[Agda] eta-equality for records and metavar resolution

[Andreas Abel]

Hi Christian,

good for bringing this up!  This is a known issue:

   http://code.google.com/p/agda/issues/detail?id=376

With Brigitte did the necessary research to fix this, see our TLCA 2011 
paper.

However, I was wondering if this issue shows up in practice.  (See 
comment #6.)  Apparently, you got stuck on it, so it seems worthwhile 
fixing.

A fix seems to be:  Whenever we see a meta-variable applied to a 
projected variable, we rewrite the context such that the projected 
variable is replaced by a "tuple" of fresh variables of the appropriate 
types.  More precisely, we solve the meta-variable by a new one which 
lives in the thus expanded context.

Cheers,
Andreas

On 26.12.13 10:19 PM, Christian Sattler wrote:
> Hi everyone,
>
> Given (A B : Set), abbreviate
>
> T = {C : A → B → Set} → Σ (A × B) (λ {(a , b) → C a b}).
>
> Given an occurrence of a variable (t : T), one would like the implicit
> parameter C to be resolvable from a given value for the resulting type Σ
> (A × B) (λ {(a , b) → C a b}) of the same shape: records are specified
> to have eta-conversion after all. We can test this hypothesis as follows:
>
> f : T → T
> f t = t
>
> Alas, the implicit argument C of t will be marked unresolved: the
> contraint resolver gets stuck at
>
> _C_11 (π₁ x) (π₂ x) =< .C (π₁ x) (π₂ x) : Set.
>
> Why is that? Should eta-expansion not allow us to regard x as a pair (a
> , b), which would simplify the contraint to _C_11 a b =< .C a b?
>
> Ironically, the situation changes if we choose
>
> T = {C : A × B → Set} → Σ (A × B) (λ {(a , b) → C (a , b)})
>
> (but not with B × A!) - because of eta!
>
> Cheers,
> Christian
>
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Theoretical Computer Science, University of Munich
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