71 lines
3.8 KiB
Markdown
71 lines
3.8 KiB
Markdown
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---
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title: Clojure More on Functions
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---
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Functions! They're pretty important. It's very difficult to do anything without a function. They're integral to any language, but especially Clojure, since it's a functional programming language that rejects object-oriented design. Let's learn some more about them!
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## Arity
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**Arity** refers to the number of arguments that your function expects.
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;; add expects 2 arguments. Its arity is 2.
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(defn add [x y] (+ x y))
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(add 2 2)
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; => 4
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;; + itself is a function, and it can have any number of arguments.
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(+ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16) ;; and so on...
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; => 136
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Clojure has some special syntax that allows you to let your function do different stuff depending on the number of arguments it receives. This is called variable arity.
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(defn foo
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([] ; if this function gets no arguments...
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(println "Lisa needs braces!")) ; do this.
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([arg1] ; if this function gets 1 argument...
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(println "Dental plan!"))) ; do this instead!
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(foo)
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; => Lisa needs braces!
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; nil
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(foo "this is a placeholder argument.")
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; => Dental plan!
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; nil
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 <a href='https://ideone.com/sXGplb' target='_blank' rel='nofollow'>IDEOne it!</a>
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## Anonymous functions
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Let's look at a really simple function: a function that adds 1 to a number.
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;; I've called this function "my-inc" so you don't confuse it with inc.
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;; inc is a built-in function that already does this for us.
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(defn my-inc [n] (+ 1 n))
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(inc' 5)
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; => 6
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This looks pretty simple. It takes a single parameter - `n` - and returns `n + 1`. Let's pick it apart.
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(def my-inc-2 (fn [n] (+ 1 n)))
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(inc' 5)
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; => 6
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You can see from this that using `defn` is just shorthand for using `(def ... (fn ...))`. But this reveals something interesting. What we're actually doing isn't 'defining a function', it's just binding an anonymous function to a special name - `inc'`. What if we don't give it a name?
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((fn [n] (+ 1 n)) 5)
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; => 6
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Bam! Boom! Kapow! Anonymous functions. This might seem useless now, but it comes in pretty handy later on for applying functions to lists using `map`, `reduce` and `filter`. Giving every function you write a name gets boring and cumbersome, fast.
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There's a shorter way to write anonymous functions, intended for very short, simple functions. It does not allow for destructuring or variable arity. However, it is quite concise.
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(#(+ 1 %) 5)
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; => 6
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`#(...)` is a shorthand way to define an anonymous function. `%` refers to the first argument to the function. If your function expects several arguments, you can use `%1, %2, ... %n`.
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(#(str %1 %2 %3) "foo" "bar" "baz")
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; => "foobarbaz"
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 <a href='https://ideone.com/roYRgS' target='_blank' rel='nofollow'>IDEOne it!</a>
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| [ Previous](//forum.freecodecamp.com/t/clojure-loop-recur/18418) | [ Home ](//forum.freecodecamp.com/t/clojure-resources/18422) | [Next ](//forum.freecodecamp.com/t/clojure-collections/18411)|
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| [Loop and Recur](//forum.freecodecamp.com/t/clojure-loop-recur/18418) | [Table of Contents](//forum.freecodecamp.com/t/clojure-resources/18422) | <a href='/http://forum.freecodecamp.com/t/clojure-collections/18411' target='_blank' rel='nofollow'>Collections</a>|
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