That’s called an as-pattern. It binds part of the pattern match to a variable. In this case you might want to do that for efficiency reasons. You can find an example of them here: https://www.haskell.org/tutorial/patterns.html I think your example isn’t correct, the `us` variable is only defined for the last clause. The first two should use `compress ys`. Without as-patterns this would look like: compress (x:y:ys’) | x==y = compress (y:ys’) | otherwise = x : compress (y:ys’) compress us = us It can be more efficient and concise to use ys@(y:_) in the pattern match and ys elsewhere instead of having to repeat (y:ys’) which without optimizations would call the : constructor again and may not share memory in the same way. -bob On Sat, Apr 27, 2019 at 07:31 Yugesh Kothari <kothariyugesh@gmail.com> wrote:

This is probably a stupid question but I can't seem to understand the use of @ in haskell pattern matching.

Ex - compress (x:ys@(y:_)) | x==y = compress us | otherwise = x : compress us compress us = us

Thanks!

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