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Here is the latest Caml Weekly News, for the week of November 27 to December 04, 2012.

### phantom types and identity function

Ivan Gotovchits asked and Jacques Garrigue replied:
```> These simple signature
>
> module type T = sig
> type 'a t constraint 'a = [< `A | `B ]
> val init: [`A] t
> val f: [`A] t -> [`B] t
> end
>
> can be used to constrain the following module
>
> module T : T = struct
> type 'a t = unit constraint 'a = [< `A | `B]
> let init = ()
> let f x = x
> end
>
> where identity function successfully satisfies the constraint
>
> [`A] t -> [`B] t
>
> but in the following module
>
> module T : T = struct
> type 'a t = {x:int} constraint 'a = [< `A | `B]
> let init = {x=0}
> let f x = x
> end
>
> the same identity function doesn't satisfy.

In the first case, a type abbreviation is used.
Since ('a t) expands to (unit), the parameter is completely
ignored, so that you can replace it by anything.

In the second case, you define a concrete type,
so that the parameter is not forgotten.
Note that you cannot even use subtyping for that:
let f x = (x : [`A] t :> [`B] t)
fails too.
But there is an easy workaround, defining in two steps:
type u = {x:int}
type 'a t = u constraint 'a = [< `A | `B]
```
```I have been a bit confused by this discussion, and found the relevant
part of the manual that may enlighten other list readers:

The OCaml Language, Type and exception definitions
http://caml.inria.fr/pub/docs/manual-ocaml/manual016.html#toc54

"If the type has either a representation or an equation, and the
parameter is free (i.e. not bound via a type constraint to
a constructed type), its variance constraint is checked but
subtyping etc. will use the inferred variance of the parameter,
which may be better; otherwise (i.e. for abstract types or
non-free parameters), the variance must be given explicitly, and the
parameter is invariant if no variance was given."

Note that this would not be needed if we had an explicit way to
express the variance of invariant (the variable appears in both
positive and negative positions) and irrelevant (the variable doesn't
appear except in irrelevant positions) explicitly. We could then
write, say:

type 0'a t = {x : int} constraint 'a = [< `A | `B ]

The absence of such a variance marker means that some OCaml code is
hard to abstract through a module boundary: in presence of the
explicit definition, the type-checker will accept to subtype between
any instances of the type (by simple expansion), but if you abstract
over its definition you cannot express this property anymore. Your
workaround corresponds to statically expressing this irrelevance
through an exported equation, but there are (arguably somewhat
unnatural) scenarios where this isn't convenient.
```
Ivan Gotovchits then asked and Gabriel Scherer replied:
```> And now I'm confused much more =). Please, could you explain the
> relevance between subtyping and type restriction?
>
> When I try to restrict type [t -> t'] by the some type [r -> r'] does
> the compiler checks that [r -> r'] is a subtype of [t -> t']? And even
> if it does, in my example
> [`A] t -> [`B] t
>
> [[`A]] is clearly not a subtype of [[`B]] (and vice versa). So I do not
> see how an explicit variance specification can help.

I was reacting mostly to Jacques' remark that "you cannot even use
subtyping for that".

The relation between subtyping and "constraint" is as explained in the
manual excerpt I quoted. The following is valid:
type 'a t = {x : 'a} ;;
let f x = (x : [ `A ] t :> [ `A | `B ] t);;

but the following is not:
type 'a t = {x : 'a} constraint 'a = [< `A | `B ];;
let f x = (x : [ `A ] t :> [ `A | `B ] t);;
Error: Type [ `A ] t is not a subtype of [ `A | `B ] t
The first variant type does not allow tag(s) `B

The reason for this behavior is that in the first case, t was inferred
covariant, while the presence of a constraint disables variance
inference (in the manual: "otherwise (ie. [...] for non-free
parameters) the variance must be given explicitly)"). You can make the
constrained version work with an explicit variance annotation:
type +'a t = {x : 'a} constraint 'a = [< `A | `B ];;
let f x = (x : [ `A ] t :> [ `A | `B ] t);;

Finally, while in this example 'a occurs positively in ('a t), in your
example 'a did not occur at all. In this case it is correct to coerce
from (foo t) to (bar t), whatever the type expression (foo) and (bar)
are -- they don't need to be in a subtyping relation. The following
works:
type 'a t = { x : int };;
let f x = (x : [ `A ] t :> [ `B ] t);;

I'm calling this form of variance "irrelevant" to highlight that the
parameter can simply be ignored when checking for subtyping (some
people call it "nonvariant", but it's kind of awkward if you already
use "invariant" for the opposite notion of appearing both in positive
and negative position).
The OCaml type checker can use some restricted form of it (eg. the
previous example), but you cannot abstract over it: there is no
variance annotation to express that.

Jacques' workaround replaces the definition of a new constrained type
with a constrained type synonym, that is not checked using variance
but direct expansion. But that works because you know what the
definition expands to. In general there may be situation when you
want to say "type 'a t, where 'a is not used" but not expose a
(potentially abstract) non-parametrized type u such that "type 'a t =
u".

While we're at it: are you sure you need to play with phantom
polymorphic variant types now that we have GADTs? I have used phantom
types in the past, but my personal use cases would be profitably
rewritten using GADTs. They tend to produce hairy error messages, but
phantom polymorphic variants are not better in this regard.
```

### Github OCaml mirror available

```There have been quite a few Git mirrors of the OCaml compiler tree floating
around for the past few years. I've grabbed the "OCaml" organisation on
Github, where key community projects can be hosted such as OASIS, the
ocaml.org website, and a mirror of the compiler itself.

You can find it at:
http://github.com/ocaml/ocaml

The conversion scripts were based on Benedikt Meurer's very useful guide,
with the exception that SVN tags and branches are converted into native Git
tags, and an authors file exists. You can find the scripts that drive it
here:
https://github.com/ocaml/ocaml.org-scripts/tree/master/vcs-sync/svn-ocaml

For those concerned about Github being a proprietary service, I'm also
setting up a git.ocaml.org mirror of all the repositories hosted on Github.
This will be ready in a few weeks. Note that the compiler Git repository is
a *read-only mirror*, and that all active development occurs within the INRIA
SVN, and issues should continue to be reported via Mantis:

http://caml.inria.fr/mantis

Thanks to Leo White and Christophe Troestler for helping with getting this
working! Please consider this experimental for a few weeks in case any
issues come up with the scripts that cause us to delete the repository and
recreate it. If anyone feels strongly about needing a Mercurial mirror,
```

### creating a module from a #use directive in the toplevel

Francois Berenger asked and Gabriel Scherer replied:
```> Is there a way to do something like this in the toplevel:
>
> module M = (#use "source_for_module_m.ml") ;;

See this discussion on the caml-list:
https://sympa.inria.fr/sympa/arc/caml-list/2012-11/msg00021.html
and in particular this message of Gerd:
https://sympa.inria.fr/sympa/arc/caml-list/2012-11/msg00024.html
and this one by Grégoire:
https://sympa.inria.fr/sympa/arc/caml-list/2012-11/msg00032.html

Grégoire's #mod_use directive has been included in trunk ( following
bug report http://caml.inria.fr/mantis/view.php?id=5825 ), so it may
be available by default in future versions of OCaml.
```

### Other Caml News

From the ocamlcore planet blog:
```Thanks to Alp Mestan, we now include in the Caml Weekly News the links to the
recent posts from the ocamlcore planet blog at http://planet.ocaml.org/.

Performance improvements in the universe checker of Coq:
http://gallium.inria.fr/~scherer/gagallium/union-find-and-coq-universes/index.html

Fixing stack overflows using continuations:
http://gallium.inria.fr/~scherer/gagallium/stack-overflow-continuations/index.html

Journée de cours Ocsigen à Paris - Développement d'applications Web en OCaml:
http://ocsigen.org/

How to implement dependent type theory III:
http://math.andrej.com/2012/11/29/how-to-implement-dependent-type-theory-iii/

A note about the performance of Printf and Format: