带记忆的惰性斐波那契数列，取自尼拉吉·辛格 http://Neeraj.Name/2008/05/10/how-hash-works-with-block-in-ruby.html:
fibs = { 0 => 0, 1 => 1 }.tap do |fibs|
fibs.default_proc = ->(fibs, n) { fibs[n] = fibs[n-1] + fibs[n-2] }
end
fibs.take(10).map(&:last).each(&method(:puts))
计数排序的实现：
module Enumerable
def counting_sort(k)
reduce(Array.new(k+1, 0)) {|counting, n| counting.tap { counting[n] += 1 }}.
map.with_index {|count, n| [n] * count }.flatten
end
end
一个实现sum又名前缀和：
module Enumerable
def scan(initial=nil, sym=nil, &block)
args = if initial then [initial] else [] end
unless block_given?
args, sym, initial = [], initial, first unless sym
block = ->(acc, el) { acc.send(sym, el) }
end
[initial || first].tap {|res|
reduce(*args) {|acc, el|
block.(acc, el).tap {|e|
res << e
}
}
}
end
end
在这里，我尝试了Hash#each yield KeyValuePairs 而不是二元Arrays。相当令人惊讶，代码量有多少still在做了如此残酷的猴子补丁之后，工作正常了。耶，鸭子打字！
class Hash
KeyValuePair = Struct.new(:key, :value) do
def to_ary
return key, value
end
end
old_each = instance_method(:each)
define_method(:each) do |&blk|
old_each.bind(self).() do |k, v|
blk.(KeyValuePair.new(k, v))
end
end
end
我一直在玩的东西是Enumerable#===执行递归结构模式匹配。我不知道这是否有任何用处。我什至不知道它是否真的有效。
module Enumerable
def ===(other)
all? {|el|
next true if el.nil?
begin
other.any? {|other_el| el === other_el }
rescue NoMethodError => e
raise unless e.message =~ /any\?/
el === other
end
}
end
end
我最近玩弄的另一件事是重新实现所有方法Enumerable，但是使用reduce代替each作为基础。在这种情况下，我know它实际上不能正常工作。
module Enumerable
def all?
return reduce(true) {|res, el| break false unless res; res && el } unless block_given?
reduce(true) {|res, el| break false unless res; res && yield(el) }
end
def any?
return reduce(false) {|res, el| break true if res || el } unless block_given?
reduce(false) {|res, el| break true if res || yield(el) }
end
def collect
reduce([]) {|res, el| res << yield(el) }
end
alias_method :map, :collect
def count(item=undefined = Object.new)
return reduce(0) {|res, el| res + 1 if el == item } unless undefined.equal?(item)
unless block_given?
return size if respond_to? :size
return reduce(0) {|res, el| res + 1 }
end
reduce(0) {|res, el| res + 1 if yield el }
end
def detect(ifnone=nil)
reduce(ifnone) {|res, el| if yield el then el end unless res }
end
alias_method :find, :detect
def drop(n=1)
reduce([]) {|res, el| res.tap { res << el unless n -= 1 >= 0 }}
end
def drop_while
reduce([]) {|res, el| res.tap { res << el unless yield el }}
end
def each
tap { reduce(nil) {|_, el| yield el }}
end
def each_with_index
tap { reduce(-1) {|i, el| (i+1).tap {|i| yield el, i }}}
end
def find_all
reduce([]) {|res, el| res.tap {|res| res << el if yield el }}
end
alias_method :select, :find_all
def find_index(item=undefined = Object.new)
return reduce(-1) {|res, el| break res + 1 if el == item } unless undefined.equals?(item)
reduce(-1) {|res, el| break res + 1 if yield el }
end
def grep(pattern)
return reduce([]) {|res, el| res.tap {|res| res << el if pattern === el }} unless block_given?
reduce([]) {|res, el| res.tap {|res| res << yield(el) if pattern === el }}
end
def group_by
reduce(Hash.new {|hsh, key| hsh[key] = [] }) {|res, el| res.tap { res[yield el] = el }}
end
def include?(obj)
reduce(false) {|res, el| break true if res || el == obj }
end
def reject
reduce([]) {|res, el| res.tap {|res| res << el unless yield el }}
end
end
