表达式/列表解析中的模式匹配

Question

为什么以下尝试在列表理解中模式匹配不起作用？

示例：同时替换术语数据类型中的原子。

的数据类型：

data Term a 
    = Atom a 
    | Compound (Term a) (Term a) 
    deriving Show 

在术语原子的取代算法（采第一匹配取代如果任何和忽略其余部分）：

subs :: [(Term a, Term a)] -> Term a -> Term a 
subs subList term = case term of 
    atom@(Atom x)  -> let substitutions = 
           [ s | s@(Atom x, _) <- subList ] 
          in if null substitutions 
           then atom 
           else snd . head $ substitutions 
    (Compound t1 t2) -> Compound (subs subList t1) (subs subList t2) 

一些测试数据：

subList = [((Atom 'a'), Compound (Atom 'b') (Atom 'c'))] 
term1 = Atom 'a' 
term2 = Atom 'x' 

运行示例结果如下：

>: subs subList term1 
Compound (Atom 'b') (Atom 'c') 

这是所期望的行为，和

>: subs subList term2 
Compound (Atom 'b') (Atom 'c') 

这是不。

Strangley明确的匹配工作的：

subs'' :: [(Term Char, Term Char)] -> Term Char -> Term Char 
subs'' subList term = case term of 
    atom@(Atom _)  -> let substitutions = 
          [ s | s@(Atom 'a', _) <- subList ] 
          in if null substitutions 
           then atom 
           else snd . head $ substitutions 
    (Compound t1 t2) -> Compound (subs subList t1) (subs subList t2) 

subs''' subList term = case term of 
    atom@(Atom _)  -> let substitutions = 
          [ s | s@(Atom 'x', _) <- subList ] 
          in if null substitutions 
           then atom 
           else snd . head $ substitutions 
    (Compound t1 t2) -> Compound (subs subList t1) (subs subList t2) 

进料与所述测试数据导致：

>: subs'' subList term1或>: subs'' subList term2
Compound (Atom 'b') (Atom 'c')

>: subs''' subList term1或>: subs''' subList term2
Atom 'x'

我错过了什么？

Answer 1

Haskell具有线性模式，这意味着模式中不能有重复的变量。此外，内部表达式中的模式变量会影响外部变量，而不是建立相同变量的相等性。

你试图做这样的事情：

charEq :: Char -> Char -> Bool 
charEq c c = True 
charEq _ _ = False 

但是，这是因为重复变量的错误。假如我们把第二c到内表达，它编译，但它仍打算不起作用：

charEq :: Char -> Char -> Bool 
charEq c d = case d of 
    c -> True 
    _ -> False 

这里内c只是一个新的变量，阴影外c，所以charEq总是返回True。

如果我们想检查平等，我们必须使用==明确：

subs :: [(Term a, Term a)] -> Term a -> Term a 
subs subList term = case term of 
    atom@(Atom x)  -> let substitutions = 
           [ s | s@(Atom x', _) <- subList, x == x' ] 
          in if null substitutions 
           then atom 
           else snd . head $ substitutions 
    (Compound t1 t2) -> Compound (subs subList t1) (subs subList t2)