Re: [Caml-list] Polymorphic Variants for big Ast?

[Jacques Carette <carette@mcmaster.ca>]

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On 04/10/2012 7:15 PM, bob zhang wrote:
> Has anyone have the experience using polymorphic variant for a big Ast?
> The benefit I can think of is  open recursion, global namespace(not in 
> a module). Did anyone give a try?

I have -- mostly because I needed subtying, since the language I was 
modelling had a kind of subtyping that polymorphic variants could track 
'for free'.

It works.  But it can be a real pain too: depending on your use cases, 
you may need a fair amount of annotations (casts).  And if you make a 
mistake, the error messages are truly frightening, especially when you 
have an AST with 4 mutually recursive parts, totaling about 25 cases, 
and the mistake is 3 or 4 levels deep -- the error messages can go on 
for pages and pages.  Buried in there will be the information you need 
to fix the mistake, but finding it can be extremely challenging.

I would say: use it only if you really really need what polymorphic 
variants 'buy' you, else stay away.  In my original code, I have 
rewritten most of it to use normal variants (except for one case) and 
use explicit open recursion (i.e. extra type variable + tying the knot) 
to get the job done.  The error messages are sane now.

Note that I don't think the error messages were incorrect in any way (I 
am sure they were not).  It might have been possible to have made them 
friendlier / more precise, but I am not even sure of that.

Jacques

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