定義 fromInteger function in Num instance on in Haskell

Question

我試圖做到這一點，以便在控制台中顯示以下內容

1:: 代數圖 Int

將生產

頂點 1

data AlgebraicGraph a
        = Empty
        | Vertex a
        | Overlay (AlgebraicGraph a) (AlgebraicGraph a)
        | Connect (AlgebraicGraph a) (AlgebraicGraph a)

instance Num a => Num (AlgebraicGraph a) where
    fromInteger x = Vertex x
    (+) x y = Overlay x y
    (*) x y = Connect x y

但是當我嘗試 fromInteger x = Vertex x 時，我得到的是

 Couldn't match type `a' with `Integer'
  `a' is a rigid type variable bound by
    the instance declaration
    at AlgebraicGraph.hs:120:10-40
  Expected type: AlgebraicGraph a
    Actual type: AlgebraicGraph Integer
* In the expression: Vertex x
  In an equation for `fromInteger': fromInteger x = Vertex x
  In the instance declaration for `Num (AlgebraicGraph a)'
* Relevant bindings include
    fromInteger :: Integer -> AlgebraicGraph a

任何幫助深表感謝！

Answer 1

fromInteger x = Vertex x

問題是x:: Integer在那里，而不是Num a => a 。 改為這樣做：

fromInteger x = Vertex (fromInteger x)