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]> 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]>: > > 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]> 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]> > >> 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] > >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > >> > >> _______________________________________________ > >> ghc-devs mailing list > >> [email protected] > >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > _______________________________________________ > ghc-devs mailing list > [email protected] > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > -- Chris Allen Currently working on http://haskellbook.com
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