問題在F＃

Question

泛型和鑄造用在以下方式

type Genome<'T> (listIn : List<'T>)= 
    member this.list = listIn 
    member this.GetValues() = 
     this.list 

我遇到了一個問題，而寫另一個代碼段實施了基因組型。

type IMutation = 
    abstract member MutateMethod: Genome<'T> -> Genome<'T> 

type DoubleGenomeMutation() = 
    member this.Mutate = (this :> IMutation).MutateMethod 
    interface IMutation with 
     member this.MutateMethod(genome: Genome<'T>) = 
      let randomIndex = MyRandom.Next(0, genome.list.Length) 
      printfn "%d" randomIndex 
      let newValue = MyRandom.NextDouble(Parameters.LowerBounds.Item(randomIndex), Parameters.UpperBounds.Item(randomIndex)) 

      let (newGenomeList : List<'double>, _) = 
       List.foldBack (fun elem acc -> 
       match acc with 
       | (l, 0) -> (newValue::l, -1) 
       | (l, i) -> (elem::l, i-1)) (genome.GetValues()) ([], genome.GetValues().Length - randomIndex - 1) 

      new Genome<'T>(newGenomeList) 

我發現不可能在這裏正確使用泛型機制。在

Error 2 Type mismatch. Expecting a 
    float list  
but given a 
    List<'a>  
The type 'float' does not match the type ''a' 

引述的樣本效應線

| (l, i) -> (elem::l, i-1)) (genome.GetValues()) ([], genome.GetValues().Length - randomIndex - 1) 

鑄造變量朝上或朝下ofter描述爲類似案件的解決方案 - 不幸的是我無法修復我的這樣的程序。我將不勝感激任何直接幫助以及任何有關所有問題根源的建議。

Answer 1

我沒有的MyRandom源，但如果我有int取代MyRandom.Next因爲它看起來像它應該是和一個double替換MyRandom.Double，那麼我可以重現你的錯誤。

問題是MyRandom.NextDouble的返回類型會不當地限制其餘代碼; MutateMethod被聲明能夠採取任何類型的參數，但是然後您使用newValue作爲(l, 0)情況下的返回列表的一部分，這也限制了另一種情況的類型。

您可能還從newValue得到了一個警告，指出它將您的'double類型變量限制爲float這是另一個線索。真的'double應該是'T，因爲這是該參數最初聲明的方式。

錯誤消失，如果我更換呼叫MyRandom.NextDouble與拋出異常（可以有任何結果類型），這證實了這一診斷：

type DoubleGenomeMutation() = 
    member this.Mutate = (this :> IMutation).MutateMethod 
    interface IMutation with 
     member this.MutateMethod(genome: Genome<'T>) = 
      let randomIndex = 1 // MyRandom.Next(0, genome.list.Length) 
      printfn "%d" randomIndex 
      let newValue = failwith "foo" 

      let (newGenomeList : List<'T>, _) = 
       List.foldBack (fun elem acc -> 
       match acc with 
       | (l, 0) -> (newValue::l, -1) 
       | (l, i) -> (elem::l, i-1)) (genome.GetValues()) ([], genome.GetValues().Length - randomIndex - 1) 

      new Genome<'T>(newGenomeList)