我已经开始通过 Erik Meijer 的13 部分讲座（和 Graham Hutton 的幻灯片）来学习 Haskell。
在第 13 页第 4 章的幻灯片中，它介绍了 n+k 模式的模式匹配语法。特别是，它说：
与数学一样，整数函数可以使用 n+k 模式定义，其中 n 是整数变量，k>0 是整数常量。
pred :: Int -> Int
pred (n+1) = n
当我在 REPL 中自己尝试此操作时，我收到一条错误消息：
*Main> let mypred (n+1) = n
<interactive>:65:13: Parse error in pattern: n + 1
同样，如果我在*.hs文件中尝试
mypred :: Int -> Int
mypred (n+1) = n
编译器给出了类似的抱怨：
/Users/pohl/Code/praxis-haskell/helloworld.hs:14:9:
Parse error in pattern: n + 1
我不了解如何使用 n+k 模式吗？
您必须通过 启用它-XNPlusKPatterns。
ghci -XNPlusKPatterns
Prelude> let mypred (n+1) = n
Prelude> mypred 2
1
同样在一个hs文件中。
{-# LANGUAGE NPlusKPatterns #-}
mypred :: Int -> Int
mypred (n+1) = n
After loading in ghci
*Main> mypred 2
1
Am I not understanding how n+k patterns are intended to be used?
Actually, nowadays n+k patterns are considered bad practice. The main reason for this is that the syntax doesn't really look like anything else in Haskell, the + part isn't really using the + that is in scope, unlike say how the do notation works. Also, the viewpatterns extension is kind of a generalization that is useful in many more settings.
There is more info here on why it was removed.