Question

当我将列表转换为其他集合类型时，是否有理由更喜欢ofList函数而不是ofSeq？

Answer 1

每种方法之间可能存在性能差异，您通常会期望更专业的转换可以提供更好的结果。

举一个例子，Array.ofList是通过此功能（https://github.com/fsharp/fsharp/blob/37a100b7caafde0f4df5a1924c9f65f4a18277a8/src/fsharp/FSharp.Core/local.fs#L300）实现的：

let toArray (l:'T list) =
    let len = l.Length
    let res = arrayZeroCreate len
    let rec loop i l =
        match l with
        | [] -> ()
        | h::t ->
            res.[i] <- h
            loop (i+1) t
    loop 0 l
    res

虽然Array.ofSeq通过Seq.toArray（https://github.com/fsharp/fsharp/blob/master/src/fsharp/FSharp.Core/seq.fs#L1231）：

let toArray (source : seq<'T>)  = 
    checkNonNull "source" source
    match source with 
    | :? ('T[]) as res -> (res.Clone() :?> 'T[])
    | :? ('T list) as res -> List.toArray res
    | :? ICollection<'T> as res -> 
        // Directly create an array and copy ourselves. 
        // This avoids an extra copy if using ResizeArray in fallback below.
        let arr = Array.zeroCreateUnchecked res.Count
        res.CopyTo(arr, 0)
        arr
    | _ -> 
        let res = ResizeArray<_>(source)                
        res.ToArray()

请注意，在这种情况下，ofSeq函数实际上会检查所提供的序列是否为list，如果是，则使用专门的转换。

ofSeq和ofList函数有什么区别？

1 个答案: