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Date: Tue, 08 Jul 2014 15:37:10 +0200
From: Alain Frisch <alain@frisch.fr>
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To: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>, 
 Gerd Stolpmann <info@gerd-stolpmann.de>
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Subject: Re: [Caml-list] Immutable strings

On 07/08/2014 03:23 PM, Jacques Garrigue wrote:
> Alain’s idea of using an extra type-only parameter (‘a is_a_type) works,
> and it doesn’t really need to be a GADT.
> But this is a bit strange to use an extra parameter where a phantom type
> on string itself would solve the problem.

I mentioned that some functions could behave differently according to 
the requested result type.  For instance, a function

  val of_bool: 'a is_a_string -> bool -> 'a

would return string literals when 'a = String and it would copy them 
when 'a = Bytes.  Similarly, a function could memoize some strings it 
produces in order to return them later again, but only when 'a = String, 
not 'a = Bytes.

Even for functions such as "copy" or "sub", it makes sense to avoid a 
copy in some cases (when both the input and the output are immutable, 
and for sub, when the range covers the entire input).

So I don't think that "'a is_a_string" can really be only a phantom type.

-- Alain