Haskell“否则”模式匹配？

Question

I have the following code:

swapInPairs :: [a] -> [a]
swapInPairs [] = []
swapInPairs [x] = [x]
swapInPairs (x:y:ys) = y : x : swapInPairs ys

is there any way to do something like

swapInPairs :: [a] -> [a]
swapInPairs (x:y:ys) = y : x : swapInPairs ys
otherwise = id

I know it's not really a big deal, and only gets rid of one line of code, but I'm curious if there's something like this for pattern matching since guards have "otherwise".

Answer 1

Pattern matching in general gets applied sequentially.

So if you have a 'catch-all' version after a more specific pattern, more specific pattern will get matched first, and if it's impossible, a 'catch-all' will do its thing.

So you can do

swapInPairs :: [a] -> [a]
swapInPairs (x:y:ys) = y : x : swapInPairs ys
swapInPairs x = x

Answer 2

Turns out order matters in pattern matching, and I didn't know that:

swapInPairs :: [a] -> [a]
swapInPairs (x:y:ys) = y : x : swapInPairs ys
swapInPairs x = x

works.