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Message-ID: <4B443038.5020204@gmail.com>
Date: Wed, 06 Jan 2010 00:39:52 -0600
From: Edgar Friendly <thelema314@gmail.com>
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To: caml-list@yquem.inria.fr
Subject: Error: In the definition of t, type ('a, 'b) t should be ('b, 'a)
 t
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X-Spam: no; 0.00; edgar:98 parameters:03 explanation:06 suggestions:07 definition:07 definition:07 error:12 error:12 should:13 should:13 type:14 type:14 what's:14 what's:14 thanks:17 

This error message was new to me, and I wondered what's going on and why:

# type ('a, 'b) t = [ `A | `T of ('b, 'a) t ];;
Error: In the definition of t, type ('a, 'b) t should be ('b, 'a) t

I can get unhelpful suggestions through foolishness with three type 
parameters:
# type ('a, 'b, 'c) t = [ `A | `T of ('a, 'c, 'b) t | `V of ('b, 'c, 'a) 
t];;
Error: In the definition of t, type ('a, 'b, 'c) t should be ('c, 'a, 'b) t
# type ('c, 'a, 'b) t = [ `A | `T of ('a, 'c, 'b) t | `V of ('b, 'c, 'a) 
t];;
Error: In the definition of t, type ('a, 'b, 'c) t should be ('b, 'c, 'a) t

Any explanation of what's going on?

Thanks,
E