Niklas,
My rationale is as follows. With the introduction of GADTs, we now
have two ways to write datatype declarations, the old simple way and
the GADTs way. The GADTs way fits better syntactically with Haskell's
other syntactic constructs, in all ways. The general style is
(somewhat simplified) "keyword type 'where' decls", where keyword can
in Haskell 98 be class or instance, but with GADTs also data. The old
simple way of defining data types is the odd one out. It certainly has
its uses though, in particular when defining some simple (but possibly
large) enum-like datatype (like cabal's Extension type incidentally),
then it obviously becomes tedious to have to restate the trivial type
signature for each constructor.
That's why one should really be allowed to group constructor's in a
type's definition:
data Colour :: * where
Red, Green, Blue :: Colour
This is consistent with what is allowed for type signatures for
functions.
More general, whatever way your proposal is going, I think you should
have it reflect that there are two, more or less unrelated, issues here:
1. The expressiveness of data types: algebraic data types <
existential data types < GADTs.
2. The syntax of type definitions: the classic, Haskell 98 syntax and
the new, cool listings-of-constructor-signature syntax. (Don't call
the latter NewTypeSyntax or anything similar in a LANGUAGE pragma;
choose something descriptive.)
These are really orthogonal issues: all three levels of expressiveness
of types can be expressed in either syntax. Therefore: keep these
issues separated in your proposal.
Just my two cents,
Stefan
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