[isabelle-dev] Typing problem in autogenerated axiom

[Florian Haftmann]

> Perhaps this is debatable, as one can always prove nonemptyness of
> arbitrary types via hidden variables:
> 
> definition
>  metaex :: "('a::{} => prop) => prop"
> where
>  "metaex P == (!! Q. (!! x. PROP (P x) ==> PROP Q) ==> PROP Q)"
> 
> lemma meta_nonempty : "PROP metaex (% x::'a::{}. x == x)"
> unfolding metaex_def
> proof -
>  fix Q
>  assume H: "(!!x::'a. x == x ==> PROP Q)"
>  have A: "y::'a::{} == y ==> PROP Q" by(rule H)
>  have B: "y::'a::{} == y" by(rule Pure.reflexive)
>  show "PROP Q" by(rule A[OF B])
>  qed

Indeed it is the other way round: the meta theory of HOL (also the
minimal HOL of Pure) demands non-emptiness of types.  Postulating an
unspecified constants of an arbitrary type is always admissible.

	Florian

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