termination for FDs and ATs

Manuel M T Chakravarty chak at cse.unsw.edu.au
Sat Apr 29 19:13:28 EDT 2006

Martin Sulzmann:
> A problem with ATs at the moment is that some terminating FD programs
> result into non-terminating AT programs.
> Somebody asked how to write the MonadReader class with ATs:
> http://www.haskell.org//pipermail/haskell-cafe/2006-February/014489.html
> This requires an AT extension which may lead to undecidable type
> inference:
> http://www.haskell.org//pipermail/haskell-cafe/2006-February/014609.html

The message that you are citing here has two problems:

     1. You are using non-standard instances with contexts containing
        non-variable predicates.  (I am not disputing the potential
        merit of these, but we don't know whether they apply to Haskell'
        at this point.)
     2. You seem to use the super class implication the wrong way around
        (ie, as if it were an instance implication).  See Rule (cls) of
        Fig 3 of the "Associated Type Synonyms" paper.

This plus the points that I mentioned in my previous two posts in this
thread leave me highly unconvinced of your claims comparing AT and FD


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