無法推斷（顯示T）

Question

我在Haskell這一段代碼，拒絕編譯：

data (Eq a, Num a, Show a) => Mat a = Mat {nexp :: Int, mat :: QT a} 
deriving (Eq, Show) 

data (Eq a , Show a) => QT a = C a | Q (QT a) (QT a) (QT a) (QT a) 
deriving (Eq, Show) 


cs:: (Num t) => Mat t -> [t] 

cs(Mat nexp (Q a b c d)) =(css (nexp-1) a c)++(css (nexp-1) b d) 
    where 
     css 0 (C a) (C b) = (a-b):[] 
     css nexp (Q a b c d) (Q e f g h) = (zipWith (+) (css (nexp-1) a c) (css (nexp-1) e g))++(zipWith (+)(css (nexp-1) b d) (css (nexp-1) f h)) 

我有這樣的錯誤：

Could not deduce (Show t) arising from a use of `Mat' 
from the context (Num t) 
bound by the type signature for cs:: Num t => Mat t -> [t] 

我在網上搜索，發現了很多類似的問題，但似乎並沒有接近我的問題。我怎樣才能使這個代碼工作？關於數據類型的

Answer 1

類約束don't mean anything.

參見this related question

上的數據類型中刪除該約束。

data Mat a = Mat {nexp :: Int, mat :: QT a} 
    deriving (Eq, Show)