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Re: type recursifs et abreviations cyclique and Co
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Date: -- (:)
From: Xavier Leroy <Xavier.Leroy@i...>
Subject: Re: recursive types
Here is the straight dope (or my view of it, anyway) about recursive
types, or more precisely, the fact that all recursive type expressions
are no longer allowed in OCaml 1.06:

1- It is true that recursive (infinite) type expressions such as

        'a where 'a = 'a list (standing for the infinite type
         ... list list list list

can be added to the ML type system without causing major theoretical
difficulties.  In particular, unification and type inference work just
as well on recursive types (infinite terms) than on regular types
(finite terms).

2- "Classic" ML does not have recursive types, just finite terms as
type expressions.  Except some versions of Objective Caml, you won't
find any implementation of ML that accepts them.

3- The reason Objective Caml supports recursive types is that they are
absolutely needed by the object stuff.  More precisely, recursive
types naturally arise when doing type inference for objects (without
prior declarations of object types).  Hence, Objective Caml performs
type inference using full recursive types (cyclic terms) internally.

4- Still (and now comes the language design issue), one may decide to
impose extra restrictions on type expressions, such as "all cycles in
a type must go through an object type", thus prohibiting recursive
types that don't involve object types, such as ... list list list above.
In OCaml, we have experimented with several such restrictions.  I
think early releases (up to 1.04) had restrictions, though I don't
remember which; 1.05 had no restrictions at all, and was strongly
criticized for that (see below); 1.06 has the "all cycles must go
through an object type" restriction.

5- The main problem with unrestricted recursive types is that they
allow type inference to give nonsensical types to clearly wrong code,
instead of issuing a type error immediately.  For instance, consider
the function

        let f x y = if ... then x @ y else x

Assume I make a typo and type "@" instead of "::" :

        let f x y = if ... then x :: y else x

Any sane ML implementation reports a type error.  But OCaml 1.05 (the
one with unrestricted types) would accept the definition above, and
infer the deeply obscure type:

val f : ('a list as 'a) -> ('b list as 'b) list -> ('c list as 'c)

Calls to the function f will probably be ill-typed, so the error will
eventually be caught, but possibly very far from the actual error (the
definition of f).

Some users of OCaml 1.05 loudly complained that unrestricted recursive
types make the language much harder to use for beginners and
intermediate programmers.  We agreed that they had a strong point
here.  You don't want types such as the above for f.  Really.  Trust me.

So we and decided to go back to recursive types restricted to objects
only --- the reasoning being that this does not reject any "classic"
ML code (which type-checks without recursive types already), but still
lets the right types for objects being inferred.

6- Of course, we forgot that users would exploit the "unrestricted
recursive types" bug of OCaml 1.05, and come back at us claiming it's
a useful feature.  So, let's see how useful are recursive types that
are not objects.  I'm taking Jason Hickey's examples here.

>     type x = x option
> the type "x" should probably be isomorphic to the natural numbers

Such a type can be written more clearly as type x = Z | S of x
(or even better type x = int, but that's a different story).

> Consider an unlabeled abstract binary tree:
> 
>         type 'a t = ('a option) * ('a option)    (* abstract *)
>         ...
>         type node = X of node t

Again, I don't see the point of the 'a t type.  A much clearer way to
describe unlabeled binary trees is:

        type node = Empty | Node of node * node

Notice: no extra boxing here.

The point I'm trying to make here is that pretty much all the time, 
recursive types can be avoided ad clarity of the code can be improved
by using the right concrete types (sums or records) to hide the
recursion, rather than using generic sum or product types such as
"option" and "*", then obtain the desired recursive structure by using
recursive type expressions.

I know of only one or two cool examples where recursive type
expressions come in handy and avoid code duplication that the regular
ML type system forces you to do otherwise.

Now, in reply to Jason Hickey's points:

> 1.  The interpretation of the general recursive type has a
>     well-defined type theoretic meaning.

Yes, but this doesn't imply it's a desirable feature in a programming
language.

> Why not issue a warning rather than forbidding the construction?

That's one option, though issuing meaningful warnings is probably
harder than just rejecting the program.  Another option we discussed
is a command-line flag that changes the behavior of the type-checker
w.r.t. recursive types.

>    For instance, the following code is
>    not forbidden:
>        let flag = (match List.length [] with 0 -> true)
>    even though constructions of this form are "prone to error,"
>    and generate warning messages.

Right.  Some of us think all warnings should be errors, though.  In
this particular case, upward compatibility with the "classic ML" way
leads to accepting the program and just issue a warning.  For
recursive types, the same argument argues in favor of rejecting the
program.

> 2.  The policy imposes a needless efficiency penalty on type
>     abstraction.

Only if you don't hide the recursion inside the abstraction, and
insist on taking fixpoints outside the abstraction.  As I've shown
before, the penalty can almost always be avoided by writing your
concrete types in a "natural" style.  Anyway (warning--silly joke
ahead), since when type theorists are worried about efficiency? (end
of silly joke).

> 3.  If the type system can be bypassed with an extraneous boxing,
>         type x = x t   ----->   type x = X of x t
>     then what is the point?

The programmers write the "X" constructor explicitely in the program,
thus making their intentions clear.  It's completely different from an
inferred recursive type, which is more often than not an unintended
consequence of a coding error.

> 4.  (Joke) All significant programs are "prone to error."  Perhaps the
>     OCaml compiler should forbid them all!

This is an old Usenet-style argument.  Another (joke) conclusion is
that for the same reasons, we should turn off all type-checking and
error checking in the compiler.

>     I use this construction extensively in Nuprl (theorem proving)
>     and Ensemble (communications) applications; do I really need
>     to change my code?

We should have released 1.06 earlier; this would have left you less
time to exploit 1.05's bugs so thoroughly...  At any rate, I'd
certainly encourage you to think about the data structures you use,
and whether you couldn't rewrite them in a clearer way by getting rid
of recursive types and using Caml's concrete datatypes (sums and
records) instead.  Of course, if you come up with convincing real-life
examples (not just type-theoretic examples) of why recursive types are
useful, we'll reconsider.

- Xavier Leroy