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Message-ID: <49ED2516.9030103@colba.net>
Date: Mon, 20 Apr 2009 21:44:54 -0400
From: Eliot Handelman <eliot@colba.net>
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Cc: caml-list@inria.fr
Subject: Polymorphism problem
References: <200904202215.27735.jon@ffconsultancy.com>
In-Reply-To: <200904202215.27735.jon@ffconsultancy.com>
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X-Spam: no; 0.00; polymorphism:01 polymorphic:01 rec:01 rec:01 surprising:01 int:01 int:01 seems:03 workaround:03 let:03 let:03 problem:05 intermediate:05 i'd:06 function:08 

Hi list,

Consider this:

type 'a x = { x_v : 'a }

and 'a y = { y_x : int kind;
         y_arr : 'a array
        }
and 'a kind =
    X of 'a x
  | Y of 'a y
    

I'd like to write a function _getter_ that's polymorphic over kind. This
doesn't work, getting int kind -> int:

let rec getter = function
    X x -> x.x_v
  | Y y -> y.y_arr.(getter y.y_x)

which seems surprising to me since getter y.y_x is an intermediate
value that's never returned. Is this just a limitation of the type system or
does this result make sense?

Here's my workaround:
      

let rec int_getter = function
    X x -> x.x_v
  | Y y -> (int_getter y.y_x)
      

let rec getter = function
    X x -> x.x_v
  | Y y -> y.y_arr.(int_getter y.y_x)

where now the type of getter is 'a kind -> 'a as needed. I have no
choice but to use this at present -- is there a better method?

thanks for wisdom,

-- eliot