Hi Hartmut, On Fri, December 4, 2009 5:38 pm, Hartmut Kaiser wrote:
Adam Butcher wrote:
Hartmut Kaiser wrote:
Well, a sequential-or is (attribute-wise) very much the same as a plain sequence: a >> b. That means if you attach an action to the whole thing _1 will refer to the attribute of the first and _2 to the attribute of the second element in that sequence. I'll add a note to the documentation making this clear.
Ah I see. So in the above snippet _1 will refer to an attribute of type optional<S> and _2 to an attribute of type optional<optional<S>>. Thanks for clearing my misunderstanding up.
Well, I expect it to be an optional<S> only as well, but I'm not sure. Normally this kind of redundancy gets collapsed away (and there is no reason for it to be an optional<optional<S>>.)
Ok, I looked: you're right, but I consider this to be a bug. This will be fixed in the next release, then.
Oh right. I had considered it desired behaviour. The grammar spec is (start || (':' >> -start)) and not (start || (':' >> start)) I would expect optional<S> for both only in the latter case. In the former I am deliberately allowing the specifier to the right of the colon to be omitted -- within the already optional right-hand-side of the sequential-or. This allows for input such as (1) $x // $x (assume it references a sequence) (2) $x[7] // element 7 of $x (3) $x[7:] // a slice of $x beginning at element 7 (4) $x[:7] // a slice of $x up to element 7 (5) $x[2:7] // a slice of $x beginning at 2 ending at 7 (6) $x[:] // equivalent to $x (unspecified bounds) In the action function, wrt to right-hand-side of the sequential-or, I need to determine: a) whether a range specifier has been provided at all (given by (bool) at_c<1>(access_spec)) b) whether the range specifier includes an explicit bound (given by (bool) *at_c<1>(access_spec)) The left-hand-side of the sequential-or is either an (optional) 'range_begin' index if a range specifier is given or a lone 'index' specifier if no range specifier is given. If the two optionals on the right-hand-side were collapsed would I not lose the ability to distinguish between (2) and (3)?
I use multi-argument sequences elsewhere so I should have really thought to try that (hindsight is such a wonderful thing!) In my experiments I had switched the '-start' for 'int_' and was convinced that my _1 had become 'int' by the time it reached the function -- this is what confused me into thinking that it was the right-hand-side that was being delivered as _1. I assume now that this must have been to do with my change causing some earlier error and the compiler had substituted 'int' for some unresolved type which had arrived in my function. 'int_' was probably a bad choice for the test!
Hmmm. Doesn't sounds like it. If you ever come across this again, please drop us a line.
Okay. I did have a go at recreating it but it behaved, annoyingly, as intended. I might have another crack at it at some point.
This lead me on to the following, rather pedantic, musing on the provision of the identity[] directive in the core library to 'flatten' composed attributes. [snip] I wondered if providing identity[] for situations like these may result in more logical user code.
That sounds good to me. Would you be willing to contribute the identity[] directive? In this case we would need some docs, tests, and an example or two as well.
Yes no problem. The implementation was attached to my original mail but when I get the time I will have a look at adding some docs/tests/example also. I've been meaning to learn QuickBook for some time -- this may be just the excuse. Regards Adam