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:[email protected]] 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 <[email protected]<mailto:[email protected]>> 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 <[email protected]<mailto:[email protected]>>: > I wanted to add that synonym expansion would be especially helpful in > error-messages like: > > Expected type: <non-expanded, small type, like "Producer a m ()"> > Actual type: <your type, like "Proxy a a' b b' m v"> > > 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" > <[email protected]<mailto:[email protected]>> 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 >> <[email protected]<mailto:[email protected]>> >> 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): <type1, synonyms are expanded> >> > Actual type: <type2> >> > (without synonyms): <type2, synonyms are expanded> >> > >> > 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 >> > [email protected]<mailto:[email protected]> >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >> >> _______________________________________________ >> ghc-devs mailing list >> [email protected]<mailto:[email protected]> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs _______________________________________________ ghc-devs mailing list [email protected]<mailto:[email protected]> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs -- Chris Allen Currently working on http://haskellbook.com
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