb_data_hash rb_struct_hash

inspect → string

Returns a string representation of self:

Measure = Data.define(:amount, :unit)

distance = Measure[10, 'km']

p distance  # uses #inspect underneath
#<data Measure amount=10, unit="km">

puts distance  # uses #to_s underneath, same representation
#<data Measure amount=10, unit="km">
Also aliased as: to_s

Source: show 

static VALUE
rb_data_inspect(VALUE s)
{
    return rb_exec_recursive(inspect_struct, s, rb_str_new2("#<data "));
}

members → array_of_symbols

Returns the member names from self as an array:

Measure = Data.define(:amount, :unit)
distance = Measure[10, 'km']

distance.members #=> [:amount, :unit]

Source: show 

#define rb_data_members_m rb_struct_members_m

to_h → hash
to_h {|name, value| ... } → hash

Returns Hash representation of the data object.

Measure = Data.define(:amount, :unit)
distance = Measure[10, 'km']

distance.to_h
#=> {:amount=>10, :unit=>"km"}

Like Enumerable#to_h, if the block is provided, it is expected to produce key-value pairs to construct a hash:

distance.to_h { |name, val| [name.to_s, val.to_s] }
#=> {"amount"=>"10", "unit"=>"km"}

Note that there is a useful symmetry between to_h and initialize:

distance2 = Measure.new(**distance.to_h)
#=> #<data Measure amount=10, unit="km">
distance2 == distance
#=> true

Source: show 

#define rb_data_to_h rb_struct_to_h

to_s → string

Returns a string representation of self:

Measure = Data.define(:amount, :unit)

distance = Measure[10, 'km']

p distance  # uses #inspect underneath
#<data Measure amount=10, unit="km">

puts distance  # uses #to_s underneath, same representation
#<data Measure amount=10, unit="km">
Alias for: inspect

with(**kwargs) → instance

Returns a shallow copy of self — the instance variables of self are copied, but not the objects they reference.

If the method is supplied any keyword arguments, the copy will be created with the respective field values updated to use the supplied keyword argument values. Note that it is an error to supply a keyword that the Data class does not have as a member.

Point = Data.define(:x, :y)

origin = Point.new(x: 0, y: 0)

up = origin.with(x: 1)
right = origin.with(y: 1)
up_and_right = up.with(y: 1)

p origin       # #<data Point x=0, y=0>
p up           # #<data Point x=1, y=0>
p right        # #<data Point x=0, y=1>
p up_and_right # #<data Point x=1, y=1>

out = origin.with(z: 1) # ArgumentError: unknown keyword: :z
some_point = origin.with(1, 2) # ArgumentError: expected keyword arguments, got positional arguments

Source: show 

static VALUE
rb_data_with(int argc, const VALUE *argv, VALUE self)
{
    VALUE kwargs;
    rb_scan_args(argc, argv, "0:", &kwargs);
    if (NIL_P(kwargs)) {
        return self;
    }

    VALUE h = rb_struct_to_h(self);
    rb_hash_update_by(h, kwargs, 0);
    return rb_class_new_instance_kw(1, &h, rb_obj_class(self), TRUE);
}