Enumerator::Lazy is a special type of Enumerator, that allows constructing chains of operations without evaluating them immediately, and evaluating values on as-needed basis. In order to do so it redefines most of Enumerable methods so that they just construct another lazy enumerator.
Enumerator::Lazy can be constructed from any Enumerable with the Enumerable#lazy method.
lazy = (1..Float::INFINITY).lazy.select(&:odd?).drop(10).take_while { |i| i < 30 } # => #<Enumerator::Lazy: #<Enumerator::Lazy: #<Enumerator::Lazy: #<Enumerator::Lazy: 1..Infinity>:select>:drop(10)>:take_while>
The real enumeration is performed when any non-redefined Enumerable method is called, like Enumerable#first or Enumerable#to_a (the latter is aliased as force for more semantic code):
lazy.first(2) #=> [21, 23] lazy.force #=> [21, 23, 25, 27, 29]
Note that most Enumerable methods that could be called with or without a block, on Enumerator::Lazy will always require a block:
[1, 2, 3].map #=> #<Enumerator: [1, 2, 3]:map> [1, 2, 3].lazy.map # ArgumentError: tried to call lazy map without a block
This class allows idiomatic calculations on long or infinite sequences, as well as chaining of calculations without constructing intermediate arrays.
Example for working with a slowly calculated sequence:
require 'open-uri' # This will fetch all URLs before selecting # necessary data URLS.map { |u| JSON.parse(URI.open(u).read) } .select { |data| data.key?('stats') } .first(5) # This will fetch URLs one-by-one, only till # there is enough data to satisfy the condition URLS.lazy.map { |u| JSON.parse(URI.open(u).read) } .select { |data| data.key?('stats') } .first(5)
Ending a chain with “.eager” generates a non-lazy enumerator, which is suitable for returning or passing to another method that expects a normal enumerator.
def active_items groups .lazy .flat_map(&:items) .reject(&:disabled) .eager end # This works lazily; if a checked item is found, it stops # iteration and does not look into remaining groups. first_checked = active_items.find(&:checked) # This returns an array of items like a normal enumerator does. all_checked = active_items.select(&:checked)
static VALUE
lazy_initialize(int argc, VALUE *argv, VALUE self)
{
VALUE obj, size = Qnil;
VALUE generator;
rb_check_arity(argc, 1, 2);
if (!rb_block_given_p()) {
rb_raise(rb_eArgError, "tried to call lazy new without a block");
}
obj = argv[0];
if (argc > 1) {
size = argv[1];
}
generator = generator_allocate(rb_cGenerator);
rb_block_call(generator, id_initialize, 0, 0, lazy_init_block_i, obj);
enumerator_init(self, generator, sym_each, 0, 0, 0, size, 0);
rb_ivar_set(self, id_receiver, obj);
return self;
}
Creates a new Lazy enumerator. When the enumerator is actually enumerated (e.g. by calling force), obj will be enumerated and each value passed to the given block. The block can yield values back using yielder. For example, to create a “filter+map” enumerator:
def filter_map(sequence) Lazy.new(sequence) do |yielder, *values| result = yield *values yielder << result if result end end filter_map(1..Float::INFINITY) {|i| i*i if i.even?}.first(5) #=> [4, 16, 36, 64, 100]
static VALUE
enumerator_with_index(int argc, VALUE *argv, VALUE obj)
{
VALUE memo;
rb_check_arity(argc, 0, 1);
RETURN_SIZED_ENUMERATOR(obj, argc, argv, enumerator_enum_size);
memo = (!argc || NIL_P(memo = argv[0])) ? INT2FIX(0) : rb_to_int(memo);
return enumerator_block_call(obj, enumerator_with_index_i, (VALUE)MEMO_NEW(memo, 0, 0));
}
Iterates the given block for each element with an index, which starts from offset. If no block is given, returns a new Enumerator that includes the index, starting from offset
offset-
the starting index to use
endif
static VALUE
lazy_super(int argc, VALUE *argv, VALUE lazy)
{
return enumerable_lazy(rb_call_super(argc, argv));
}
Like Enumerable#chunk, but chains operation to be lazy-evaluated.
static VALUE
lazy_compact(VALUE obj)
{
return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_compact_funcs);
}
Like Enumerable#compact, but chains operation to be lazy-evaluated.
static VALUE
lazy_drop(VALUE obj, VALUE n)
{
long len = NUM2LONG(n);
VALUE argv[2];
argv[0] = sym_each;
argv[1] = n;
if (len < 0) {
rb_raise(rb_eArgError, "attempt to drop negative size");
}
return lazy_add_method(obj, 2, argv, n, rb_ary_new3(1, n), &lazy_drop_funcs);
}
Like Enumerable#drop, but chains operation to be lazy-evaluated.
static VALUE
lazy_drop_while(VALUE obj)
{
if (!rb_block_given_p()) {
rb_raise(rb_eArgError, "tried to call lazy drop_while without a block");
}
return lazy_add_method(obj, 0, 0, Qfalse, Qnil, &lazy_drop_while_funcs);
}
Like Enumerable#drop_while, but chains operation to be lazy-evaluated.
static VALUE
lazy_eager(VALUE self)
{
return enumerator_init(enumerator_allocate(rb_cEnumerator),
self, sym_each, 0, 0, lazy_eager_size, Qnil, 0);
}
Returns a non-lazy Enumerator converted from the lazy enumerator.
static VALUE
lazy_filter_map(VALUE obj)
{
if (!rb_block_given_p()) {
rb_raise(rb_eArgError, "tried to call lazy filter_map without a block");
}
return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_filter_map_funcs);
}
Like Enumerable#filter_map, but chains operation to be lazy-evaluated.
(1..).lazy.filter_map { |i| i * 2 if i.even? }.first(5) #=> [4, 8, 12, 16, 20]
static VALUE
lazy_flat_map(VALUE obj)
{
if (!rb_block_given_p()) {
rb_raise(rb_eArgError, "tried to call lazy flat_map without a block");
}
return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_flat_map_funcs);
}
Returns a new lazy enumerator with the concatenated results of running block once for every element in the lazy enumerator.
["foo", "bar"].lazy.flat_map {|i| i.each_char.lazy}.force #=> ["f", "o", "o", "b", "a", "r"]
A value x returned by block is decomposed if either of the following conditions is true:
-
xresponds to both each and force, which means thatxis a lazy enumerator. -
xis an array or responds to to_ary.
Otherwise, x is contained as-is in the return value.
[{a:1}, {b:2}].lazy.flat_map {|i| i}.force
#=> [{:a=>1}, {:b=>2}]
static VALUE
lazy_grep(VALUE obj, VALUE pattern)
{
const lazyenum_funcs *const funcs = rb_block_given_p() ?
&lazy_grep_iter_funcs : &lazy_grep_funcs;
return lazy_add_method(obj, 0, 0, pattern, rb_ary_new3(1, pattern), funcs);
}
Like Enumerable#grep, but chains operation to be lazy-evaluated.
static VALUE
lazy_grep_v(VALUE obj, VALUE pattern)
{
const lazyenum_funcs *const funcs = rb_block_given_p() ?
&lazy_grep_v_iter_funcs : &lazy_grep_v_funcs;
return lazy_add_method(obj, 0, 0, pattern, rb_ary_new3(1, pattern), funcs);
}
Like Enumerable#grep_v, but chains operation to be lazy-evaluated.
static VALUE
lazy_lazy(VALUE obj)
{
return obj;
}
Returns self.
static VALUE
lazy_map(VALUE obj)
{
if (!rb_block_given_p()) {
rb_raise(rb_eArgError, "tried to call lazy map without a block");
}
return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_map_funcs);
}
Like Enumerable#map, but chains operation to be lazy-evaluated.
(1..Float::INFINITY).lazy.map {|i| i**2 } #=> #<Enumerator::Lazy: #<Enumerator::Lazy: 1..Infinity>:map> (1..Float::INFINITY).lazy.map {|i| i**2 }.first(3) #=> [1, 4, 9]
static VALUE
lazy_reject(VALUE obj)
{
if (!rb_block_given_p()) {
rb_raise(rb_eArgError, "tried to call lazy reject without a block");
}
return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_reject_funcs);
}
Like Enumerable#reject, but chains operation to be lazy-evaluated.
static VALUE
lazy_select(VALUE obj)
{
if (!rb_block_given_p()) {
rb_raise(rb_eArgError, "tried to call lazy select without a block");
}
return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_select_funcs);
}
Like Enumerable#select, but chains operation to be lazy-evaluated.
static VALUE
lazy_take(VALUE obj, VALUE n)
{
long len = NUM2LONG(n);
int argc = 0;
VALUE argv[2];
if (len < 0) {
rb_raise(rb_eArgError, "attempt to take negative size");
}
if (len == 0) {
argv[0] = sym_cycle;
argv[1] = INT2NUM(0);
argc = 2;
}
return lazy_add_method(obj, argc, argv, n, rb_ary_new3(1, n), &lazy_take_funcs);
}
Like Enumerable#take, but chains operation to be lazy-evaluated.
static VALUE
lazy_take_while(VALUE obj)
{
if (!rb_block_given_p()) {
rb_raise(rb_eArgError, "tried to call lazy take_while without a block");
}
return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_take_while_funcs);
}
Like Enumerable#take_while, but chains operation to be lazy-evaluated.
static VALUE lazy_to_a(VALUE self)
{
}
Expands lazy enumerator to an array. See Enumerable#to_a.
static VALUE
lazy_to_enum(int argc, VALUE *argv, VALUE self)
{
VALUE lazy, meth = sym_each, super_meth;
if (argc > 0) {
--argc;
meth = *argv++;
}
if (RTEST((super_meth = rb_hash_aref(lazy_use_super_method, meth)))) {
meth = super_meth;
}
lazy = lazy_to_enum_i(self, meth, argc, argv, 0, rb_keyword_given_p());
if (rb_block_given_p()) {
enumerator_ptr(lazy)->size = rb_block_proc();
}
return lazy;
}
Similar to Object#to_enum, except it returns a lazy enumerator. This makes it easy to define Enumerable methods that will naturally remain lazy if called from a lazy enumerator.
For example, continuing from the example in Object#to_enum:
# See Object#to_enum for the definition of repeat r = 1..Float::INFINITY r.repeat(2).first(5) # => [1, 1, 2, 2, 3] r.repeat(2).class # => Enumerator r.repeat(2).map{|n| n ** 2}.first(5) # => endless loop! # works naturally on lazy enumerator: r.lazy.repeat(2).class # => Enumerator::Lazy r.lazy.repeat(2).map{|n| n ** 2}.first(5) # => [1, 1, 4, 4, 9]
static VALUE
lazy_uniq(VALUE obj)
{
const lazyenum_funcs *const funcs =
rb_block_given_p() ? &lazy_uniq_iter_funcs : &lazy_uniq_funcs;
return lazy_add_method(obj, 0, 0, Qnil, Qnil, funcs);
}
Like Enumerable#uniq, but chains operation to be lazy-evaluated.
static VALUE
lazy_with_index(int argc, VALUE *argv, VALUE obj)
{
VALUE memo;
rb_scan_args(argc, argv, "01", &memo);
if (NIL_P(memo))
memo = LONG2NUM(0);
return lazy_add_method(obj, 0, 0, memo, rb_ary_new_from_values(1, &memo), &lazy_with_index_funcs);
}
If a block is given, returns a lazy enumerator that will iterate over the given block for each element with an index, which starts from offset, and returns a lazy enumerator that yields the same values (without the index).
If a block is not given, returns a new lazy enumerator that includes the index, starting from offset.
offset-
the starting index to use
static VALUE
lazy_zip(int argc, VALUE *argv, VALUE obj)
{
VALUE ary, v;
long i;
const lazyenum_funcs *funcs = &lazy_zip_funcs[1];
if (rb_block_given_p()) {
return rb_call_super(argc, argv);
}
ary = rb_ary_new2(argc);
for (i = 0; i < argc; i++) {
v = rb_check_array_type(argv[i]);
if (NIL_P(v)) {
for (; i < argc; i++) {
if (!rb_respond_to(argv[i], id_each)) {
rb_raise(rb_eTypeError, "wrong argument type %"PRIsVALUE" (must respond to :each)",
rb_obj_class(argv[i]));
}
}
ary = rb_ary_new4(argc, argv);
funcs = &lazy_zip_funcs[0];
break;
}
rb_ary_push(ary, v);
}
return lazy_add_method(obj, 0, 0, ary, ary, funcs);
}
Like Enumerable#zip, but chains operation to be lazy-evaluated. However, if a block is given to zip, values are enumerated immediately.