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Message-ID: <4A5B2C76.9060603@gmail.com>
Date: Mon, 13 Jul 2009 07:45:42 -0500
From: Edgar Friendly <thelema314@gmail.com>
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To: Dario Teixeira <darioteixeira@yahoo.com>
Cc: caml-list@inria.fr, oleg@okmij.org
Subject: Re: [Caml-list] GADTs in OCaml
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X-Spam: no; 0.00; ocaml:01 ocaml:01 invariants:01 printf:01 scanf:01 node:01 node:01 variants:01 edgar:98 polymorphic:01 wrote:01 caml-list:01 href:97 data:02 structures:02 

Dario Teixeira wrote:
> Hi,
> 
>> We present a simple, pure, magic-free implementation of a form of
>> GADTs in OCaml that is sufficient for the common applications of GADTs
>> such as data structures with embedded invariants, typed printf/scanf,
>> tagless interpreters. (...)
> 
> Very interesting -- clever, yet so simple!  But note that as far as the
> link-nodes-shall-not-be-ancestors-of-their-kind problem is concerned, there
> is one problem with the presented implementation.  Consider the following
> declarations:
> 
> let t1 = text "ola"
> let t2 = href "http"
> let t3 = bold [t1; t2]
> let t4 = mref "http" [t1]
> 
> This causes an error upon t4.  The reason is that because of t3, t1 was
> unified as "node_link node_t", whereas t4 requires it to be "node_nolink node_t".  I think the solution is to revert to using polymorphic variants
> for the phantom type, and take advantage of their open-ended nature.
> (Though I'm guessing some extra massaging will be required -- it's Sunday
> and I don't have much time to look into this).
> 
> Anyway, perhaps the Batteries folks will consider including both versions
> of the Eq module?  It will certainly prove useful for many people. (Am I
> correct in assuming you are releasing the code into the public domain or
> at least some open-source friendly license?)
> 
> Best regards,
> Dario Teixeira
> 
> 

Absolutely - this kind of thing seems perfect for batteries included.  I
still need to wrap my head around exactly how to use it, but I'm very
happy to have more flexible types available.

E