Re: [Caml-list] Labels and polymorphism

[Eric Cooper <ecc@cmu.edu>]

On Thu, Mar 08, 2007 at 10:31:46AM -0800, Nathaniel Gray wrote:
> # let f ~x = x;;
> val f : x:'a -> 'a = <fun>
> # f ~x:1;;
> - : int = 1
> # f 1;;
> - : x:(int -> 'a) -> 'a = <fun>
> 
> Can somebody make sense of this for me?  Is there a paper somewhere on
> labels in ocaml that I should read?

Section 6.7.1 of the manual, subsection "Function application", says:
    As a special case, if the function has a known arity, all the
    arguments are unlabeled, and their number matches the number of
    non-optional parameters, then labels are ignored and non-optional
    parameters are matched in their definition order.

The problem is that if the result type of a function f is 'a, then
it's not of known arity -- an application of f could produce a
function, which could then be applied to more arguments.  For example:

# let id x = x;;
val id : 'a -> 'a = <fun>
# id id id id 17;;
- : int = 17

In your example, if you constrain the type of f, it works as expected:

# (f : x:int->int) 1;;
- : int = 1

-- 
Eric Cooper             e c c @ c m u . e d u