Re: Why is this not allowed?

[Pierre Weis <Pierre.Weis@inria.fr>]

> 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/