DIP 1009--Improve Contract Usability--Preliminary Review Round 1

[MysticZach via Digitalmars-d]

On Friday, 23 June 2017 at 09:06:59 UTC, Solomon E wrote:
> What I expected from my impression of existing D syntax was 
> that something like this might be coming up:
>
> T foo(T)(T x, T y)
>     in (x > 0, y > 0)
>     out (r; r > 0)
> {
>     return x % y + 1;
> }
>
> `out ()` has syntax similar to `foreach` and `for`, allowing 
> mentioning something about the return type, or unpacking a 
> return value, then testing it. (When there's no semicolon, an 
> `out` block follows.)

This is a good idea, and I didn't think of it. Points:

* It would be great if you were allowed to omit the identifier 
before the semicolon in cases where you didn't need it. The 
existing foreach grammar does not allow this [1]. I'm not sure if 
that is intended to prevent bugs, or if it was just never thought 
to be useful.

* The grammar for what's inside the parens should also mimic 
what's inside `assert`s, allowing an error message. So the new 
grammar would actually be a mix of the existing `foreach` and 
`assert` grammars, e.g.

out(r; r >0, "contract violated!")

Comparing with the double parens solution:

out(result)(result > 0, "violation!")
out()(otherData > 0, "prohibited!")

vs.

out(result; result > 0, "violation!")
out(; otherData > 0, "prohibited!")

It's a close call. Nice job!

[1] https://dlang.org/spec/statement.html#foreach-statement