haskell parse error in pattern for n+k pattern

Question

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 intro

Answer 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

Answer 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.