"The input sequence has an insufficient number of elements" 在 Seq.length 通话中？

Question

我正在研究 Matasano 加密挑战，但我运行遇到了一个奇怪的错误。我有一个整数序列，在调用 Seq.length（或 Seq.sum）时，我得到一个 'System.InvalidOperationException' 并解释为 "The input sequence has an insufficient number of elements." 通常此错误与调用相关Seq.take 或类似的空 Seq，但此 Seq 不为空，通常会导致此类异常的调用也不为空。

let HammingDistance (b1:seq<byte>) (b2:seq<byte>) : int =
    let bytePairs = Seq.zip b1 b2
    let HammingWeight (b:byte) =
        let bitPositions = {0 .. 7}
        let isSet (byte1:byte) (offset:int) = if (byte1 &&& ((byte 1) <<< offset)) > (byte 0) then 1 else 0
        Seq.map (isSet b) bitPositions |> Seq.sum
    let HammingDistanceBytes (byte1, byte2) =
        HammingWeight (byte1 ^^^ byte2)
    let distances = Seq.map HammingDistanceBytes bytePairs
    printfn "Distances: %A" distances
    printfn "Distances length: %d" (Seq.length distances)
    Seq.sum distances

异常前的最后几行输出如下：

Distances length: 1435
Distances: seq [3; 3; 1; 5; ...]
Distances length: 1436
Distances: seq [3; 6; 2; 1; ...]
Distances length: 1437
Distances: seq [0; 3; 2; 3; ...]
Distances length: 1438
Distances: seq [3; 3; 4; 2; ...]

任何人都可以解释为什么会发生此异常，或者提供我应该采取的任何步骤来进一步诊断问题吗？

Answer 1

这里的问题与懒惰求值有关 - b2 不可能首先生成，因为它是从一个更大的列表派生的：

// message:seq<byte>        
let firstKeysizeBytes = Seq.take keysize message
let secondKeysizeBytes = Seq.skip keysize message |> Seq.take keysize

在最终序列被强制求值之前，异常不会弹出。错误消息的位置非常混乱，但这就是惰性评估的生活。我可能应该转向使用严格的顺序类型。

"The input sequence has an insufficient number of elements" 在 Seq.length 通话中？

"The input sequence has an insufficient number of elements" on a Seq.length call?

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