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This 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.