Re: Why is this not allowed?

Re: Why is this not allowed?

[Damien Doligez]

>From: Claudio Sacerdoti Coen <sacerdot@students.cs.unibo.it>
[...]
>can you explain me why is this not allowed?
>I can't see a problematic counterexample.
[...]
># let rec id'' = id;;
>This kind of expression is not allowed as right-hand side of `let rec'

Because we don't know how to compile "let rec x = x" or
  let rec x = y
  and y = x

Moreover, you can just remove the "rec" and it works.  It is possible
to implement "let rec var1 = var2", but the usefulness is quite small
compared to the amount of code we would need to write.

-- Damien

Re: Why is this not allowed?

[John Prevost]

Damien Doligez <Damien.Doligez@inria.fr> writes:

> ># let rec id'' = id;;
> >This kind of expression is not allowed as right-hand side of `let rec'
> 
> Because we don't know how to compile "let rec x = x" or
>   let rec x = y
>   and y = x
> 
> Moreover, you can just remove the "rec" and it works.  It is possible
> to implement "let rec var1 = var2", but the usefulness is quite small
> compared to the amount of code we would need to write.

Well, you can't quite.  Because of the value restriction, id'' defined
like this:

let id'' = id;;

has type:

id'' : '_a -> '_a

(That is, it'll only work for a single type.)

John.

Re: Why is this not allowed?

[Pierre Weis]

> Damien Doligez <Damien.Doligez@inria.fr> writes:
> 
> > ># let rec id'' = id;;
> > >This kind of expression is not allowed as right-hand side of `let rec'
> > 
> > Because we don't know how to compile "let rec x = x" or
> >   let rec x = y
> >   and y = x
> > 
> > Moreover, you can just remove the "rec" and it works.  It is possible
> > to implement "let rec var1 = var2", but the usefulness is quite small
> > compared to the amount of code we would need to write.
> 
> Well, you can't quite.  Because of the value restriction, id'' defined
> like this:
> 
> let id'' = id;;
> 
> has type:
> 
> id'' : '_a -> '_a
> 
> (That is, it'll only work for a single type.)
> 
> John.

There is some confusion here: since «id» is an expression that is a
mere variable, it is not expansive; hence it can be safely generalized;
hence it can be used with different uncompatible types:

# let id x = x;;
val id : 'a -> 'a = <fun>
# let id'' = id;;
val id'' : 'a -> 'a = <fun>
# id'' 1, id'' true;;
- : int * bool = 1, true

Rule of thumb concerning the polymorphism generalisation restriction:

the restriction is performed by matching the right hand side expression e
of a let definition:

1 if e is a constant, a variable expression, a function (that is a
syntactically explicit function, introduced by the keyword fun or
function), or a constructor application, then its type can safely be
generalised.

2 otherwise the type of e is not generalized.

Condition 1 is a rough approximation of the property ``the execution
of e cannot create a polymorphic mutable value''; in practice,
condition 2 means that if e is a function application then its type is
not generalized.

Best regards,

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://cristal.inria.fr/~weis/

Why is this not allowed?

[Claudio Sacerdoti Coen]

Hi,

can you explain me why is this not allowed?
I can't see a problematic counterexample.

        Objective Caml version 2.04

# let id x = x;;
val id : 'a -> 'a = <fun>
# let rec id' x = id x;;
val id' : 'a -> 'a = <fun>
# let rec id'' = id;;
This kind of expression is not allowed as right-hand side of `let rec'

							T.I.A.
							C.S.C.

-- 
-----------------------------------------
Real Name: Claudio Sacerdoti Coen
Address: via del Colle n.6
	 S. Lazzaro di Savena (BO)
	 Italy
e-mail:  sacerdot@cs.unibo.it
-----------------------------------------