Thank you Alexis for the clear explanation. I now understand how to use ~between and it is working for me.
One small hitch I encountered is a custom syntax class I defined doesn't work in the ~between statement but works elsewhere within the same syntax pattern. This isn't a huge issue for me as I just copied the pattern in place of the syntax class but I am curious why the :integer syntax class works and my custom one doesn't. Once again, thanks for taking the time to explain this! -- Jonathan On Thursday, October 10, 2019 at 11:17:53 PM UTC-4, Alexis King 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 <jjsi...@gmail.com > <javascript:>> 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. 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