Re: Offset of a value's element by name

[Jose E. Marchesi]

Given the lack of feedback I just went ahead and implemented this as
`eoffset' in the standard library.

> Hi people.
>
> During one of the demos at the binary tools summit we felt the need of,
> given a Poke struct value, get the offset of one of its fields by name.
>
> We already support the following value attributes:
>
>   VAL'length
>     Evals to the number of elements (array elements or struct fields) of
>     the given composite value.
>
>   VAL'eoffset (IDX)
>     Evals to the offset of the element (array element or struct field)
>     at the given position IDX in the given composite value.
>   
>   VAL'ename (IDX)
>     Evals to the name of the element (array element or struct field)
>     at the given position IDX in the given composite value.  This is
>     "[IDX]" for array elements, and the empty string "" for unnammed
>     struct fields.
>
> Now, we could add a new attribute:
>
>   VAL'eoffset (STRING)
>     Evals to the offset of the element (array element or struct field)
>     having the given name.
>
> But, it is way easier to do it in Poke instead of adding a new
> language-level construction:
>
>   fun eoffset = (any v, string n) offset<uint<64>,b>:
>   {
>     for (var i = 0UL; i < v'length; ++i)
>       if (n == v'ename (i))
>         return v'eoffset (i);
>     raise E_inval;
>   }
>
> Example usage:
>
> (poke) type Foo = struct { int i; int; int j; }
> (poke) var f = Foo {}
> (poke) eoffset_by_name (f, "j")
> 0x40UL#b
> (poke) eoffset_by_name (f, "i")
> 0x0UL#b
>
> Now the question is: should we add the above function to the standard
> library?  With what name?
>
> Opinions?