Clojure is a functional programming language. Creating and using functions–and creating functions that use functions–is what Clojure programers do. Clojure has a number of ways to create a function. Defining a function with defn is the most popular approach.

The syntax is:

(defn name doc-string? attr-map? [params*] body)

It may not be clear how to write a function based on that syntax. It is a good idea to look at how other functions are defined. For example, let’s look at the source code of + function.

  • Advice to coaches

    Please make sure console is opened if attendees are using LightTable. On other repl, in some cases, clojure.repl might not be loaded. If so, don’t forget (use '[clojure.repl]). The source and doc functions will be available after that.

user> (source +)
(defn +
  "Returns the sum of nums. (+) returns 0. Does not auto-promote
  longs, will throw on overflow. See also: +'"
  {:inline (nary-inline 'add 'unchecked_add)
   :inline-arities >1?
   :added "1.2"}
  ([] 0)
  ([x] (cast Number x))
  ([x y] (. clojure.lang.Numbers (add x y)))
  ([x y & more]
    (reduce1 + (+ x y) more)))

Compare the defn syntax and the actual function code.

  • name is +
  • doc-string? is “Returns the sum of nums…(snip)…See also: +’”
  • attr-map? is {:inline (nary-inline...(snip)...:added "1.2"}
  • [params*]s are [], [x], [x y], and [x y & more]
  • bodys follow each param. 0, (cast Number x) and other

We use + function like:

user=> (+)            ; params []
user=> (+ 3)          ; params [x]
user=> (+ 3 4)        ; params [x y]
user=> (+ 3 4 5 6 7)  ; params [x y & more]

You can look up the function’s documentation by using the doc function. doc-string? and other components will be displayed.

user=> (doc +)
([] [x] [x y] [x y & more])
  Returns the sum of nums. (+) returns 0. Does not auto-promote
  longs, will throw on overflow. See also: +'

Clojure has another way to look at functions. meta function shows the definition in attr-map? along with other information. See the example below:

user> (meta #'+)
{:arglists ([] [x] [x y] [x y & more]), :ns #<Namespace clojure.core>, :name +, :column 1, :added
 "1.2", :inline-arities #<core$_GT_1_QMARK_ clojure.core$_GT_1_QMARK_@f73b0ec>, :doc "Returns the
 sum of nums. (+) returns 0. Does not auto-promote\n  longs, will throw on overflow. See also: +'
", :line 936, :file "clojure/core.clj", :inline #<core$nary_inline$fn__3961 clojure.core$nary_inl

See def for what #'+ is.

  • Advice to coaches

    Mention that defn is the same as (def name (fn [params* ] exprs*)) or (def name (fn ([params* ] exprs*)+)), which are explained in (doc defn).

Let’s get back to the defn syntax, (defn name doc-string? attr-map? [params*] body).

  • name is required.
  • doc-string? and attr-map? are optional.
  • [params*] can take multiple forms, [], [x], [x y], [x y & more], [x & more], or [& more]. We need at least one of them.

    In some programming languages, you can overload operators or define multiple versions of the same method with different numbers of arguments. The number of arguments is called arity. Clojure can do that in a single definition.

So far, we learned how to create a function. It’s time to try your own function. Here’s an example of multiple arities:

user> (do-something)
user> (do-something 1)
user> (do-something 1 1)
"hm, ok, will do"
user> (do-something 1 1 1)
"oh, no, so many!"

The first example doesn’t use given arguments. The next example uses arguments.

This add-up function:

  • sums up the given initial value and all elements in the given collection (vector or list)
  • recursively calls itself unless the given collection is empty
  • returns the summed up value when the given collection becomes empty
  • prints out val and coll to visualize what’s going on during the recursive call (in let binding)

(see let binding, cond )

user> (add-up 1 '(1 2))
val: 1, coll: (1 2)
val: 2, coll: (2)
val: 4, coll: clojure.lang.PersistentList$EmptyList@1
user> (add-up 3 [1 2 3])
val: 3, coll: [1 2 3]
val: 4, coll: (2 3)
val: 6, coll: (3)
val: 9, coll: clojure.lang.PersistentList$EmptyList@1

The example above is just practice in creating functions. In real life, you could use reduce in Clojure to write simpler code. With reduce, we can get the same results with the code below:

user> (reduce + 1 '(1 2))
user> (reduce + 3 [1 2 3])

It’s always good to check the Clojure Cheat Sheet or search online for functions.