Ruby中实现stream也是可以做到可惜就是没有尾递归优化
按照sicp首要是两个
:delay和force:def mem_proc(exp)
alread_run=false
result=false
lambda{
!alread_run
result=exp.call
alread_run=true
result
result
end
}
end
def force(delayed_object)
delayed_object.call
end
def delay(exp)
mem_proc(lambda{exp})
end delay
返回延时对象就是对于未来某个时间求值表达式承诺；force以延时对象为参数进行相应求值工作这里mem_proc用于记忆已经求值过表达式定义streamconstructor和selector:def cons_stream(a,b)
a,delay(b)
end
def stream_car(s)
s[0]
end
def stream_cdr(s)
force(s[1])
end
def stream_null?(s)
s.nil? or s
end 用Ruby中
充当“粘合剂”stream_car直接返回第
个元素而stream_cdr需要用force求值表达式履行承诺另外将空作为the-empty-stream再定义几个高阶map和foreach其他如filter和此类似:def stream_enumerate_
erval(low,high)
low>high
cons_stream(low,stream_enumerate_erval(low.succ,high))
end
end
def stream_ref(s,n)
n0
stream_car(s)
stream_ref(stream_cdr(s),(n-1))
end
end
def stream_map(proc,s)
stream_null?(s)
cons_stream(proc.call(stream_car(s)),stream_map(proc,(stream_cdr(s))))
end
end
def stream_for_each(proc,s)
stream_null?(s)
:done
proc.call(stream_car(s))
stream_for_each(proc,stream_cdr(s))
end
end
def display_stream(s)
stream_for_each(lambda{|item| puts item},s)
end
def stream_filter(pred,s)
stream_null?(s)
els pred.call(stream_car(s))
cons_stream(stream_car(s),stream_filter(pred,stream_cdr(s)))
stream_filter(pred,stream_cdr(s))
end
end
最后看下例子:puts "s:"
s=stream_enumerate_erval(1,5)
display_stream(s)
puts "odd_s:"
odd_s=stream_filter(lambda{|x| x%21},s)
display_stream(odd_s)
puts "ss:"
ss=stream_map(lambda{|x|x*x},s)
display_stream(ss)
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