[Caml-list] polymorphism in general

[Caml-list] polymorphism in general

[Mihamina Rakotomandimby]

Hello,

I have a question about polymorphism in general.

Let's take this example:
  val polymorph1 : 'a * 'b -> string = <fun>

Is it polymorph?
I have a doubt because the result is always a "string", not something 
related to either "'a" or "'b".

So another way to ask the question: is polymorphism only qualified by 
the parameter?

Thank you for your help.

-- 
RMA.

Re: [Caml-list] polymorphism in general

[Martin Koch]

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A function is polymorphic if it accepts _arguments_ with different types
(the return value is not considered). Thus

# let f x = x + 1;;
val f : int -> int = <fun>

is not polymorphic; it only accepts an integer.

Your function, on the other hand, is polymorphic. It accepts a two-tuple,
where any type may appear on either position, e.g.

# let f (a,b) = "hi there";;
val f : 'a * 'b -> string = <fun>

So I can use f on both, say a string and a bool pair, or an int and an
empty list.

# f("Blue Stilton", false);;
- : string = "hi there"
# f(42, []);;
- : string = "hi there"

/Martin

On Fri, Dec 23, 2011 at 9:30 AM, Mihamina Rakotomandimby <mihamina@rktmb.org
> wrote:

> Hello,
>
> I have a question about polymorphism in general.
>
> Let's take this example:
>  val polymorph1 : 'a * 'b -> string = <fun>
>
> Is it polymorph?
> I have a doubt because the result is always a "string", not something
> related to either "'a" or "'b".
>
> So another way to ask the question: is polymorphism only qualified by the
> parameter?
>
> Thank you for your help.
>
> --
> RMA.
>
> --
> Caml-list mailing list.  Subscription management and archives:
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>
>

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Re: [Caml-list] polymorphism in general

[David Baelde]

Hi,

I would say that any value whose type contains a generalized type
variable ('a, 'b, etc.) is polymorphic.

The empty list (of type 'a list) is polymorphic:
  let empty = [] in (1::empty, "2"::empty)

And so is the function always returning the empty list, even though it
has no polymorphic argument:
  let empty () = []

My 2 cents,
-- 
David