An example of substituability test that is recursive

forax at forax at
Thu Jan 31 18:46:30 UTC 2019

> De: "John Rose" <john.r.rose at>
> À: "Remi Forax" <forax at>
> Cc: "Karen Kinnear" <karen.kinnear at>, "valhalla-spec-experts"
> <valhalla-spec-experts at>
> Envoyé: Jeudi 31 Janvier 2019 19:05:33
> Objet: Re: An example of substituability test that is recursive

> On Jan 31, 2019, at 6:34 AM, [ mailto:forax at | forax at ]
> wrote:

>> The other solution is to say that == should do an upcall to equals (after the
>> null checking and the class checking), if equals throw a StackOverflow, it's
>> the expected behavior because the user is in control of that behavior.

> What you are doing here, I think, is exposing a requirement
> that we *don't* use the control stack for recursion on subst.
> testing (or hashing). That's a reasonable requirement.
> It leads to a worklist algorithm for doing this tricky thing,
> just like we had to do many times in the JIT.

IMO that the other solution, 
solution 1: you use a worklist (and also perhaps a marking algorithm to avoid to crawle the DAG) 
solution 2: you claim it's too complex and you just let the user deal with it by calling equals() (and provide a way for a user to call the default subst). 


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