流是通過延時(shí)求值實(shí)現(xiàn)的，Ruby中實(shí)現(xiàn)stream也是可以做到，可惜就是沒有尾遞歸優(yōu)化。按照sicp，首要的是兩個(gè)函數(shù)：delay和force：
def mem_proc(exp)
alread_run=false
result=false
lambda{
if !alread_run
result=exp.call
alread_run=true
result
else
result
end
}
end
def force(delayed_object)
delayed_object.call
end
def delay(exp)
mem_proc(lambda{exp})
end
delay函數(shù)返回延時(shí)對(duì)象，就是對(duì)于未來某個(gè)時(shí)間求值表達(dá)式的承諾；force函數(shù)以延時(shí)對(duì)象為參數(shù)，進(jìn)行相應(yīng)的求值工作，這里的mem_proc用于記憶已經(jīng)求值過的表達(dá)式。定義stream的constructor和selector函數(shù)：
def cons_stream(a,b)
return 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中的數(shù)組充當(dāng)“粘合劑”，stream_car直接返回第一個(gè)元素，而stream_cdr需要用force求值表達(dá)式，履行承諾。另外，將空數(shù)組[]作為the-empty-stream。再定義幾個(gè)高階函數(shù)，map和foreach，其他如filter與此類似：
def stream_enumerate_interval(low,high)
if low>high
return []
else
cons_stream(low,stream_enumerate_interval(low.succ,high))
end
end
def stream_ref(s,n)
if n==0
stream_car(s)
else
stream_ref(stream_cdr(s),(n-1))
end
end
def stream_map(proc,s)
if stream_null?(s)
[]
else
cons_stream(proc.call(stream_car(s)),stream_map(proc,(stream_cdr(s))))
end
end
def stream_for_each(proc,s)
if stream_null?(s)
:done
else
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)
if stream_null?(s)
[]
elsif pred.call(stream_car(s))
cons_stream(stream_car(s),stream_filter(pred,stream_cdr(s)))
else
stream_filter(pred,stream_cdr(s))
end
end
最后，看下例子：
puts "s:"
s=stream_enumerate_interval(1,5)
display_stream(s)
puts "odd_s:"
odd_s=stream_filter(lambda{|x| x%2==1},s)
display_stream(odd_s)
puts "ss:"
ss=stream_map(lambda{|x|x*x},s)
display_stream(ss)