[isabelle] Exception in code generator


I'm now trying to define an overloaded Cryptol constant that is supposed to return zero on all types, but quickcheck's code generator seems to be failing on one of the overloaded definitions. Here is a stripped-down example:

  class czero_ty = type +
    fixes czero :: "'a"

  instance bool :: czero_ty
    czero_bool_def:  "czero == False"

  instance "fun" :: (type,czero_ty)czero_ty
    czero_fun_def:  "czero n == czero"

When I try to test "(czero :: int => bool) n = False" with quickcheck, I get the following error:

  *** Unable to generate code for term:
  *** czero
  *** required by:
  *** czero_quickcheck.czero_ty_class.czero def1, <Top>
  *** At command "quickcheck".

I think the problem is in czero_fun_def, because if I try to execute this:

  declare czero_fun_def[code]

then Isabelle returns this exception:

  *** exception TERM raised: dest_Trueprop
  *** At command "declare".

I'm using Isabelle2007.


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