我有一个Result<'T, 'E> list
,我想按照这些规则变成一个Result<'T list, 'E>
:
Result
为Error
,则结果应为Error
Error
,则该列表应为列表中的第一个Error
OK
,那么结果应该是Ok
并且应保持列表顺序所以我有一个去实现如下:
let all xs =
let folder = fun state next ->
match (state, next) with
| (Result.Ok ys, Result.Ok y) -> ys |> List.append [ y ] |> Result.Ok
| (Result.Error e, _) -> Result.Error e
| (_, Result.Error e) -> Result.Error e
Seq.fold folder (Result.Ok []) xs
然而,这似乎已经在标准库中实现了。有吗?
其次,我有Result
的计算表达式,如下所示:
type ResultBuilder () =
member this.Bind(x, f) =
match x with
| Result.Ok o -> f o
| Result.Error e -> Result.Error e
member this.Return(value) = Result.Ok value
member this.ReturnFrom(value) = value
let result = new ResultBuilder()
我可以在all
内使用result { ... }
,但可以进一步整合吗?例如,通过实施ResultBuilder.For
?
答案 0 :(得分:6)
您有一个Result<'a, 'e> list
,想要一个Result<'a list, 'e>
。这听起来像https://fsharpforfunandprofit.com/posts/elevated-world-4/中描述的sequence
函数(与seq
无关,尽管名称听起来像)。快速检查https://github.com/fsharp/fsharp/blob/master/src/fsharp/FSharp.Core/result.fs表明此功能尚未在标准FSharp.Core库中实现,因此您需要自己实现它。
关于你的第二个问题,你能提供一些更具体的细节吗?例如,您希望ResultBuilder.For
出现什么行为? for
表达式的正常行为是采用Result<'a, 'e> list
(或seq或数组)并对列表或seq或数组中的每个Result<'a, 'e>
运行一次内部块。如果您尝试在此处使用all
函数,则Result<'a, 'e>
(这是F#期望CE的.For
方法生成的)和{{1}之间的类型不匹配这是您的Result<'a list, 'e>
方法返回的内容。具体您希望all
方法做什么?
答案 1 :(得分:3)
此功能由 FsToolkit.ErrorHandling
包提供。函数 List.sequenceResultM
将返回:
Result.Ok
包含所有 Ok
值的列表Result.Error
值的 Error
还有一个变体 List.sequenceResultA
返回发现的所有错误的列表。
#r "nuget: FsToolkit.ErrorHandling, 2.0.0"
open FsToolkit.ErrorHandling
let xs : Result<int, string> list =
[
Ok 123
Ok 456
]
let xa = List.sequenceResultA xs
let xm = List.sequenceResultM xs
printfn "xa: %A" xa
printfn "xm: %A" xm
let ys =
[
Ok 123
Ok 456
Error "abc"
Ok 789
]
let ya = List.sequenceResultA ys
let ym = List.sequenceResultM ys
printfn "ya: %A" ya
printfn "ym: %A" ym
let zs =
[
Ok 123
Error "abc"
Error "def"
Ok 456
]
let za = List.sequenceResultA zs
let zm = List.sequenceResultM zs
printfn "za: %A" za
printfn "zm: %A" zm
xa: Ok [123; 456]
xm: Ok [123; 456]
ya: Error ["abc"]
ym: Error "abc"
za: Error ["abc"; "def"]
zm: Error "abc"
在计算表达式方面(也由 FsToolkit.ErrorHandling
提供),你可以这样做:
#r "nuget: FsToolkit.ErrorHandling, 2.0.0"
open FsToolkit.ErrorHandling
let printAll xs =
result {
let! xs = List.sequenceResultA xs
for x in xs do
printfn "%A" x
}
答案 2 :(得分:0)
这是我的解决方案,需要一个结果序列(而不是一个列表),因此验证是懒惰的。
let takeTo<'T> predicate (source: 'T seq) =
seq {
use en = source.GetEnumerator()
let mutable isDone = false
while isDone = false && en.MoveNext() do
yield en.Current
isDone <- predicate en.Current
}
let fromResults rs =
rs
|> Seq.scan (fun (vs, err) i ->
match i with
| Ok v -> (v::vs,err)
| Error e -> (vs, Some e)) ([], None)
|> Seq.takeTo (fun (vs, err) -> err.IsSome)
|> Seq.last
|> fun (vs, err) ->
match err with
| None -> vs |> List.rev |> Ok
| Some err -> Error err