我正在通过Chris Allen Haskell编程书来学习Haskell。我被困在使用scanl和自引用列表定义Fibonacci系列的一部分上。 我了解对以下代码的评估：
scanl (+) 1 [1..3]
1 : (scanl (+) ((+) 1 1) [2,3])
我读到（+）在两个参数上都是严格的，因此下一步将（（+）1 1）替换为2。
1 : (2 : (scanl (+) ((+) 2 2) [3]))
1 : (2 : (4 : (scanl (+) ((+) 4 3) [])))
1 : (2 : (4 : (7 : (scanl (+) 7 []))))
1 : (2 : (4 : (7 : [])))
我无法理解以下代码的评估以及它如何产生正确的斐波那契数列。
fibs = 1 : scanl (+) 1 fibs.
我很难找到非空列表（x：xs）的模式匹配如何用于自引用列表。 感谢帮助和任何资源，以更好地了解自引用/无限列表。
答案 0 :(得分：2)
我们可以使用由1:引起的相位差来仅使用scanl进行递归后向引用，如您在上一个示例中所示。
let fib = 1 : scanl (+) 1 fib in take 5 fib
以这种形式替换scanl的定义时，我们首先看到(x:xs)与现有的:相匹配：
1 : 1 : scanl (+) 2 (drop 1 fibs)
此时，第二个元素存在，因此我们可以提取另一个x。
1 : 1 : 2 : scanl (+) 3 (drop 2 fibs)
1 : 1 : 2 : 3 : scanl (+) 5 (drop 3 fibs)
1 : 1 : 2 : 3 : 5 : scanl (+) 8 (drop 4 fibs)
我们并不是真正使用drop，而是通过为每个元素匹配:来沿着我们正在构建的相同列表前进。我必须给其余列表命名，以递归地引用它，并发现drop形式可以方便地表示生成和起源。实际上，像这样使用drop会使整个列表保留在内存中，并从头开始遍历不必要的遍历以查找下一个元素。实际上，只有小块形状像递归let表达式那样作为尾巴，直到我们对其求值：
fibsfrom a b = let morefibs = a : scanl (+) b morefibs in morefibs
如果将此函数以1 1或其他任意位置作为种子，则可以通过连续的一对斐波那契数对种子来获得整个列表。尝试使用例如2 3或5 8.尾巴的公式先回到列表的前一个元素，因此可以始终将其拆分为头尾。
有时将引用作为图形查看会更清楚。我们将递归视为一个循环，但是仍然可以进行评估，因为我们只需要head list即可执行下一步。这是前几个步骤的graphviz点渲染（为回旋处的“运行代码段-全页”方法表示歉意，堆栈溢出无法整齐地处理点或svg）。
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