Re: Proposal: Deprecate ExistentialQuantification
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 ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: Proposal: Deprecate ExistentialQuantification
In other words, in your 2x3 grid of syntactic x expressiveness, I want
the two points corresponding to classic syntax x {existential
quantification, GADTs} to be removed from the language. My second
semi-proposal also makes each of the three points corresponding to the
new cool syntax a separate extension.
I see, but why are you opposed to have the classic syntax still support
existentials (though foralls) and GADTs (through equality constraints). I
would make sense to me to keep this support around.
I am opposed since
a) it requires the addition of extra syntax to the language, and
b) we have another, better, way to do it.
Somewhat pointed, I don't think the C++ way of putting all imaginable
ways to do the same thing into the language is a sound design
principle. If we have two ways to do the same thing, and one of them
is considered prefered, then I see no reason at all to keep the other
around. What I'm arguing here is that the GADT style syntax is truly
preferable, and thus the other should be removed.
Cheers,
/Niklas
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Re: Proposal: Deprecate ExistentialQuantification
Niklas,
I am opposed since
a) it requires the addition of extra syntax to the language, and
b) we have another, better, way to do it.
Somewhat pointed, I don't think the C++ way of putting all imaginable
ways to do the same thing into the language is a sound design
principle. If we have two ways to do the same thing, and one of them
is considered prefered, then I see no reason at all to keep the other
around. What I'm arguing here is that the GADT style syntax is truly
preferable, and thus the other should be removed.
I agree. But ;-) since it's obvious not possible to get rid of the
classic syntax completely, I see no harm in having it support
existentials and GADTs as well. In an ideal word, in which there
wasn't a single Haskell program written yet, I'd indeed like to throw
the classic syntax out altogether.
Cheers,
Stefan
On Jun 28, 2009, at 12:32 PM, Niklas Broberg wrote:
In other words, in your 2x3 grid of syntactic x expressiveness, I
want
the two points corresponding to classic syntax x {existential
quantification, GADTs} to be removed from the language. My second
semi-proposal also makes each of the three points corresponding to
the
new cool syntax a separate extension.
I see, but why are you opposed to have the classic syntax still
support
existentials (though foralls) and GADTs (through equality
constraints). I
would make sense to me to keep this support around.
I am opposed since
a) it requires the addition of extra syntax to the language, and
b) we have another, better, way to do it.
Somewhat pointed, I don't think the C++ way of putting all imaginable
ways to do the same thing into the language is a sound design
principle. If we have two ways to do the same thing, and one of them
is considered prefered, then I see no reason at all to keep the other
around. What I'm arguing here is that the GADT style syntax is truly
preferable, and thus the other should be removed.
Cheers,
/Niklas
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Re: Proposal: Deprecate ExistentialQuantification
While I agree in principle that GADTs are the way forward, I have to vote against deprecating anything using the existing syntax in any kind of a hurry. There are syntactic extensions which don't (yet?) work with GADTs that I am loathe to lose, even if they do leave a lot to be desired. Not that I have any real suggestions on what to replace them with, but I think it'd be a good idea to settle that now, before implementing the code that would anyhow need to handle it in half a dozen Haskell implementations. Well, I suppose that's my cue to start noting down ideas. Such a fascinating topic, theorizing about language extensions... -- Svein Ove Aas ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
