解析輸入錯誤'let'

Question

該代碼需要一個四叉樹並插入一個矩形。但是，我得到一個語法分析錯誤。我想調用函數newExtent而不必添加所有參數。所以newExtent TopLeft而不必撥打newExtent TopLeft extent rectangle。我如何使它工作？

insert :: QuadTree -> Rectangle -> QuadTree 
insert (Qt extent horizontal vertical qTL qTR qBL qBR) rectangle  
    | quadPart extent rectangle == VerLine = Qt extent horizontal (rectangle:vertical) qTL qTR qBL qBR 
    | quadPart extent rectangle == HorLine = Qt extent (rectangle:horizontal) vertical qTL qTR qBL qBR 
    | quadPart extent rectangle == TopLeft && qTL == EmptyQuadTree = Qt extent horizontal vertical (newExtent TopLeft) qTR qBL qBR 
    | quadPart extent rectangle == TopLeft = Qt extent horizontal vertical (insert qTL rectangle) qTR qBL qBR 
    | quadPart extent rectangle == TopRight && qTR == EmptyQuadTree = Qt extent horizontal vertical qTL (newExtent TopRight) qBL qBR 
    | quadPart extent rectangle == TopRight = Qt extent horizontal vertical qTL (insert qTR rectangle) qBL qBR 
    | quadPart extent rectangle == BottomLeft && qBL == EmptyQuadTree = Qt extent horizontal vertical qTL qTR (newExtent BottomLeft) qBR 
    | quadPart extent rectangle == BottomLeft = Qt extent horizontal vertical qTL qTR (insert qBL rectangle) qBR 
    | quadPart extent rectangle == BottomRight && qBR == EmptyQuadTree = Qt extent horizontal vertical qTL qTR qBL (newExtent BottomRight) 
    | otherwise = Qt extent horizontal vertical qTL qTR qBL (insert qBR rectangle) 

    let (Rect eL eT eR eB) = extent 

    in newExtent :: TreeParts -> Rectangle -> Rectangle -> QuadTree 
     newExtent part (Rect eL eT eR eB) rectangle 
      | part == TopLeft = insert (emptyQtree (Rect eL eT ((eL + eR) `div` 2) ((eT + eB) `div` 2))) rectangle 
      | part == TopRight = insert (emptyQtree (Rect ((eL + eR) `div` 2) eT eR ((eT + eB) `div` 2))) rectangle 
      | part == BottomLeft = insert (emptyQtree (Rect eL ((eT + eB) `div` 2) ((eL + eR) `div` 2) eB)) rectangle 
      | part == BottomRight = insert (emptyQtree (Rect ((eL + eR) `div` 2) ((eT + eB) `div` 2) eR eB)) rectangle 

Answer 1

1. 不要縮進插入

2. 的類型簽名in後，您不定義表達式功能。讓 語句允許您在in之後並且僅在那裏定義可用於 表達式的內容。在這種情況下，where聲明可能會更容易。與這裏的let語句相比，您可以在函數定義之後定義一些東西，並且可以在函數中使用它們，這看起來像是您正在嘗試執行的操作。因此，試圖通過

   where (Rect eL eT eR eB) = extent 
        newExtent part 
         | part == TopLeft = insert (emptyQtree (Rect eL eT ((eL + eR) `div` 2) ((eT + eB) `div` 2))) rectangle 
         | part == TopRight = insert (emptyQtree (Rect ((eL + eR) `div` 2) eT eR ((eT + eB) `div` 2))) rectangle 
         | part == BottomLeft = insert (emptyQtree (Rect eL ((eT + eB) `div` 2) ((eL + eR) `div` 2) eB)) rectangle 
         | part == BottomRight = insert (emptyQtree (Rect ((eL + eR) `div` 2) ((eT + eB) `div` 2) eR eB)) rectangle