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patterned

Available via clojars
Leiningen: [patterned "0.2.0"]

Provides defpatterned to define Clojure functions in a somewhat Haskell-like way.

Here is a simple example showing an implementation of that has a single-argument main interface and a two-step recursive function that uses a collecting parameter.

(defpatterned factorial
  [n] (factorial 1 n)
  [so-far 0] so-far
  [so-far n] (factorial (* n so-far) (dec n)))

Matches are made recursively when the pattern contains subvectors:

(defpatterned deep
  [:a [1]]          "a simple match"
  [s [1 [1 [n]]]]   (str "a match of " s " and " n))

user=> (deep :a [1])
"a simple match"
user=> (deep :b [1 [1 [3]]])
"a match of :b and 3"

Rest arguments are supported. Here's a typical recursive implementation of a sequence function:

(defpatterned count-sequence
  [[]] 0
  [[head & tail]] (inc (count-sequence tail)))

user=> (count-sequence [10 9 8 7 6 5 4 3 2 1])
10

A list containing :when may be used to match any argument that passes a predicate. An optional :bind argument names a symbol that will be bound on the right-hand side:

(defpatterned classify-number
  [(:when odd? :bind n)] (str "odd number " n)
  [(:when even?)] (str "some even number"))

user=> (classify-number 5)
"odd number 5"
user=> (classify-number 6)
"some even number"

A list containing :in will match any one of a number of literals. :bind can be used to find out which value matched. Here is a factorial that uses both :in and :when:

(defpatterned factorial
  [(:when (partial > 0) :bind n)] (oops! "Bad number" :n n)
  [(:in [0 1])] 1
  [n] (* n (factorial (dec n))))

The patterned macro is the pattern-matching equivalent of fn:

( (patterned
    [1] "one"
    [n] "other")
  8888)

The letpatterned macro is the pattern-matching equivalent of letfn:

(letpatterned
   [(f1 [0] 0 [n] (f2 (dec n)))
    (f2 [0] 1 [n] (f1 (dec n)))]
 (f2 88))

This library is in support of Functional Programming for the Object-Oriented Programmer. Feel free to use it for other things. Please note:

  • This is not based on clojure.core.match because I had trouble getting that working.

  • Speed was not a concern.

  • Right now, all you can do is write functions. There's no support for matching expressions inside a function.