Yes I agree with Tom; it's at least a bit unexpected because unlike with records, there is (if I understand well) nothing I can write inside the {} that would actually be valid for that type. On Thu, 30 Nov 2023 at 14:06, Tom Ellis < tom-lists-haskell-cafe-2023@jaguarpaw.co.uk> wrote:

It was unexpected to me because A1 and A2 were not defined as record constructors. (Since I discovered it I now use it regularly; it's very nice.)

On Thu, Nov 30, 2023 at 02:58:43PM +0100, David Kraeutmann wrote:

...

Yes. You're just using record pattern match with no records. Why would it be unexpected?

On 30.11.2023 14:46, Noon van der Silk wrote:

...

Sometimes I have a type like:

data A = A1 Int | A2 Int Int

Then if I want to do pattern matching and ignore the parameters I do:

f (A1 _) = .. f (A2 _ _) = ...

But that's annoying; I need to remember how many parameters each one has!

Yesterday I learned I can just do this:

f A1 {} = ... f A2 {} = ...

And GHC is happy.

Is this expected? Am I the last to learn about this trick?

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-- Noon van der Silk, ن http://silky.github.io/ "My programming language is kindness."

Like others, this caught me by surprise, but now I use it regularly. I use hlint to advise me on the quality of my Haskell code.  I have learned many, many good ideas from hlint. As of a few years ago, hlint begins to recommend changing f (A3 _ _ _) = ... to f A3 {} = ... as soon as there are three or more components. On Thursday, November 30, 2023 at 09:12:09 AM EST, Brent Yorgey wrote: Yes, you're the last to learn about it; we were all wondering when you would figure it out. ;-) It's definitely "folklore", I can't remember where I first learned about it.  I agree with Tom that it's surprising (but nice) that it works even with data types that were not declared using record syntax.  I also always find it surprising that record update or matching binds more tightly than function application, so that no parentheses are needed.  Sometimes I feel like it would actually look nicer to write  f (A1 {}) = ... but then hlint yells at me.  (Yes, I'm aware I can turn off individual hlint warnings. =) -Brent On Thu, Nov 30, 2023 at 7:47 AM Noon van der Silk wrote: Sometimes I have a type like:     data A = A1 Int | A2 Int Int Then if I want to do pattern matching and ignore the parameters I do:     f (A1 _) = ..    f (A2 _ _) = ... But that's annoying; I need to remember how many parameters each one has! Yesterday I learned I can just do this:     f A1 {} = ...    f A2 {} = ... And GHC is happy. Is this expected? Am I the last to learn about this trick? -- Noon van der Silk, ن http://silky.github.io/ "My programming language is kindness."_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.