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You can reach the person managing the list at beginners-ow...@haskell.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Beginners digest..." Today's Topics: 1. Pattern Matching (Yugesh Kothari) 2. Re: Pattern Matching (Bob Ippolito) ---------------------------------------------------------------------- Message: 1 Date: Sat, 27 Apr 2019 20:01:18 +0530 From: Yugesh Kothari <kothariyug...@gmail.com> To: beginners@haskell.org Subject: [Haskell-beginners] Pattern Matching Message-ID: <cacsi4wal+7orvz9tobxhfazsvz827bpjmm8ychmyf+q4jsd...@mail.gmail.com> Content-Type: text/plain; charset="utf-8" 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! -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.haskell.org/pipermail/beginners/attachments/20190427/e3ded698/attachment-0001.html> ------------------------------ Message: 2 Date: Sat, 27 Apr 2019 08:24:07 -0700 From: Bob Ippolito <b...@redivi.com> To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners@haskell.org> Subject: Re: [Haskell-beginners] Pattern Matching Message-ID: <cacwmpm_5rx0d+joxwqky_c9aw9_062+jx05wxthzbywkpu-...@mail.gmail.com> Content-Type: text/plain; charset="utf-8" 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 <kothariyug...@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! > > _______________________________________________ > Beginners mailing list > Beginners@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.haskell.org/pipermail/beginners/attachments/20190427/ebc71c3d/attachment-0001.html> ------------------------------ Subject: Digest Footer _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners ------------------------------ End of Beginners Digest, Vol 130, Issue 8 *****************************************