Re: assert() inside type x = struct { ...

[apache2]

On Fri, Apr 08, 2022 at 05:12:24PM +0200, Jose E. Marchesi wrote:
> 
> > Is it intentional that assert() does not work inside type definitions like 
> > this?
> >
> > type foo = struct { string yo; };
> > type x = struct {
> >   uint<8> len;
> >   union {
> >     foo myfoo;
> >     byte[len] unknown;
> >   } payload;
> >   assert(payload'size'magnitude/8 == len);
> > };
> 
> Statements inside struct types are not supported, no.
> Only declarations.

So what would be the idiomatic way to achieve this?
It looks like I can call it from inside a lambda in a constraint like this:

#!!# type foo = struct { string yo; };
#!!# type x = struct {
  uint<8> len;
  union {
   foo myfoo;
   byte[len] unknown;
  } payload : (lambda int:{assert(payload'size'magnitude/8 == len);return 
1;})();
}
#!!# var r = open("*data*");
#!!# var z = byte[] @ r : 0#b;
#!!# z[0]=1;
#!!# x @ r : 0#b;
x {
  len=0x1UB,
  payload=struct {
    myfoo=foo {
      yo=""
    }
  }
}

This way I can get a crash (from the unhandled assertion failure) instead of a 
mere
constraint violation, so e.g. this will fail rather than decode as "int yy":
#!!# type ee = union { x xx; int yy; };
#!!# ee @ r : 0#b;
unhandled assertion failure exception
<stdin>:6:28: 

Another way to trigger that lambda without exposing it could be to capture the 
result
in a variable I guess.