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Today's Topics:
1. Pattern Matching (Yugesh Kothari)
2. Re: Pattern Matching (Bob Ippolito)
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Message: 1
Date: Sat, 27 Apr 2019 20:01:18 +0530
From: Yugesh Kothari <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Pattern Matching
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<cacsi4wal+7orvz9tobxhfazsvz827bpjmm8ychmyf+q4jsd...@mail.gmail.com>
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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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Message: 2
Date: Sat, 27 Apr 2019 08:24:07 -0700
From: Bob Ippolito <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Pattern Matching
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<cacwmpm_5rx0d+joxwqky_c9aw9_062+jx05wxthzbywkpu-...@mail.gmail.com>
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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 <[email protected]>
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
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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