On Tue, Feb 13, 2018 at 1:30 PM, Steven Watanabe via Boost < boost@lists.boost.org> wrote:
AMDG
On 02/13/2018 09:44 AM, Zach Laine wrote:
On Mon, Feb 12, 2018 at 10:13 PM, Steven Watanabe via Boost < boost@lists.boost.org> wrote:
On 02/12/2018 06:09 PM, Zach Laine wrote:
On Mon, Feb 12, 2018 at 12:08 PM, Steven Watanabe via Boost < boost@lists.boost.org> wrote:
[snip]
I think you're forgetting that the terminal transform is applied before calling operator()(xxx_tag, ...). Thus, the result is actually:
make_expression<expr_kind::call>(checked_add{}, transform(transform(1_p, *this), *this), transform(transform(2_p, *this), *this)).
Since the extra transform tacks on another call expr, we end up up coming right back around to operator()(call_tag).
I was forgetting that! Thanks.
The bottom line is, any transform which is not idempotent for terminals is badly broken if you unwrap terminals automatically.
If you don't apply the terminal transform, but still unwrap terminals, then it's *still* broken, because then the terminal transform gets skipped entirely.
Actually, the code I posted fails to compile (even after fixing the obvious typos) because the recursion makes it impossible for auto to deduce the return type.
Right, and although this saves the user from runtime infinite recursion, it is a very obscure failure mode. This more than outweighs the convenience of the current interface. I'm convinced the auto-evaluating, and in fact all terminal unwrapping in tag-transforms, should be removed. Zach