Syntax and Native Data Types

Let's get our hands dirty with native data types and common operations.

Syntax and native data types#

Clojure affords developers the standard collection of data types. All data structures are immutable. All data structures are manipulated using interfaces. This allows for easy interoperability with the underlying host.

In this module, we'll learn about native data types and some common operations. We suggest that you start the Clojure REPL and try these out for yourself. You can start the REPL using the clj command:

Reading Clojure code#

If you don't have a background in LISP, Clojure code will feel unnatural at first. This is because the brackets are in a different position.

In general, when you see a Clojure form like:

In your mind, you should think of it as a list where the first element is an operation to execute and other elements are arguments to that operation. This operation can be a function, a macro, or something else.

type function#

The type function tells us the native data type of a value or variable. It can be used as follows:

1 can be a variable or any other data type.


Any data type can be nil. The actual implementation of nil is dependent on the host. For Clojure, it's Java's null, for ClojureScript it's JavaScript's null.

The logical value of nil is false.

You can determine if a variable is nil by using the nil? function. A function suffixed with a question mark ? is conventionally a predicate.


These represent logical true and false values. On the JVM these correspond to java.lang.boolean.TRUE and java.lang.boolean.FALSE respectively.

The functions true? and false? can be used to ascertain if a value is truthy or falsy.


Like all other data types, numbers are also just the host's implementation of numbers. However, the differences across hosts are ironed out. Leaving the implementation details aside, the following types of numbers are supported.


All numbers in Clojure are Long by default. You can check this in the REPL by using the type function.


Non-terminating decimals like 22/7 are stored as ratios. The data type for the ratio is clojure.lang.Ratio. This signifies that Ratio is a Clojure data type and not dependent on the host.

Clojure takes care of the implementation detail across hosts.

BigInt and BigDecimals#

In case a number is too large for the Long type, it can be suffixed with an N to signify clojure.lang.BigInt. For example 44N.

Numbers suffixed with M signify java.math.BigDecimal or the equivalent host implementation of Java's BigDecimal.

Clojure operations intelligently wrap and transform numbers into the correct data type. For example, if you divide 22 by 7 using the divide / function, you'll get a Ratio:

Did you notice how we nested two functions (type and /) together?

Common numerical functions#

Since Clojure has no concept of syntax, numerical operations that you expect from a language are implemented as functions.

For computation, we have +, -, *, /, min, max, quot, and rem.

Algebraic functions can take more than one argument:

The core also ships with the functions inc and dec which add or subtract 1 to a number respectively:

Common comparison functions are also baked into the core. These are ==, >, <, >=, <=, zero?, pos?, neg?.

Like arithmetic operators, some comparison operators also take more than one argument:

The < function above expects the sequence of arguments to be in increasing order. The same is true for other inequality functions.


Strings are denoted with double quotes "like this".

We already know the println function. Some other common string functions are:

  • prn : Like println but doesn't include a terminating newline

  • pr-str: Prints the given string and returns it, unlike prn which returns nil

  • str: Can be used to coerce a value to a string. Can also be used to join multiple strings

  • string?: Determines if a variable is string or not


Characters are used to represent a single glyph, for example, \newline or \a.

Some common char functions are:

  • char: Coerces a variable to char or throws an exception

  • char?: Determines if a variable is char or not


Clojure code is organized in namespaces. We'll see this as we start writing more advanced programs. A namespace is a collection of symbols. Each of these symbols might be used to represent a function or value. Symbols can be imported across namespaces.

When you start the Clojure REPL in the terminal, you will see a prompt user=>. This signifies that you are in the user namespace. If you are familiar with JS modules or Java packages, think of Clojure namespaces as the same concept.

Defining variables#

The function def is used to define static values. All defs are namespaced.

This is equivalent to ES6's const age = 20.


Symbols are identifiers that are used to refer to something else. They are defined as a string and can be namespaced. A symbol can contain only some special characters * + ! / . : - _ ?. A symbol cannot start with a colon.

Here fname is a namespaced symbol, ie it is defined in user namespace. The character / is used to separate the symbol from the namespace. Symbols are namespaced by default. In this case, the term #'user/fname tells us that fname is a symbol defined in user namespace.

You can refer to the symbol by adding a single tick as a prefix: 'fname. The function symbol? can be used to determine if an identifier is a symbol or not.


Keywords are similar to symbols, but unlike symbols that evaluate to the assigned values, keywords evaluate to themselves. They are written with a colon prefix :like-so and are not namespaced by default. The symbol fname above holds a string value, however, the keyword :like-so has no explicit value.

Keywords provide fast equality checks and can be used in places where symbols might not be viable, for example, in the case of enums, where you want a value belonging to a set of predefined values.

Keywords implement the IFn interface, ie are callable, like functions.

Observe how we used :age as a function on map m. The default behavior is to return the value where the key is the keyword being used as the function.

Keywords, like symbols, can be namespaced using a forward slash/. The literal :user/m denotes a keyword in the user namespace. To automatically add the current namespace to a keyword, two colons can be used. ::m is the same as :user/m provided you are in the user namespace.

Some common keyword functions are:

  • keyword?: Check if a variable is keyword or not

  • keyword: Try to coerce a variable to keyword

  • namespace: Get the namespace of the keyword


Composite data structures are loosely termed as collections. These include Lists, Vectors, Hash-Maps, and Sets. Collections are immutable, thread-safe, and are represented as abstractions.

Modifying collections yield new collections that may or may not be of the same type as the collection we started with. The yield will however have the same logical interface.

Collection manipulation is efficient because data is not copied to new collections, only pointed back to. Collections are sequenceable, ie implement the ISeq interface.

Some common collection functions are:

  • count: Counts the number of elements in a collection

  • seq: Returns a sequence that can be looped over. Output sequences might differ for different data types.


Lists in Clojure are defined using parentheses (). Since lists are used to interpret function calls, something like (1 2 3) will throw an error in the REPL. This is because Clojure will try to evaluate this form considering the first element of the list (1) as a function and the remaining elements as the arguments.

To bypass this issue, a single quote can be used. '(1 2 3) can be evaluated in a REPL with no issues.

Lists implement the ISeq interface. New elements are added to the front of the list using conj. Elements can be of any data type.

Lists are conceptually similar to Linked Lists. Appending to the head or tail of the list is very efficient. Lists are also a good choice if you want to access the variable as a sequence.

Some common list operations are:

  • list : Create a list

  • list?: Determine if a variable is a list

  • pop: Get the last element of a list, same as last but more efficient

  • count: Get the total number of elements in a list

  • conj: Add an element to the start of the list

Manipulation of lists is immutable - ie a new list is created each time. However, in terms of implementation, the new list is a view on top of the original list, making operations more efficient.


Vectors are similar to lists, except they are indexed. This allows for faster lookups compared to lists. The lookup complexity of a vector is O(1). Vectors are defined using square brackets [].

A vector is a collection so count, conj, and other collection functions work as intended. Elements are added to the end of a vector using conj.

Vectors also implement the IFn interface, hence can be called as a function. They take a single argument of an index number, and return the value at that index.

Where an index is out-of-bounds an exception is raised.

Some common vector functions are:

  • first: Gets the first element of the vector

  • second: Guess what this would do?

  • take: Return a new vector of the first n elements of the original vector

  • drop: Return a new vector with all but the first n elements of the original vector

  • nth: Get element at index

The functions used in the example above are collection functions ie they can be applied to all data structures that implement the ISeq interface. They will work fine with lists too, although efficiency may vary.


....or just Maps - these are key-value pairs. Maps are collections, hence can be sequenced, counted, and manipulated like collections.

Maps are expressed using curly brackets:

Maps should always have an even number of forms. In the example above, :a, "Hello", and :b, "World" are key-value pairs.

Notice how we don't need any commas or colons to segregate elements. It is a convention for map keys to be Clojure Keywords, however, the keys and values of a map can be any data type. The following map is perfectly fine:

Some common map functions are:

  • assoc: Short for associate. Adds key/value pairs to a map

  • dissoc: Removes key/value pairs from the map

  • keys: Gets all map keys

  • vals: Guess what values do?

  • get: Get the value for a given key

  • get-in: Get the value for a nested key

  • contains?: Check if the map contains a key


Sets are a collection of unique values and can be defined using #{:a :b :c} literal. All collection functions are available. Some common set functions are:

  • union : Combines two sets

  • intersection: Computes set intersection

  • difference: Computes set difference

Union is not defined?! See, I pulled a sneaky on you. Set operations are not available in Clojure core, but need to be imported from clojure.set namespace.

The syntax to require a namespace is as follows:

There are more ways to import namespaces but we'll cover this topic in the modules to come. For now, having imported these functions, you can try to run them yourself.

Defining functions#

The macro defn is used to define functions. We'll learn more about macros, but for now, you can think of macros as a type of function.

The last element of the list invoking the defn function is the return value of that function.

The adder that we implemented in JavaScript, in the Homoiconicity section of this chapter, would look as follows:

Observe how we don't need to specify a return value. The last element of the list is returned automatically.

Commas are optional#

You don't need to use commas as they are considered to be white space. You can add them if you wish, however there is a very strong convention to not use commas at all.

Brackets are used to define the order of invocation#

There is no concept of BODMAS in Clojure and you explicitly need to specify the order of invocation.


Any form starting with a semi-colon is a comment. There are conventionally two kinds of comments:

  • those that start with double semi-colons ;; - used when the entire line is dedicated to the comment

  • those that start with a single semi-colon ; - used when a comment is in line with code


C-style languages have a clear distinction between data structures and code. Consider the following JavaScript code:

The words function and return and the curly brackets are part of the language' syntax.

Clojure (and Lisp in general) doesn't have special forms for syntax. Plain data structures like lists and vectors are used instead. This property is called Homoiconicity or Autoiconicity. The above code in Clojure would look like:

The same data structures are used to define data and code. The code is data.


We've learned about Clojure's Lispy syntax and standard data types. We got familiar with the REPL and saw that all Clojure data structures are immutable. We also discovered that Collections are defined as abstractions and different collections implement different interfaces.

We highly recommend that you get comfortable using the functions showcased in this module. We also recommend that you get a sense of the syntax and the REPL before proceeding.

This module highlights only a subset of operations available for each data type. The Clojure API Docs is a good place to learn more about all available namespaces and methods.

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