termination for FDs and ATs

[Ross Paterson]

Thanks for clarifying this.

On Sat, Apr 29, 2006 at 05:49:16PM -0400, Manuel M T Chakravarty wrote:
> So, we get the following type for f
> 
>   f :: (Mul a b, [Result a b] = b) => Bool -> a -> b -> [Result a b]
> 
> Given the instance definitions of that example, you can never satisfy
> the equality [Result a b] = b, and hence, never apply the function f.

That would be the case if the definition were

	f b x y = if b then x .*. [y] else y

You'd assign f a type with unsatisfiable constraints, obtaining
termination by deferring the check.  But the definition

	f = \ b x y -> if b then x .*. [y] else y

will presumably be rejected, because you won't infer the empty context
that the monomorphism restriction demands.  So the MR raises the reverse
question: can you be sure that every tautologous constraint is reducible?

> So, clearly termination for ATs and FDs is *not* the same.  It's quite
> interesting to have a close look at where the difference comes from.
> The FD context corresponding to (*) above is
> 
>   Mul a [b] b
> 
> Improvement gives us
> 
>   Mul a [[c]] [c] with b = [c]
> 
> which simplifies to 
> 
>   Mul a [c] c
> 
> which is where we loop.  The culprit is really the new type variable c,
> which we introduce to represent the "result" of the type function
> encoded by the type relation Mul.  So, this type variable is an artifact
> of the relational encoding of what really is a function,

It's an artifact of the instance improvement rule, but one could define
a variant of FDs without that rule.  Similarly one could have a variant
of ATs that would attempt to solve the equation [Result a b] = b by
setting b = [c] for a fresh variable c.  In both cases there is the
same choice: defer reduction or try for more precise types at the risk
of non-termination for instances like these.