Re: expanding type synonyms in error messages

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