问题
I have started working my way through Erik Meijer's 13-part lectures (and Graham Hutton's slides) to learn Haskell.
On the slides for Chapter 4, on page 13, it introduces the pattern-matching syntax for n+k patterns. In particular, it says:
As in mathematics, functions on integers can be defined using n+k patterns, where n is an integer variable and k>0 is an integer constant.
pred :: Int -> Int
pred (n+1) = n
When I tried this on my own in the REPL I get an error message:
*Main> let mypred (n+1) = n
<interactive>:65:13: Parse error in pattern: n + 1
Similarly, if I try it in a *.hs file
mypred :: Int -> Int
mypred (n+1) = n
The compiler gives a similar complaint:
/Users/pohl/Code/praxis-haskell/helloworld.hs:14:9:
Parse error in pattern: n + 1
Am I not understanding how n+k patterns are intended to be used?
回答1:
You have to enable it by -XNPlusKPatterns.
ghci -XNPlusKPatterns
Prelude> let mypred (n+1) = n
Prelude> mypred 2
1
Similarly in a hs file.
{-# LANGUAGE NPlusKPatterns #-}
mypred :: Int -> Int
mypred (n+1) = n
After loading in ghci
*Main> mypred 2
1
回答2:
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.
来源：https://stackoverflow.com/questions/14250717/haskell-parse-error-in-pattern-for-nk-pattern