F# 类型注释

我正在努力解决Euler#8项目。以下是我的解决方案：

open System

let product_of_digits num =
    Seq.fold (fun acc elem -> acc * elem) 1 [for x in num -> int32(x) - 48]

let all_possible_strings number =
    [ for x in [0 .. number.Length - 2] -> number.Substring(x, 2) ]

let problem_8 number =
    Seq.max(
        Seq.map (fun x -> (product_of_digits x)) (all_possible_strings number)
        )

[<EntryPoint>]
let main(args : string[]) =    
    printfn "result = %d" (problem_8 "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450")
    0

---

如何修复它？

正如错误所说，您需要向编译器提供有关函数参数类型的更多信息。如果我数对行，函数“problem_8”中的变量“number”看起来是不确定类型。要解决此问题，请将函数定义重新格式化如下（假设希望“number”参数为字符串）：

---

为了进一步了解，F#编译器在推断

所有可能的字符串中的
number
类型时遇到困难的原因是，您在
number
上使用的是.NET面向对象实例属性和方法，这些属性和方法没有提供足够的类型信息。但是，您可以为F#function应用程序
String.Length number
交换属性调用
number.Length
，该应用程序进行级联，以便现在可以完全推断所有函数和参数

D:\datahub\Dropbox\development\myprojects\project-euler\Problem_8\problem_8.fs(7,22): error FS0072: Lookup on object of
indeterminate type based on information prior to this program point. A type annotation may be needed prior to this progr
am point to constrain the type of the object. This may allow the lookup to be resolved.

D:\datahub\Dropbox\development\myprojects\project-euler\Problem_8\problem_8.fs(7,44): error FS0072: Lookup on object of
indeterminate type based on information prior to this program point. A type annotation may be needed prior to this progr
am point to constrain the type of the object. This may allow the lookup to be resolved.

let problem_8 (number : string) =