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@sharplet
Created August 7, 2015 10:44
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Lazy `#reduce` and `#join` in Ruby
require "rspec/autorun"
class Enumerator::Lazy
def reduce(*)
Lazy.new([nil]) do |yielder, _|
yielder << super
end
end
def join(separator="")
reduce do |str, each|
"%s%s%s" % [str, separator, each]
end
end
end
RSpec.describe Enumerator::Lazy do
shared_examples_for "a lazy method" do
it "doesn't call its block" do
called = false
method.call do
called = true
end
expect(called).to be_falsy
end
end
subject { [1, 2, 3].lazy }
describe "#reduce" do
it_behaves_like "a lazy method" do
let(:method) { subject.public_method(:reduce) }
end
it "accepts the same number of arguments as Enumerable#reduce" do
enumerable = Object.new.extend(Enumerable).public_method(:reduce)
lazy = subject.public_method(:reduce)
expect(lazy.arity).to eq(enumerable.arity)
end
it "yields the result of calling reduce when forced" do
eager_sum = [1, 2, 3].reduce(0, :+)
lazy_sum = subject.reduce(0, :+)
expect(lazy_sum.first).to eq(eager_sum)
end
end
describe "#join" do
it_behaves_like "a lazy method" do
let(:method) { subject.public_method(:join) }
end
it "stringifies and joins values" do
expect(subject.join(",").first).to eq("1,2,3")
end
end
end
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