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 <omeraga...@gmail.com> 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 > 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