haskell - 构造函数模式匹配haskell

Question

我有这种数据类型

data Struct val =  Empty | Exec1 val
                         | Exec2 val 

和两个虚拟函数

apply :: Struct -> String
apply (Empty) = "matched Empty"  
apply (exec struct) = "matched Exec1 or Exec2"              

apply' :: Struct val -> String
apply' (Empty)   = "matched Empty"  
apply' (Exec1 _) = "matched Exec1"
apply' (Exec2 _) = "matched Exec2"

第二个工作正常，但第一个导致错误:“Parse error in pattern: exec”。你能解释一下为什么我不能以这种方式匹配构造函数:
应用(执行结构)= ...？

当我的数据类型中有多个构造函数并且必须分别对它们进行模式匹配时，它会导致大量样板代码。

Answer 1

一般来说，如果您有多个共享数据的构造函数，那么通常最好将数据声明重构为类似

data Struct val = Empty | NonEmpty StructType val
data StructType = Exec1 | Exec2

现在您可以在 apply 中进行模式匹配像这样

apply :: Struct -> String
apply (Empty) = "matched Empty"  
apply (NonEmpty exec struct) = "matched Exec1 or Exec2"

并且您仍然可以与特定的 Exec 类型进行模式匹配

apply' :: Struct val -> String
apply' (Empty)   = "matched Empty"  
apply' (NonEmpty Exec1 _) = "matched Exec1"
apply' (NonEmpty Exec2 _) = "matched Exec2"