[Caml-list] mystified by typing of optional arguments

[Caml-list] mystified by typing of optional arguments

[Francois Rouaix]

Hi all,
The following example mystifies me

let f ?opt x =
   match opt with
   | None -> x
   | Some dx -> x + dx

let g x = x + 1


let h1 = function
   | 0 -> [g]
   | _ -> [f; g]


let h2 = function
   | _ -> [f; g]


h1 is typable but not h2.
Why is that ?

--f
PS: the example code is not meant to be useful in any way other than 
reproducing the behavior of type checking.

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Re: [Caml-list] mystified by typing of optional arguments

[Francois Thomasset]

> Hi all,
> The following example mystifies me
> 
> let f ?opt x =
>    match opt with
>    | None -> x
>    | Some dx -> x + dx
> 
> let g x = x + 1
> 
> 
> let h1 = function
>    | 0 -> [g]
>    | _ -> [f; g]
> 
> 
> let h2 = function
>    | _ -> [f; g]
> 
> 
> h1 is typable but not h2.
> Why is that ?
You cant' type [f;g] either !
# [ f ; g ];;
This expression has type int -> int but is here used with type
  ?opt:int -> int -> int
While [g;g] is a list of funs
FT



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Re: [Caml-list] mystified by typing of optional arguments

[Ville-Pertti Keinonen]

> The following example mystifies me
>
> let f ?opt x =
>   match opt with
>   | None -> x
>   | Some dx -> x + dx
>
> let g x = x + 1
>
>
> let h1 = function
>   | 0 -> [g]
>   | _ -> [f; g]
>
>
> let h2 = function
>   | _ -> [f; g]
>
>
> h1 is typable but not h2.
> Why is that ?

The type of the list is inferred from the first element of the first 
instance.

Similarly:

let x = [ g; f ] (* works *)

let x = [ f; g ] (* doesn't work *)

let x = [ (f : int -> int); g ] (* works *)

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Re: [Caml-list] mystified by typing of optional arguments

[Eric C. Cooper]

On Thu, Apr 03, 2003 at 04:50:44PM +0200, Francois Rouaix wrote:
> The following example mystifies me
> 
> let f ?opt x =
>   match opt with
>   | None -> x
>   | Some dx -> x + dx
> 
> let g x = x + 1
> 
> let h1 = function
>   | 0 -> [g]
>   | _ -> [f; g]
> 
> 
> let h2 = function
>   | _ -> [f; g]
> 
> 
> h1 is typable but not h2.
> Why is that ?

Even simpler: [g; f] type-checks, but [f; g] does not.

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

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RE: [Caml-list] mystified by typing of optional arguments

[Jacques Carette]

This can be simplified further:

let f ?opt x =
     match opt with
     | None -> x
     | Some dx -> x + dx

let g x = x + 1

Then (in ocaml 3.06)

# [f;g];;
Characters 3-4:
  [f;g];;
     ^
This expression has type int -> int but is here used with type
  ?opt:int -> int -> int
# [g;f];;
- : (int -> int) list = [<fun>; <fun>]

In other words it looks like unification of optional types is not
commutative !  This is indeed surprising.  I can understand that this could
simplify implementation, but is there a theoretical reason that underlies
this ?

Jacques

-----Original Message-----
From: owner-caml-list@pauillac.inria.fr
[mailto:owner-caml-list@pauillac.inria.fr] On Behalf Of Francois Rouaix
Sent: April 3, 2003 9:51 AM
To: caml-list@inria.fr
Subject: [Caml-list] mystified by typing of optional arguments 


Hi all,
The following example mystifies me

let f ?opt x =
   match opt with
   | None -> x
   | Some dx -> x + dx

let g x = x + 1


let h1 = function
   | 0 -> [g]
   | _ -> [f; g]


let h2 = function
   | _ -> [f; g]


h1 is typable but not h2.
Why is that ?

--f
PS: the example code is not meant to be useful in any way other than 
reproducing the behavior of type checking.

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Re: [Caml-list] mystified by typing of optional arguments

[Jacques Garrigue]

From: Francois Rouaix <francois@rouaix.org>

> Hi all,
> The following example mystifies me
> 
> let f ?opt x =
>    match opt with
>    | None -> x
>    | Some dx -> x + dx
> 
> let g x = x + 1
> 
> 
> let h1 = function
>    | 0 -> [g]
>    | _ -> [f; g]
> 
> 
> let h2 = function
>    | _ -> [f; g]
> 
> 
> h1 is typable but not h2.
> Why is that ?

As stated in many places in the manual, type inference for optional
arguments is not principal.
But there is a now a -principal option to ocaml and ocamlc, which
attempts to recover principality, and gives more information in such
cases.

% ocaml -principal
        Objective Caml version 3.06+27 (2003-03-31)

[...]
# let h1 = function
     | 0 -> [g]
     | _ -> [f; g]
             ^
  ;;
Warning: Eliminated optional argument without principality
val h1 : int -> (int -> int) list = <fun>


Note that -principal itself is available in ocaml 3.06, but would not
provide the above warning.

Jacques Garrigue

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