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Reuse subpatterns in other patterns

(m/defsyntax ending-with [end]
  ['_ '... end])

(m/rewrite
  [1 2 3 4 5]
  (ending-with ?x)
  ?x)
;;=> 5

(m/rewrite
  [:a :b [1 2 3]]
  [:a :b (ending-with ?x)]
  ?x)
;;=> 3

Self referential patterns

(m/match {:pair [2 [3 [4 5]]]}
 (m/with [%pair [!as (m/or %pair !bs)]]
   {:pair %pair})
 [!as !bs])
;; => [[2 3 4] [5]]

Tokenize a sequence (partitioning)

You can use ..!n as a subsequence grouping facility, and with to define a recursive pattern.

(m/rewrite [1 2 3 0 4 5 6 0 7 8 0 9]
  (m/with [%split (m/or [!xs ..!n 0 & %split]
                        [!xs ..!n])]
    %split)
  [[!xs ..!n] ...])
;; => [[1 2 3] [4 5 6] [7 8] [9]]

Multiple variable length sub-sequences

You can use m/cata to recursively apply the same pattern for identifying a separator and subsequent values. Here we group odd numbers after even numbers together.

(m/rewrite [2 3 5 4 3 2]
  [] [] ; The base case for no values left
  [(m/pred even? ?x) . (m/pred odd? !ys) ... & ?more]
  [[?x [!ys ...]] & (m/cata ?more)])
;; => [[2 [3 5]] [4 [3]] [2 []]]

Get all keys and values from a map

(m/match {1 2 3 4 5 6}
  {& (m/seqable [!ks !vs] ...)}
  [!ks !vs])
;; => [[1 3 5] [2 4 6]]

Webscrape HTML

Use a library like hickory to parse the HTML into data structures, then you can match either the DOM or hiccup. $ is a convenient way to search for matches in sub-trees.

(m/search (fetch-as-hiccup company-directory-page)
  (m/$ [:div {:class "directory-tables"}
        . _ ...
        [:h3 _ ?department & _]])
  ?department)

Recursion, reduction, and aggregation

Patterns can call themselves with the m/cata operator. This is like recursion. You can leverage self recursion to accumulate a result.

(m/rewrite [() '(1 2 3)] ;; Initial state
  ;; Intermediate step with recursion
  [?current (?head & ?tail)]
  (m/cata [(?head & ?current) ?tail])

  ;; Done
  [?current ()]
  ?current)
;; => (3 2 1)

Use the same value from a memory variable twice

When you have a match pattern that contains a memory varible !n and a substitution pattern where you want to make use of the variable in multiple ways, you can't do that directly because [!n !n] would take 2 different values out of !n instead of the same value twice. However, you can easily create two names for the same value in the search pattern with (m/and !n !n2) which will match a single value, but create 2 memory variables.

(me/rewrite [[:a 1] [:b 2] [:c 3]]
  [[!k (me/and !n !n2)] ...]
  [[!k !n (me/app str !n2)] ...])
;; => [[:a 1 "1"] [:b 2 "2"] [:c 3 "3"]]

Replace all occurrences

You can use meander.strategy.epsilon/top-down or bottom-up to find and replace.

(def p
  (s/top-down
    (s/match
      (m/pred string? ?s) (keyword ?s)
      ?x ?x)))
(p [1 ["a" 2] "b" 3 "c"])
;; => [1 [:a 2] :b 3 :c]

Optional values

Say you want to match a number that may be followed by a string, and then a keyword:

[1 "this is fine" :foo]
[1 :foo]

Zeta will include a regex style ? operator.

Prior to zeta there are 3 ways to handle optional values:

a) Write separate patterns:

(m/match [1 "this is fine" :foo]
  [(m/pred number? ?n) (m/pred string? ?s) ?k]
  "first case!"
  [(m/pred number? ?n) (m/pred keyword? ?k)]
  "second case!")
;; => "first case!"

b) Use recursion:

(m/match [1 "this is fine" :foo]
  [(m/pred number? ?n) & (m/with [%tail [(m/pred keyword? ?k)]]
                           (m/or [(m/pred string? ?s) & %tail]
                                 (m/and (m/let [?s nil])
                                        %tail)))]
  [?n ?s ?k])
;; => [1 "this is fine" :foo]

c) Constrain a memory variable to length <= 1:

(m/match [1 "this is fine" :foo]
  (m/and
    [(m/pred number? ?n) . (m/pred string? !s) ..?sn (m/pred keyword? ?k)]
    (m/guard (<= ?sn 1)))
  [?n (first !s) ?k])
;; => [1 "this is fine" :foo]