Re: [RFC] Syntax for fields with both explicit constraints and initializers

[Jose E. Marchesi]

Hi Matt.

> As a solution I propose the following new syntax for struct fields:
>
> TYPE NAME [=INITVAL] [: EXPR]
>
> where INITVAL is the default field value that is used if none is
> provided, and EXPR is a constraint expression. Here's how I would
> propose handling the different combinations of INITVAL and EXPR:
>
> - No INITVAL or EXPR: No change
> - Only INITVAL: No change
> - Only EXPR: No change
> - Both INITVAL and EXPR: INITVAL is treated as only a default value,
> and no implicit constraint is created based on it. The only constraint
> that will exist is the one declared by EXPR. The explicit constraint
> overrides the implicit constraint that would normally be created.
>
> For example we could make the comp_type field above have a default of 
> COMP_NONE like so:
>
> ...
> uint<8> comp_type = COMP_NONE : comp_type in [COMP_NONE, COMP_LZO, COMP_ZLIB];
> ...
>
> and then you can construct a chunk with just:
>
> var chunk = Chunk {}
> assert(chunk.comp_type == COMP_NONE);
>
> Thoughts?

I like this solution.

I just just add a little thing: if both INITVAL and EXPR are provided,
then INITVAL should be checked against EXPR.

In other words, this should raise a constraint expression:

    int i = QUUX : i in [FOO, BAR, BAZ];