Re: expanding type synonyms in error messages

Update: I have a patch, it's not quite ready for reviews, but I'm now getting this error message: Main.hs:17:26: error: Couldn't match type ‘[Char]’ with ‘()’ Expected type: Proxy () String () X IO () (aka. Proxy () [Char] () X IO ()) Actual type: Consumer String IO String (aka. Proxy () [Char] () X IO [Char]) In the second argument of ‘(>->)’, namely ‘doubleUp’ In the second argument of ‘($)’, namely ‘loop >-> doubleUp’ cabal: Error: some packages failed to install: ghc-ty-patch-0.1.0.0 failed during the building phase. The exception was: ExitFailure 1 I'll tidy the code a bit, add a command line flag etc. and submit for reviews. 2015-06-19 10:13 GMT-04:00 Kostiantyn Rybnikov :

Great, thanks for doing this! I'm afraid even if you succeed with patch we would still need more "real-world examples" that show the need for this feature, and I think this is separate from GHC tests, as they don't need to be realistic, but of course please continue and hopefully more examples will come.

19 черв. 2015 16:19 "Ömer Sinan Ağacan" пише:

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Done: https://ghc.haskell.org/trac/ghc/ticket/10547

2015-06-19 9:12 GMT-04:00 Richard Eisenberg :

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Don't forget to make a Feature Request on Trac (https://ghc.haskell.org/trac/ghc/newticket) with a link to the wiki page. Trac is really the only way things like this don't get lost.

Thanks!

Richard

On Jun 19, 2015, at 9:07 AM, Ömer Sinan Ağacan wrote:

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Great, thanks Kostiantyn! I'm looking for simple examples that we can add to GHC testsuite, if I find something I'll update the wiki page also.

I made some progress on the patch, I think I can hopefully submit something this weekend for reviews.

2015-06-19 5:16 GMT-04:00 Kostiantyn Rybnikov :

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Created some initial wiki-page with just one example, will keep it in mind to add more as seen.

https://wiki.haskell.org/Expanding_type_synonyms_in_error_messages_proposal

On Fri, Jun 19, 2015 at 10:42 AM, Simon Peyton Jones wrote:

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On this thread, a representative collection of *reproducible examples* with the actual error message and the desired one, would be tremendously helpful. Lacking that, it’s hard to know where to begin. (Creating the examples takes a bit of work, I know.)

Start a wiki page! Stuff in email threads gets lost

Simon

From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Christopher Allen Sent: 19 June 2015 04:27 To: Ömer Sinan Ağacan Cc: ghc-devs Subject: Re: expanding type synonyms in error messages

Just to add my own +1, having this when working with streaming libraries (I've needed it less with lens, oddly) would be a tremendous help for people learning them I think. I think I've run into the same thing as Kostiantyn in the past.

On Thu, Jun 18, 2015 at 9:42 PM, Ömer Sinan Ağacan wrote:

It's good to see that I'm not the only one who wants this. I'm doing some GHC hacking nowadays and I'll give it a try.

2015-06-18 4:41 GMT-04:00 Kostiantyn Rybnikov : > I wanted to add that synonym expansion would be especially helpful > in > error-messages like: > > Expected type: > Actual type: > > I would be glad if we could have an expansions enabling flag in GHC, > and > could consider turning it on by default if it will look good for > that. > > 16 черв. 2015 22:44 "Richard Eisenberg" пише: > >> GHC tries hard to preserve type synonyms where possible, but of >> course, >> it >> can't preserve all of them. The general rule it tries to follow is: >> preserve >> vanilla type synonyms; expand type families. This is true both in >> expected >> types and actual types. >> If you have a case where you believe that GHC could preserve a type >> synonym in an expected type, submit a bug report. (Note that >> constraint >> synonyms are particularly hard to preserve!) >> >> It would be very easy to report both the synonym-preserving form >> and >> the >> expanded form in an error report, at the cost of making error >> reports >> even >> more verbose. You're welcome to submit a feature request, and this >> would >> likely make a good first patch to GHC if you want to get your hands >> dirty. >> I'd personally prefer the feature to be protected behind a flag (to >> avoid >> seeing that `String` expands to `[Char]` everywhere, for example), >> but >> others may feel differently here. >> >> Richard >> >> On Jun 16, 2015, at 11:20 AM, Ömer Sinan Ağacan >> >> wrote: >> >>> Hi all, >>> >>> While working with complex types with lots of arguments etc. >>> errors >>> are >>> becoming annoying very fast. For example, GHC prints errors in >>> this >>> way: >>> >>> Expected type: <type without any synonyms> >>> Actual type: <type with synonyms> >>> >>> Now I have to expand that synonym in my head to understand the >>> error. >>> >>> I was wondering if implementing something like this is possible: >>> >>> In type error messages, GHC also prints types that are cleaned >>> from >>> type >>> synonyms. Maybe something like this: >>> >>> Expected type: <type1> >>> (without synonyms): >>> Actual type: <type2> >>> (without synonyms): >>> >>> If this is not always desirable for some reason, we can hide this >>> behavior >>> behind a flag. >>> >>> What do GHC devs think about this? Is this, in theory, possible to >>> do? >>> How hard >>> would it be to implement this? >>> >>> Thanks. >>> _______________________________________________ >>> ghc-devs mailing list >>> ghc-devs@haskell.org >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >> >> _______________________________________________ >> ghc-devs mailing list >> ghc-devs@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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Chris Allen

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