Generalized Algebraic Datatypes

Generalized Algebraic Datatypes

[Jacques Le Normand]

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Dear Caml list,

I am pleased to announce an experimental branch of the O'Caml compiler:
O'Caml extended with Generalized Algebraic Datatypes. You can find more
information on this webpage:

https://sites.google.com/site/ocamlgadt/


And you can grab the latest release here:

svn checkout https://yquem.inria.fr/caml/svn/ocaml/branches/gadts


Any feedback would be very much appreciated.

Sincerely,

Jacques Le Normand

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Re: [Caml-list] Generalized Algebraic Datatypes

[bluestorm]

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It's very interesting.

First, I'm curious of the "historical" aspects of this work : where does it
come from ? Apparently there is work from you and Jacques Garrigue, but it's
hard to tell. Is it new, or a long-running experiment ?

In your "intuition" section (btw. there is a typo here, it should be (type
s) (x : s t)), you seem to present GADT as directly related to the new (type
s) construct. It's a bit surprising because it's difficult to know the
difference between this and classic type variables. I suppose it is because
only (type s) guarantee that the variable remains polymorphic, and you use
that to ensure that branch-local unifications don't "escape" to the outer
level ? Could you be a bit more explicit on this ?

It's also a bit difficult to know what's the big deal about "exhaustiveness
checks". As I understand it, you remark that with GADTs some case cannot
happen due to typing reasons, but the exhaustive check doesn't know about
it. Could you be a bit more explicit about what the exhaustiveness checker
does :
- is it exactly the same behavior as before, ignoring GADT specificities ?
(ie. you haven't changed anything)
- if not, what have you changed and how can we try to predict its reaction
to a given code ?
- what can we do when it doesn't detect an impossible case ? I suppose we
can't a pattern clause for it, as the type checker would reject it.

I'm not sure I understand the example of the "Variance" section.
Why is the cast in that direction ? It seems to me that even if we could
force t to be covariant, this cast (to a less general type) shouldn't work :

  # type +'a t = T of 'a;;
  # let a = T (object method a = 1 end);;
  # (a :> < m : int; n : bool > t);;
  Error: Type < a : int > t is not a subtype of < m : int; n : bool > t

Again, you "Objects and variants" and "Propagation" subsections are a bit
vague. Could you give example of code exhibiting possible problems ?

Finally, a few syntax trolls, in decreasing order of constructivity :

- is it a good idea to reproduce the "implicit quantification" of ordinary
types ? It seems it could be particularly dangerous here.
  for example, changing
    type _ t = Id : 'a -> 'a t
  to
    type 'a t = Id : 'a -> 'a t | Foo of 'a
  introduce, if I'm not mistaken, a semantic-changing variable captures.
  (I thought other dark corners of the type declarations already had this
behavior, but right now I can't remember which ones)

- if I understand it correctly, (type a . a t -> a) is yet another syntax
for type quantification. Why ? I thought (type a) was used to force
generalization, but ('a . ...)-style annotation already force polymorphism
(or don't they ?). Is it a semantic difference with ('a . 'a t -> 'a), other
than its interaction with gadts ? Can we use (type a . a t -> a) in all
places where we used ('a . 'a t -> 'a) before ?

- is there a rationale for choosing Coq-style variant syntax instead of just
adding a blurb to the existing syntax, such as
    | IntLit of int : int t
  or
    | IntList of int return int t
  ?


Thanks.

On Mon, Oct 25, 2010 at 10:39 AM, Jacques Le Normand <rathereasy@gmail.com>
 wrote:

> Dear Caml list,
>
> I am pleased to announce an experimental branch of the O'Caml compiler:
> O'Caml extended with Generalized Algebraic Datatypes. You can find more
> information on this webpage:
>
> https://sites.google.com/site/ocamlgadt/
>
>
> And you can grab the latest release here:
>
> svn checkout https://yquem.inria.fr/caml/svn/ocaml/branches/gadts
>
>
>
> Any feedback would be very much appreciated.
>
> Sincerely,
>
> Jacques Le Normand
>
>
> _______________________________________________
> Caml-list mailing list. Subscription management:
> http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list
> Archives: http://caml.inria.fr
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs
>
>

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Re: [Caml-list] Generalized Algebraic Datatypes

[Jacques Le Normand]

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On Mon, Oct 25, 2010 at 6:44 PM, bluestorm <bluestorm.dylc@gmail.com> wrote:

> It's very interesting.
>
> First, I'm curious of the "historical" aspects of this work : where does it
> come from ? Apparently there is work from you and Jacques Garrigue, but it's
> hard to tell. Is it new, or a long-running experiment ?
>
>
The history: the algorithm was developed, in part, for my PhD research. I've
been working on it with Jacques Garrigue for the last two months.


> In your "intuition" section (btw. there is a typo here, it should be (type
> s) (x : s t)), you seem to present GADT as directly related to the new (type
> s) construct. It's a bit surprising because it's difficult to know the
> difference between this and classic type variables. I suppose it is because
> only (type s) guarantee that the variable remains polymorphic, and you use
> that to ensure that branch-local unifications don't "escape" to the outer
> level ? Could you be a bit more explicit on this ?
>
>
I don't know what you mean by "remains polymorphic". However, (type s) and
polymorphism are quite distinct concepts. Consider the following examples:

# let rec f (type s) (x : s) : s = f x;;
Error: This expression has type s but an expression was expected of type s
       The type constructor s would escape its scope

# fun (type s) ( f : s -> s) ( x : s) -> f x;;
- : ('_a -> '_a) -> '_a -> '_a = <fun>


The reason I chose to use newtypes, ie (type s), is that I needed a type
variable which did not change (I believe the Haskell people call it rigid),
so I decided to use type constructors. Another option, previously
implemented, was to use polymorphic variables, ie:

let rec foo : 'a. 'a t -> t =
    function
        | IntLit x -> x


However, this has several disadvantages, the biggest of which is that  the
variable 'a cannot be referenced inside the expression since its scope is
the type in which it was introduced.




> It's also a bit difficult to know what's the big deal about "exhaustiveness
> checks". As I understand it, you remark that with GADTs some case cannot
> happen due to typing reasons, but the exhaustive check doesn't know about
> it. Could you be a bit more explicit about what the exhaustiveness checker
> does :
> - is it exactly the same behavior as before, ignoring GADT specificities ?
> (ie. you haven't changed anything)
> - if not, what have you changed and how can we try to predict its reaction
> to a given code ?
> - what can we do when it doesn't detect an impossible case ? I suppose we
> can't a pattern clause for it, as the type checker would reject it.
>
>
This problem is not new in O'Caml. For example:

# type t = { x : 'a . 'a list } ;;
type t = { x : 'a. 'a list; }
# let _ = function { x = [] } -> 5;;
Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
{x=_::_}

however, try creating a value of type ('a. 'a list) satisfying the pattern _
:: _

What I've done is written a second pass to the exhaustiveness checker. The
first pass does the same thing as before, but ignores GADTs completely. The
second pass exhaustively checks every possible generalized constructor
combination.

For example, in the code

type 'a t = Foo : int t | Bar : bool t | Baz : float t

let f : type s. s t * s t * s t -> s =
    function
         Foo, Foo, Foo
      | ....

My code will check all 9 possible patterns and will output any which were
missed. The pattern returned by my algorithm is a valid pattern.


> I'm not sure I understand the example of the "Variance" section.
> Why is the cast in that direction ? It seems to me that even if we could
> force t to be covariant, this cast (to a less general type) shouldn't work :
>
>   # type +'a t = T of 'a;;
>   # let a = T (object method a = 1 end);;
>   # (a :> < m : int; n : bool > t);;
>   Error: Type < a : int > t is not a subtype of < m : int; n : bool > t
>
>
I apologize, that should be:

type -'a t = C : < m : int > -> < m : int > t

or, as a constraint:

type -'a t = C of 'a constraint 'a = < m : int >


> Again, you "Objects and variants" and "Propagation" subsections are a bit
> vague. Could you give example of code exhibiting possible problems ?
>
>
Propagation:

Currently, this code doesn't compile:

    let rec baz : type s . s t -> s =
      fun (type z) ->
function
    IntLit x -> x : s
  | BoolLit y -> y : s

so you need to add the annotation:

    let rec baz : type s . s t -> s =
      fun (type z) ->
((function
    IntLit x -> x
  | BoolLit y -> y) : s t -> s)

objects (and polymorphic variants):

the following will not compile:

    let rec eval : type s . s t -> s =
      function
| IntLit x -> ignore (object method m : s = failwith "foo" end : < m : int;
..>) ; x

polymorphic variants in patterns:

the following will not compile:

    let rec eval : type s . [`A] * s t -> unit =
      function
| `A, IntLit _ -> ()
| `A, BoolLit _ -> ()


> Finally, a few syntax trolls, in decreasing order of constructivity :
>
> - is it a good idea to reproduce the "implicit quantification" of ordinary
> types ? It seems it could be particularly dangerous here.
>   for example, changing
>     type _ t = Id : 'a -> 'a t
>   to
>     type 'a t = Id : 'a -> 'a t | Foo of 'a
>   introduce, if I'm not mistaken, a semantic-changing variable captures.
>   (I thought other dark corners of the type declarations already had this
> behavior, but right now I can't remember which ones)
>

type 'a t = Id : 'a -> 'a t | Foo of 'a

is the same as

type 'b t = Id : 'a -> 'a t | Foo of 'b

In other words, the type variables in generalized constructor definitions
are distinct from the type parameters.


>
> - if I understand it correctly, (type a . a t -> a) is yet another syntax
> for type quantification. Why ? I thought (type a) was used to force
> generalization, but ('a . ...)-style annotation already force polymorphism
> (or don't they ?). Is it a semantic difference with ('a . 'a t -> 'a), other
> than its interaction with gadts ? Can we use (type a . a t -> a) in all
> places where we used ('a . 'a t -> 'a) before ?
>

(type s) does not force generalization (see above); this is why this new
syntax is needed. You can use (type a . a t -> a) anywhere you used ('a. 'a
t -> 'a) could before, assuming that you don't have any types a that you
don't want hidden. This syntax extension is purely syntactic sugar.


>
> - is there a rationale for choosing Coq-style variant syntax instead of
> just adding a blurb to the existing syntax, such as
>     | IntLit of int : int t
>   or
>     | IntList of int return int t
>   ?
>
>
The only rationale is that I want to make it clear that the type variables
found inside generalized constructor definitions are distinct from the type
parameters. In your second example, return is not a keyword in O'Caml. I
could very well have chosen your first example. If there is a consensus on
some alternate syntax, I have no qualms about changing it.

Thank you for the feedback. I will add some of these things to my webpage.

Sincerely,

Jacques Le Normand


> Thanks.
>

> On Mon, Oct 25, 2010 at 10:39 AM, Jacques Le Normand <rathereasy@gmail.com
> > wrote:
>
>> Dear Caml list,
>>
>> I am pleased to announce an experimental branch of the O'Caml compiler:
>> O'Caml extended with Generalized Algebraic Datatypes. You can find more
>> information on this webpage:
>>
>> https://sites.google.com/site/ocamlgadt/
>>
>>
>> And you can grab the latest release here:
>>
>> svn checkout https://yquem.inria.fr/caml/svn/ocaml/branches/gadts
>>
>>
>>
>>
>> Any feedback would be very much appreciated.
>>
>> Sincerely,
>>
>> Jacques Le Normand
>>
>>
>> _______________________________________________
>> Caml-list mailing list. Subscription management:
>> http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list
>> Archives: http://caml.inria.fr
>> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
>> Bug reports: http://caml.inria.fr/bin/caml-bugs
>>
>>

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Re: [Caml-list] Generalized Algebraic Datatypes

[Florian Hars]

Am 25.10.2010 10:39, schrieb Jacques Le Normand:
> I am pleased to announce an experimental branch of the O'Caml compiler:
> O'Caml extended with Generalized Algebraic Datatypes.

Of course, some would claim than 3.12 is already almost there:
http://okmij.org/ftp/ML/first-class-modules/#naive-GADTs
(not that I usually understand what Oleg does...)

- Florian

Re: Generalized Algebraic Datatypes

[Sylvain Le Gall]

On 31-10-2010, Lukasz Stafiniak <lukstafi@gmail.com> wrote:
> On Sun, Oct 31, 2010 at 3:35 PM, Sylvain Le Gall <sylvain@le-gall.net> wrote:
>> On 31-10-2010, Wojciech Daniel Meyer <wojciech.meyer@googlemail.com> wrote:
>>> bluestorm <bluestorm.dylc@gmail.com> writes:
>>>
>>>> It was actually the case in Caml Light : each datatype constructor
>>>> implicitly declared a constructor function with the same name. I
>>>> don't exactly know why this feature was dropped in Objective Caml,
>>>> but I think I remember (from a previous discussion) that people
>>>> weren't sure it was worth the additional complexity.
>>>
>>> Would that be not possible now with Camlp4 extension?
>>>
>>
>> I am pretty sure, it is possible to implement them with camlp4. Just a
>> matter of time -- and motivation.
>>
>> The only limitation I can see, is that the generated constructors won't
>> be capitalized. E.g.:
>>
>> type t = MyConstr | YourConstr of int
>>
>> =>
>>
>> type t = MyConstr | YourConstr of int
>>
>> let myConstr = MyConstr
>> let yourConstr i = YouConstr i
>>
>
> Why do you say so? HOL Light uses capitalized identifiers for values,
> for example. It's probably possible to do whatever one reasonably
> wants.
>

Function names and values are "low id" in OCaml (first letter must be
uncapitalized). If you try to define "let MyConstr = 0" in an OCaml
toplevel, you will get a syntax error... 

The code generated by camlp4 must be syntactically correct.

But maybe you are talking about a deeper integration?

E.g. whenever you encounter the constructor "YourConstr" in expr, you
transform it into "fun i -> YourConstr i". This should work but since
camlp4 is limited to a single module, you won't be able to use this
outside the module, because you won't have access to the definition of
YouConstr and won't be able to determine his arity...

But if you have an idea about how to solve this, just tell us.

Regards,
Sylvain Le Gall

Re: Generalized Algebraic Datatypes

[Sylvain Le Gall]

On 31-10-2010, Wojciech Daniel Meyer <wojciech.meyer@googlemail.com> wrote:
> bluestorm <bluestorm.dylc@gmail.com> writes:
>
>> It was actually the case in Caml Light : each datatype constructor
>> implicitly declared a constructor function with the same name. I
>> don't exactly know why this feature was dropped in Objective Caml,
>> but I think I remember (from a previous discussion) that people
>> weren't sure it was worth the additional complexity.
>
> Would that be not possible now with Camlp4 extension?
>

I am pretty sure, it is possible to implement them with camlp4. Just a
matter of time -- and motivation.

The only limitation I can see, is that the generated constructors won't
be capitalized. E.g.:

type t = MyConstr | YourConstr of int 

=>

type t = MyConstr | YourConstr of int

let myConstr = MyConstr
let yourConstr i = YouConstr i

Regards,
Sylvain Le Gall

Re: Generalized Algebraic Datatypes

[Stefan Monnier]

> type _ t =
>   | IntLit : int -> int t
>   | BoolLit : bool -> bool t
>   | Pair : 'a t * 'b t -> ('a * 'b) t
>   | App : ('a -> 'b) t * 'a t -> 'b t
>   | Abs : ('a -> 'b) -> ('a -> 'b) t 

> There's something "Haskellish" about this syntax, in the sense that type
> constructors are portrayed as being like functions.

Indeed IIRC OCaml does not accept "App" as an expression (you have to
provide arguments to the construct).  Maybe this is a good opportunity
to lift this restriction.

> While this does make sense in Haskell, in Ocaml it feels a bit out of
> place, because you cannot, for example, partially apply
> a type constructor.

The types above don't allow partial applications either.  They use the
OCaml/SML style of constructors were partial application is not possible
because the various arguments are not provided in a curried way.


        Stefan

Re: Generalized Algebraic Datatypes

[Sylvain Le Gall]

On 29-10-2010, Jacques Le Normand <rathereasy@gmail.com> wrote:
>
> I didn't know about this alternate syntax; can you please describe it?
> cheers
> --Jacques

It is on page 14:
http://gallium.inria.fr/~xleroy/talks/cug2008.pdf

And around 14:22 in the video:
http://video.google.com/videoplay?docid=1704671501085578312&hl=en#

Regards
Sylvain Le Gall

Generalized algebraic datatypes

[Jacques Le Normand]

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Dear caml-list,
I'm writing a toy compiler that compiles into ocaml and my toy compiler
supports Generalized Algebraic Datatypes, so I need to compile into a
language which also supports them.
Does ocaml support Generalized Algebraic datatypes? If not, are there any
caml based compilers that support it?
cheers
--Jacques

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