id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
3306,Improve syntax for GADT + records,simonpj,simonpj,"This [http://www.haskell.org/pipermail/glasgow-haskell-users/2009-June/017391.html email thread] raised the question of defining data constructors that
 * use GADT syntax
 * and record syntax
 * and have class constraints
This combination isn't supported in GHC 6.10, but it's annoying that it isn't.  The problem is just coming up with a plausible syntax. Probably the most plausible possibilities are
{{{
(A)   data RecContTest a where
         Show a => C { showable :: a } :: RecContTest a

(B)   data RecContTest a where
         C :: Show a => { showable :: a } -> RecContTest a
}}}
The latter (B) looks best to me. I dislike (A) because part of the type (the ""Show a =>"") occurs before the constructor name C, and part appears after.  On the other hand, (B) has something that looks vaguely like a type
{{{
   { x::ty, y::ty } -> ty
}}}
but that's not really valid type syntax.  (Mind you, the ! marks in a constructor signature aren't part of valid types either, so maybe it's not so bad to have a special form in constructor declarations.)

But if we were going to adopt (B), then even when there is no class context we should really say
{{{
(B)  data RecTest a where
        B :: { arg :: a } -> RecTest a
}}}
rather than the current syntax which is
{{{
(A)  data RecTest a where
       B { arg :: a } :: RecTest a
}}}
[Note the different placement of the double colon and arrow in (B).]


My take on this
  * (B) looks nicer, but it would represent a breaking change
  * But perhaps not many people use record-style syntax + GADT-style syntax
  * And better to make breaking changes sooner than later

Question for everyone:

 * are (A) and (B) the only choices?
 * do you agree (B) is best

There are some replies on the above email thread. Please add further opinions as comments to this ticket.",feature request,closed,normal,6.12.1,Compiler,6.10.2,fixed,,,Unknown/Multiple,Unknown/Multiple,,Unknown,,,,