ghc6 behavior with circular instance declaration

Kenny haskellmail at
Thu Oct 23 18:53:23 EDT 2003

Hi Simon,

 --- Simon Peyton-Jones <simonpj at> wrote:
> I don't know why you would possibly want this.

yes, in this example there isn't any obvious reason to
motivate us writing such instance, but I am interested
in how GHC now handles coinduction in type class. It
will be neat if we can express coinductive algorithm
in terms of Haskell type class. Currently I am working
a project with Dr Martin Sulzmann in which we think
such an extension is neccessary.

> So far as I can see from your message, the recursive
> dictionary
> you are trying to build is exactly the same as what
> GHC will build
> automatically from the "conventional" instance
> declaration.
Yes, exactly, I realized that after I compiled the
code with -fext-core flag. Is it because it tries to
avoid building infinite evidence constructors?

Thanks for pointing out. 

Kenny LU Zhuo Ming

Research Assistant
School of Computing
National University of Singapore

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