類型註釋

Question

我想解決項目歐拉＃8。這裏是我的解決方案：

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 

所有工作正常Fsi.exe，但是當我試圖編譯代碼我有錯誤：

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. 

我怎樣才能解決這個問題？

Answer 1

正如錯誤所述，您需要爲編譯器提供有關函數參數類型的更多信息。如果我正確地計算這些行，它看起來像函數「problem_8」中的變量「number」是不確定類型的。爲了解決這個問題，重新制定功能定義，就像這樣（假設你希望你的「數字」參數是一個字符串）：

let problem_8 (number : string) = 

Answer 2

爲了進一步瞭解，爲什麼F＃編譯器有困難推斷的number類型的原因在all_possible_strings是因爲您使用的是面向對象的.NET實例屬性和方法，它們不提供有關該類型的足夠信息。但是，您可以將F＃函數應用程序String.length number的屬性調用number.Length替換爲級聯，以便現在可以完全推斷所有函數和參數。