Lexical Conventions

All configuration files read by Dune use a simple syntax that’s similar to S-expressions. The Dune language can represent three kinds of values: atoms, strings, and lists. By combining these, it’s possible to construct arbitrarily complex project descriptions.

A Dune configuration file is a sequence of atoms, strings, or lists separated by spaces, newlines, and comments. The other sections of this manual describe how each configuration file is interpreted, and we illustrate the syntax below:


The Dune language only has end of line comments. A semicolon introduces end of line comments and span up to the end of the current line. The system ignores everything from the semicolon to the end of the line. For instance:

; This is a comment


An atom is a non-empty contiguous sequences of character other than special characters. Special characters are:

  • spaces, horizontal tabs, newlines and form feed

  • opening and closing parenthesis

  • double quotes

  • semicolons

For instance hello or + are valid atoms.

Note that backslashes inside atoms have no special meaning and Dune always interprets them as plain backslash characters.


A string is a sequence of characters surrounded by double quotes. A string represent the exact text between the double quotes, except for escape sequences. A backslash character introduces escape sequences. Dune recognizes and interprets the following escape sequences:

  • \n to represent a newline character

  • \r to represent a carriage return (character with ASCII code 13)

  • \b to represent ASCII character 8

  • \t to represent a horizontal tab

  • \NNN, a backslash followed by three decimal characters to represent the character with ASCII code NNN

  • \xHH, a backslash followed by two hexadecimal characters to represent the character with ASCII code HH in hexadecimal

  • \\, a double backslash to represent a single backslash

  • \%{ to represent %{ (see Variables)

Additionally, you can use a backslash just before the end of the line. This skips the newline leading up to the next non-space character. For instance, the following two strings represent the same text:


In most places where Dune expects a string, it will also accept an atom. As a result, it’s possible to write most Dune configuration files using very few double quotes. This is very convenient in practice.

End of Line Strings

You can also write string using end of line strings. They are a convenient way to write blocks of text inside a Dune file.

The characters "\| or "\> introduce end of line strings and span to the end of the current line. If the next line also starts with "\| or "\>, Dune reads it as a continuation of the same string. For readability, either leave the text following the delimiter empty or start it with a space (that will be ignored).

For instance:

"\| this is a block
"\| of text

represents the same text as the string "this is a block\nof text".

Escape sequences are interpreted in text that follows "\| but not in text that follows "\>. Both delimiters can be mixed inside the same block of text.


Lists are sequences of values enclosed by parentheses. For instance (x y z) is a list containing the three atoms x, y and z. Lists can be empty, for instance: ().

Lists can be nested, allowing arbitrary representation for complex descriptions. For instance:

 (head (title "Hello world!"))
   This is a simple example of using S-expressions))