使用約束創建自定義數據類型

Question

我正嘗試創建自定義數據類型。作爲一個例子

data Time = Second Int deriving (Show) 

但是，這是太有限（我們可以說以後需要毫秒）。我想，而不是定義是這樣的：

data Time = Second Num deriving (Show) 

這並不編譯，因爲Num有種* -> ghc-prim-0.4.0.0:GHC.Prim.Constraint

如何設置Time這樣Second可以包含任何Num？

Answer 1

有一個名爲數據類型的上下文棄用的功能，可以讓你做到這一點：

{-# LANGUAGE DatatypeContexts #-} 
data Num a => Time a = Second a deriving (Show) 
t = Second (5 :: Int) 
main = print t 

這對執行GHC 7.8.3（對不起，沒有7.10檢查），但警告你這個折舊：

t.hs:1:14: Warning: 
    -XDatatypeContexts is deprecated: It was widely considered a 
misfeature, and has been removed from the Haskell language. 
Second 5 

非過時的方式來做到這一點是使用Generalized Algebraic Datatypes（GADTs）（並且你需要獨立deriving以及）：

{-# LANGUAGE GADTs, StandaloneDeriving #-} 
data Time a where 
    Second :: Num a => a -> Time a 
deriving instance Show a => Show (Time a) 
t = Second (5 :: Int) 
main = print t 

如果試圖創建一些變量非Num，你會得到一個編譯錯誤：

t = Second "a" 

t.hs:12:5: 
    No instance for (Num [Char]) arising from a use of ‘Second’ 
    In the expression: Second "a" 
    In an equation for ‘t’: t = Second "a" 

Answer 2

之一的，爲什麼這可能不是那麼理想的最好的例子是found here在維基教科書關於類和類型的部分。他們說：

Type constraints in data declarations are less useful than it might seem at first. Consider:

data (Num a) => Foo a = F1 a | F2 a String 

Here, Foo is a type with two constructors, both taking an argument of a type a which must be in Num. However, the (Num a) => constraint is only effective for the F1 and F2 constructors, and not for other functions involving Foo. Therefore, in the following example...

fooSquared :: (Num a) => Foo a -> Foo a 
fooSquared (F1 x) = F1 (x * x) 
fooSquared (F2 x s) = F2 (x * x) s 

... even though the constructors ensure a will be some type in Num we can't avoid duplicating the constraint in the signature of fooSquared

這表明，你一個合理的選擇是隻創建Time與通用參數，再後來確保上Time數據操作模塊的功能總是有必要的約束爲Num。

這不會是這麼多一份擔心有人熄滅，愚蠢使得Time String什麼的 - 如果他們這樣做，那麼所有的提供的模塊功能都將是對他們有幫助的，所以也沒有非常重要。

還有幾個選項查找與GADTs的{-# LANGUAGE GeneralizedNewtypeDeriving #-}編譯和{-# LANGUAGE DatatypeContexts #-}編譯。但通常情況下，這些都會導致不必要的額外複雜度，特別是如果你是像我這樣的Haskell新手。