+1 for adding the explanation to the Guide. On Fri, Oct 11, 2019 at 9:02 AM David Storrs <david.sto...@gmail.com> wrote: > > Alexis, is there any way that we can get this explanation and some of > the ones you've given me added to the Guide? I've tried to add things > to the documentation before, or even just submit typo reports but, as > I've mentioned before, the way the docs are organized (random little > snippets scattered in unintuitive locations in a huge code base > scattered across multiple repositories) presents too high a barrier to > entry to be worth it for simple changes. Presumably someone who knows > the system better could copy/paste this into an appropriate file and > submit a pull request. > > Your explanations of things are extremely helpful, and really REALLY > need to be part of the docs somewhere. I would add them myself if the > method of doing so were straightforward and/or clearly documented. > Sadly, it is not. > > Dave > > PS: IMO, a vastly better system for the docs would be to have a set > of complete pages that actually correspond to the pages / URLs that > appear in the presentation. I'm not sure why it isn't done that way, > although I'm sure there's a reason. I do think that it's a major > problem that is denying a lot of the value of open-source > documentation. > > On Thu, Oct 10, 2019 at 11:17 PM Alexis King <lexi.lam...@gmail.com> wrote: > > > > tl;dr: You need to use an ellipsis, so your pattern should be ((~between > > x:integer 3 3) ...). A (much) more detailed explanation of why follows. > > > > ~between is an ellipsis-head pattern. The most common ellipsis-head > > pattern, ~optional, also works as a plain head pattern, but ~between does > > not. What’s the difference? > > > > Let’s start by answering what a head pattern is. The simplest kind of > > syntax/parse pattern is a single-term pattern, which (as the name implies) > > only matches a single syntax object at a time. Head patterns are special in > > that they can match zero or more consecutive syntax objects in the head of > > a list. What is the head of a list? Well, if you have a list like '(1 2 3 > > 4), its head is the sequence of elements “1 2 3 4” and its tail is simply > > the empty list, '(). It’s possible to write the list '(1 2 3 4 . ()) to > > make that more explicit. > > > > So when you have a head pattern like (~optional x:integer), it might parse > > an integer, but it also might parse nothing. In the latter case, the next > > head pattern in the sequence would get a chance to parse the same element > > that (~optional x:integer) did. Head patterns are able to do this because > > lists introduce a kind of linear sequencing (not just tree-like nesting), > > so “skipping” an element is an operation that makes sense. > > > > But what about ellipsis-head patterns? These are patterns that don’t just > > appear inside a list pattern, they appear inside a list pattern and under > > an ellipsis. For example, in the pattern (x y ... z), x and z are head > > patterns, but y is an ellipsis-head pattern. While head patterns introduce > > the ability to consume one or more elements at a time, ellipsis-head > > patterns extend that with the power to match elements in the list out of > > order. This is most useful when parsing keyword options, such as in the > > following pattern: > > > > ((~alt (~once (~seq #:foo foo:integer)) (~once (~seq #:bar > > bar:string))) ...) > > > > The above pattern will match (#:foo 1 #:bar "two") or (#:bar "two" #:foo > > 1), but not (#:foo 1) or (#:foo 1 #:foo 2 #:bar "three"). This is because > > ~alt introduces a set of alternatives that can be matched, but unlike a > > simple ~or* pattern, it also keeps track of how many times each case > > matched, and patterns like ~once, ~optional, and ~between introduce > > constraints on the number of times a given case must match for the overall > > parse to be successful. > > > > Interestingly, note that pattern variables bound under ~once and ~optional > > don’t have an ellipsis depth of 1, they have an ellipsis depth of 0. This > > is why, in the given example, you can refer to the foo and bar pattern > > variables in a template without any ellipses. ~between, however, still > > increments the ellipsis depth, since the pattern can actually match > > multiple times. > > > > In the pattern I suggested at the beginning of this email, ((~between > > x:integer 3 3) ...), you’re creating an ellipsis-head context with exactly > > one alternative: (~between x:integer 3 3). That is exactly what you want, > > so everything works out fine. > > > > The one remaining question, however, is why ~between is only allowed as an > > ellipsis-head pattern, but ~optional is also allowed as a head pattern. I > > can’t say for certain, since you can think of ((~optional x:integer)) as > > being sort of implicitly expanded to ((~optional x:integer) ...), and the > > same could be done for ~between. However, my guess is that it isn’t allowed > > because ~between increments the ellipsis depth of its sub-pattern, and Ryan > > thought it would be confusing for a pattern variable’s ellipsis depth to be > > incremented despite there not actually being any ellipses in the pattern. > > Therefore, when using ~between, you have to write the ellipsis explicitly. > > > > Alexis > > > > On Oct 10, 2019, at 20:37, Jonathan Simpson <jjsim...@gmail.com> wrote: > > > > This seems like it should be simple but I've never been able to figure out > > how to do this. What I've been doing instead is this: > > > > (x:integer ...+) to match two or more integers. > > > > (x:integer y:integer ...+) to match three or more. > > > > And so on. > > > > I'm at a point now where I need to build patterns dynamically to match an > > exact number of elements. I'd also like to avoid having to create unique > > names for a bunch of pattern variables. ~between seems like what I want but > > I haven't been able to get it to work. I've been using ~seq without issue > > but that isn't exactly what I need. > > > > Example of an attempt to use ~between: > > > > (syntax-parse #'(1 1 1) [((~between x 3 3)) #'(x ...)]) > > ; stdin::2631: syntax-parse: pattern keyword not allowed here > > ; at: ~between > > > > > > Can anyone give me a quick example of how to do this, using ~between or > > otherwise? I'm using syntax-parse, if that makes a difference. > > > > Thanks! > > > > -- Jonathan > > > > > > -- > > You received this message because you are subscribed to the Google Groups > > "Racket Users" group. > > To unsubscribe from this group and stop receiving emails from it, send an > > email to racket-users+unsubscr...@googlegroups.com. > > To view this discussion on the web visit > > https://groups.google.com/d/msgid/racket-users/7CD020EC-82DF-4117-8C56-A88CDD2EF818%40gmail.com. > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to racket-users+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-users/CAE8gKoeAf5DAO_PFWTQMpGXvHtk1S6rAT7tCZaLQhn-EWBHe%2Bw%40mail.gmail.com.
-- Abraço, Wanderley Guimarães -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CAAmHZofQKC0ta0TYSdAns6CF2OZAsRXu3sNXRe%3DeRzQ0h%3Di8Xg%40mail.gmail.com.
