Pattern Matching

• Operators
  • match
  • search
  • find
• Pattern Syntax
  • Literals
  • ClojureScript Literals
  • Variables
    • Logic Variables
    • Memory Variables
    • Any Variables
    • Mutable Variables
  • Operators
    • and
    • or
    • pred
    • guard
    • app
    • let
    • not
    • scan
    • with
  • Subsequences
    • Zero or More
    • N or More
    • Partition
  • Escaping
    • Unquote

Operators

The primary operators for pattern matching and searching are available in meander.epsilon.

(require '[meander.epsilon :as m])

match

The match operator provides traditional pattern matching. It takes an expression to "match" followed by a series of pattern/action clauses.

(m/match x  ;; 1
  pattern   ;; 2
  action    ;; 3
  ,,,)

1. x is the expression.
2. pattern is the pattern to match against the expression. Patterns have special syntax that is important to understand.
3. action is the action expression to be evaluated if pattern matches successfully. Certain patterns can bind variables and, if a match is successful, will be available to the action expression.

A pair of pattern and action is called a clause.

(m/match [1 2 1]
  [1 2]
  :first-action

  [1 2 1]
  :second-action)
;; =>
:second-action

(m/match []
  [1 2]
  :first-action

  [1 2 1]
  :second-action)
;; #error {:cause "non exhaustive pattern match" }

Like clojure.core/case, if no patterns match an exception will be thrown.

search

The search operator is an extended version match which returns a sequence of all action values which satisfy their pattern counterparts. Map patterns with variable keys, set patterns with variable subpatterns, or two side-by-side zero or more subsequence patterns, are all examples of patterns which may have multiple matches for a given value. search will find all such matches and, unlike match, will not throw when a pattern match could not be made. In essence, search allows you to query arbitrary data.

;; Find all pairs of an odd number followed by an even number in the
;; collection.
(m/search [1 1 2 2 3 4 5]
  [_ ... (m/pred odd? ?a) (m/pred even? ?b) . _ ...]
  [?a ?b])
;; =>
([1 2] [3 4])

find

The find operator is similar to search, however, returns only the first search result. If it cannot be found, find returns nil.

;; Find the first pair of an odd number followed by an even number in
;; the collection.
(m/find [1 1 2 2 3 4 5]
  [_ ... (m/pred odd? ?a) (m/pred even? ?b) . _ ...]
  [?a ?b])
;; =>
[1 2]

Pattern Syntax

Literals

The simplest patterns to express are literal patterns. Literal patterns are

1. scalar data types such as numbers, strings, booleans, keywords;
2. symbols that are not considered special by Meander such as variables;
3. and lists and vectors composed of literals and do not contain maps, sets, or subsequence operators.

Notice that maps and sets are not included in the above list, we'll come back to that in a moment.

Literal patterns match exactly themselves. For example, the literal pattern

(fn [] "foo")

matches a list where the first element is the symbol fn, the second is the empty vector, and the third is the string "foo".

List and vector patterns may also qualify as literal patterns if they contain no map or set patterns.

([1] [2] [3])

is a literal pattern, however,

[{:foo 1} #{:bar :baz} {:quux 3}]

is not. This is because map and set patterns express submap and subset patterns respectively. The pattern

{:foo 1}

expresses the value being matched is a map containing the key :foo with value 1. That means that there may be more keys. For example the above pattern would match the following list: {:foo 1 :bar 2}.

The pattern

#{:foo :bar}

expresses the value being matched is a set containing the values :foo and :bar. Again, this does not mean that these are the only elements in the set.

ClojureScript Literals

In ClojureScript it is possible to pattern match on JavaScript Arrays and Object using the #js [] and #js {} literal syntaxes respectively.

(m/match #js [1 2 1]
  #js [?x ?y ?x]
  ?x)
;; => 1

(m/match js/process
  #js {:version ?version, :platform ?platform}
  [?version ?platform])
;; =>
["v11.4.0" "darwin"]

#js {} pattern matching is very liberal and matches against any non nil equivalent JavaScript object.

Variables

Pattern variables are variables which may or may not bind symbols to the values they match. In the case of variables which bind, the bindings are made available for use in pattern actions, substitutions, and even within patterns. There are two types of pattern variables which bind, logic variables and memory variables, and one type of variable which does not, the so-called any variable also known as a wild card.

Logic Variables

Logic variables are variables which express an equivalent, but not necessarily identical, value everywhere within a pattern. They are represented by a simple symbol prefixed with the ? character.

To express any 2-tuple composed of equivalent elements we would write the following.

[?x ?x]

This pattern will match a value like

[1 1]

and bind ?x to 1 but will not match a value like

[1 2]

since the second occurrence of ?x is not equal to 1.

Note that a logic variable in place of a map's value might have a surprising result:

(m/match {:a 1}
  {:b ?b}
  ?b)
;; =>
nil

One might expect this pattern to fail. This behavior is useful in other situations though since in Clojure it is idiomatic to leave a key out completely when there's no reasonable value for it:

(doseq [person [{:name "John Doe" :title "MD"}
                {:name "Mike Foe"}]]
  (m/match person
    {:name ?name :title ?title}
    (println (str ?name (when ?title (str ", " ?title))))))
;; =>
John Doe, MD
Mike Foe
nil

If you wish to match a key's value to a non-nil value you can use:

(m/match {:name "Mike Foe"}
  {:name (m/pred some? ?name)
   :title (m/pred some? ?title)}
  [?name ?title])
;; =>
Execution error (ExceptionInfo) at user/eval4134$fail (REPL:1).
non exhaustive pattern match

Or if you just need to ensure a key is present without binding it:

(m/match {:name "John Doe" :title "MD"}
  (m/pred #(contains? % :title) ?p)
  :has-a-title)
;; =>
:has-a-title

pred will be discussed shortly.

Memory Variables

Memory variables are variables which "remember" or collect values during pattern matching. They are represented by a simple symbol prefixed with the ! character. Because they collect multiple values it is idiomatic to employ a plural naming convention e.g. !xs or !people.

To collect values from a 4-tuple such that we collect the first and last elements in one container and the middle elements in another we would write the following.

[!xs !ys !ys !xs]

This pattern will match a value like

[:red :green :yellow :blue]

and bind !xs to [:red :blue] and !ys to [:green :yellow].

Any Variables

Any variables are variables which match anything but do not bind the values they match. They are represented as simple symbols prefixed with the _ character e.g. _, _first-name, and so on. Any variables commonly appear in the last clause of a match expression as a catch-all when all other patterns fail to match.

Mutable Variables

Mutable variables are variables which, like any variables, will match anything but, unlike any variables will bind the values they match. They are represented as simple symbols prefixed with the * character e.g. *scratch. Mutable variables were introduced as a primitive in order to derive specific features cleanly.

Matching the pattern

[*m *m]

against

[1 2]

would first bind *m to 1, and then ultimately to 2.

Operators

guard

(m/guard expr)

matches whenever expr is truthy.

(m/match :anything
  (m/guard (= 1 1)) :okay)
;; => :okay

pred

(m/pred pred-fn pat-0 ,,, pat-n) matches whenever pred-fn applied to the current value being matched returns a truthy value and all of pat-0 through pat-n match.

(m/match 42
  (m/pred even?)
  :okay)
;; => :okay

(m/match [42 43]
  [(m/pred even? ?x) (m/pred odd?)]
  ?x)
;; => 42

app

(m/app fn-expr pat-0 ,,, pat-n)

matches whenever fn-expr applied to the current value being matched matches pat-0 through pat-n.

(m/match 42
  (m/app inc (m/pred odd? ?x))
  :even

  _
  :odd)
;; =>
:even

(m/match (list 1 2 3)
  (m/and (m/app first ?x) (m/app rest ?xs))
  {:x ?x, :xs ?xs})
;; =>
{:x 1, :xs (2 3)}

let

(meander.epsilon/let [pattern clojure-expr ,,,])

(meander.epsilon/let [pattern clojure-expr ,,,]
  target-pattern)

matches when all of the patterns matches the result of evaluating their corresponding clojure-expr. This allows pattern matching on an arbitrary expression. Optionally, target-pattern may be provided to pattern match on the current pattern matching target.

(m/match :not-a-pair
  (m/or [?x ?y]
        (m/let [?x 1, ?y 2]))
  [?x ?y])
;; => [1 2]

not

(meander.epsilon/not pattern)

is the negation of a pattern. It will match anything that does not match pattern.

(m/match 12
  (m/not 42)
  :yep)
;; => :yep

(m/match 42
  (m/not 42)
  :yep

  _
  :nope)
;; => :nope

and

(meander.epsilon/and pat-0 ,,, pat-n)

matches when all of pat-0 through pat-n match.

(m/match 42
  (m/and ?x (m/guard (even? ?x)))
  ?x)
;; => 42

or

(meander.epsilon/or pat-0 ,,, pat-n)

matches when any one of pat-0 through pat-n match.

(m/match 42
  (m/or 43 42 41)
  true)
;; =>
true

Note that unbound variables must be shared by pat-0 through pat-n.

(m/match [1 2 3]
  (m/or [?x ?y]
        [?x ?y ?z])
  [?x ?y])
;; Every pattern of an or pattern must have references to the same
;; unbound variables.
;; {:pat (m/or [?x ?y] [?x ?y ?z]),
;;  :env #{},
;;  :problems [{:pat [?x ?y], :absent #{?z}}]}

scan

(meander.epsilon/scan pattern)

searches a sequence for elements that match pattern.

(m/search [1 2 3]
  (m/scan ?x)
  ?x)
;; =>
(1 2 3)

(m/search {:x 1 :y 2 :z 3}
  (m/scan [?a ?b])
  {?b ?a})
;; => ({1 :x} {2 :y} {3 :z})

$

(meander.epsilon/$ pattern)
(meander.epsilon/$ context pattern)

recursively searches for any subtree that matches pattern.

(m/search [[1] 2 [[3 4] 5]]
  (m/$ [?a ?b])
  [?a ?b])
;; => ([[3 4] 5] [3 4])

Additionally, you can optionally specify a context variable that, when called with an argument, returns the toplevel collection with all matched variables replaced with the argument.

(m/search [[1] 2 [[3 4] 5]]
  (m/$ ?context [?a ?b])
  (?context [9])
;; => ([[1] 2 [9]] [[1] 2 [[9] 5]])

with

The with pattern operator enables patterns to be bound much like clojure.core/let.

(m/with [%ref1 pat1 
         ,,,
         %refn patn]
  pat)

These pattern bindings are called "references" and are named with simple symbols prefixed by the % character. References may be specified in any order and may also be recursive. In essence, the with operator allows for novel and powerful feature: the ad-hoc construction and matching of recursive grammars.

(let [hiccup [:div
              [:p {"foo" "bar"}
               [:strong "Foo"]
               [:em {"baz" "quux"} "Bar"
                [:u "Baz"]]]
              [:ul
               [:li "Beef"]
               [:li "Lamb"]
               [:li "Pork"]
               [:li "Chicken"]]]]
  ;; meander.match.delta/find
  (m/find hiccup
    (m/with [%h1 [!tags {:as !attrs} . %hiccup ...]
             %h2 [!tags . %hiccup ...]
             %h3 !xs
             %hiccup (m/or %h1 %h2 %h3)]
      %hiccup)
    [!tags !attrs !xs]))
;; =>
[[:div :p :strong :em :u :ul :li :li :li :li]
 [{"foo" "bar"} {"baz" "quux"}]
 ["Foo" "Bar" "Baz" "Beef" "Lamb" "Pork" "Chicken"]]

In the example above, with is used to (naively) describe and match hiccup. Notice that references %h1, %h2, and %hiccup refer to each other in their definitions. The "body" of the with form says we wish to match %hiccup against the current value being matched, in this case hiccup. When the match executes it does so recursively and, as we can see, correctly.

Subsequences

When matching subsequences it is often useful to express the notions of zero or more and n or more things. The postfix operators ... or ..n respectively provide this utility.

Zero or more

The ... postfix operator matches the subsequence of patterns to its left (up to the first . or start of the collection) zero or more times.

(m/match [1 2 1 2 2 3]
  [1 2 ... ?x ?y]
  [?x ?y])
;; =>
[2 3]

(m/match [:A :A :A :B :A :C :A :D]
  [:A !xs ...]
  !xs)
;; =>
[:A :B :C :D]

Note that multiple unbounded sequences are allowed in a pattern only for search and find, match will throw an exception since the result is non-determinitic.

(m/search [1 1 2 3]
  [1 ... !rest ... ]
  !rest)
;; =>
([1 1 2 3] [1 2 3] [2 3])

(m/match [1 1 2 3]
  [1 ... !rest ... ]
  !rest)
;; =>
Syntax error macroexpanding mm/match at (REPL:1:1).
A variable length subsequence pattern may not be followed by another variable length subsequence pattern.

N or more

The ..n postfix operator matches the subsequence of patterns to its left (up to the first . or start of the collection) n or more times where n is a positive natural number.

(m/match [1 1 1 2 3]
  [1 ..3 ?x ?y]
  [?x ?y])
;; =>
[2 3]

(m/match [1 2 3]
  [1 ..3 ?x ?y]
  [:okay [?x ?y]]
  _
  [:fail])
;; =>
[:fail]

(m/match [1 1 1 2 3]
  [1 ..3 ?x ?y]
  [:okay [?x ?y]]
  _
  [:fail])
;; =>
[:okay [2 3]]

Partition

The . operator, read as "partition", partitions the collection into two parts: left and right. This operator is used primarily to delimit the start of a variable length subsequence. It is important to note that both ... and ..n act as partition operators as well.

(m/match [3 4 5 6 7 8]
  [3 4 . !xs !ys ...]
  [!xs !ys])
;; =>
[[5 7] [6 8]]

Had the pattern [3 4 . !xs !ys ...] in this example been written as [3 4 !xs !ys ...] the match would have failed. This is because the latter pattern represents a subsequence of 4 elements beginning with the sequence 3 4.

(m/search [3 0 0 3 1 1 3 2 2]
  [_ ... 3 . !ys ...]
  {:!ys !ys})
;; =>
({:!ys [0 0 3 1 1 3 2 2]}
 {:!ys [1 1 3 2 2]}
 {:!ys [2 2]})

This example demonstrates how search finds solutions for patterns which have sequential patterns which contain variable length subsequences on both sides of a partition. The pattern [_ ... 3 . !ys ...] says find every subsequence in the vector being matched after any occurrence of a 3.

Escaping

unquote

In some cases you may want to "parameterize" a pattern by referencing an external value. This can be done using Clojure's unquote operator (unquote-splicing is currently not implemented).

(def x 2)

(defn match-my-map [m]
  (m/match m
    {:x ~x :y ?y}
    [:okay ?y]

    _
    [:fail]))

(match-my-map {:x 1 :y 3})
;; =>
[:fail]

(match-my-map {:x 2 :y 3})
;;=>
[:okay 3]

;; The first two elements summed together equals the third.
(let [f (fn [z]
          (m/match z
            [?x ?y ~(+ (nth z 0) (nth z 1))]
            :yes
            _
            :no))]
  [(f [1 2 3])
   (f [2 1 4])
   (f [1 3 4])])
;; =>
[:yes :no :yes]