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Like many lisps, Urn uses ; to mark a line comment. This will result in the parser ignoring all input until the next new line. There is no block comment syntax, meaning you will have to comment each line individually.

Number literals

Numbers follow a similar format as Lua, with additional support for binary literals.

For example, 2, #x2, #b10, 2.e0, .2e1 and 20e-1 are all the same value.


Strings are delimited by double quotes. Characters can be escaped using the \ character, with the following escape codes defined:

Note: Using single quotes may not always be a syntax error. However, it will definitely not give you want you want. For instance, 'foo' is a quoted version of the foo' symbol.

Urn also allows decimal and hexadecimal escape codes. You can specify up to three decimal digits, or x followed by two hexadecimal digits. It is a parser error for the resulting value to exceed 255 (so \300 will not parse). Consequently, "A", "\65" and "\x41" are all equivalent strings.

Strings can also span multiple lines. In this case, all successive lines must be aligned to the character after the opening quote mark. This is especially convenient for documentation strings.

(print! "Hello,

Should you wish to split a string over multiple lines, but not insert a new line character, you can terminate each line with \:

;; Equivalent to (print! "Hello, World!")
(print! "Hello, \


Symbols represent a reference to a specific variable. Symbols can be composed of almost any character:

It is worth noting that symbols are case sensitive: foo is not the same as Foo. You should try to keep variable names case sensitive, using kebab-case should you need to separate words.

Urn also has a couple of naming conventions:

Booleans and nil.

The standard values true, false and nil take no special meaning with Urn: they are just another variable which can be shadowed at will.


Keys are a rather weird type. They follow the same syntactic form as symbols, though must be prefixed with :.

(print! :foo)

Keys primary purpose is as indexes in structures. For instance, you will commonly see code like (.> foo :bar), which is equivalent to Lua’s Generally, you can consider keys to be equivalent to strings. However, when quoted, keys are compiled to an object with the type key.


Lists are delimited by matching pairs of () or []. They can contain any number of elements, as long as the list is terminated.

() ;; The empty list
(foo bar 23 "foo" [:baz]) ;; Another list, including a series of constants and nested lists.


Structs are Urn’s equivalent of tables, holding key value pairs. They are by specifying key value pairs inside {}. For more information, see the const-struct documentation.

{ :foo 2
  :bar 3 }

“Shortcut” characters

Urn offers several shortcut characters, which expand into a list which calls a given variable with the next form. These are as follows:

'foo ;; A quoted symbol, which expands to (quote foo)

`(foo bar) ;; A syntax-quoted list (expands to (syntax-quote (foo bar)) ).

,23 ;; An unquoted number, which expands to (unquote 23).