Overlapping, undecidable, incoherent -- or worse?

Alex Ferguson abf at cs.ucc.ie
Thu May 20 23:50:00 EDT 2004

On Thu, May 20, 2004 at 09:25:20PM +0100, MR K P SCHUPKE wrote:
> Thats it... Neither GHC nor Hugs pay any attention to the 
> dependancies when choosing which instance to use. The
> dependancies are only considered after the decision has
> been irrevocably made. If the dependancies don't hold, the
> only option the compiler has is to bail out with a compile
> time error.

Let's recap.  You observed that taking dependencies into account leads to
"further problems of the compiler possible not terminating".  I didn't
dispute that ghc and hugs _do not_ take such dependencies into account,
I pointed out that the type system _may already_ not terminate (and not
just hypothetically, but for the option I was using and the class of
instance in my original example, with or without the duplicate heads).
So from a theoretical pov I didn't, and still don't, see how this would
be 'worse'.

That this is clearly not what ghc does or is likely in the near future
to do probably puts this in the category of language list fodder,

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