Chapter 12 Language extensions

13 Extension nodes

(Introduced in OCaml 4.02, infix notations for constructs other than expressions added in 4.03, infix notation (e1 ;%ext e2) added in 4.04. )

Extension nodes are generic placeholders in the syntax tree. They are rejected by the type-checker and are intended to be “expanded” by external tools such as -ppx rewriters.

Extension nodes share the same notion of identifier and payload as attributes ‍12.12.

The first form of extension node is used for “algebraic” categories:

extension::= [%attr-idattr-payload]
expr::= ...
typexpr::= ...
pattern::= ...
module-expr::= ...
module-type::= ...
class-expr::= ...
class-type::= ...

A second form of extension node can be used in structures and signatures, both in the module and object languages:

item-extension::= [%%attr-idattr-payload]
definition::= ...
specification::= ...
class-field-spec::= ...
class-field::= ...

An infix form is available for extension nodes when the payload is of the same kind (expression with expression, pattern with pattern ...).


let%foo x = 2 in x + 1     === [%foo let x = 2 in x + 1]
begin%foo ... end          === [%foo begin ... end]
x ;%foo 2                  === [%foo x; 2]
module%foo M = ..          === [%%foo module M = ... ]
val%foo x : t              === [%%foo: val x : t]

When this form is used together with the infix syntax for attributes, the attributes are considered to apply to the payload:

fun%foo[@bar] x -> x + 1 === [%foo (fun x -> x + 1)[@bar ] ];

An additional shorthand let%foo x in ... is available for convenience when extension nodes are used to implement binding operators (See 12.23.4).

Furthermore, quoted strings {|...|} can be combined with extension nodes to embed foreign syntax fragments. Those fragments can be interpreted by a preprocessor and turned into OCaml code without requiring escaping quotes. A syntax shortcut is available for them:

{%%foo|...|}               === [%%foo{|...|}]
let x = {%foo|...|}        === let x = [%foo{|...|}]
let y = {%foo bar|...|bar} === let y = [%foo{bar|...|bar}]

For instance, you can use {%sql|...|} to represent arbitrary SQL statements – assuming you have a ppx-rewriter that recognizes the %sql extension.

Note that the word-delimited form, for example {sql|...|sql}, should not be used for signaling that an extension is in use. Indeed, the user cannot see from the code whether this string literal has different semantics than they expect. Moreover, giving semantics to a specific delimiter limits the freedom to change the delimiter to avoid escaping issues.

13.1 Built-in extension nodes

(Introduced in OCaml 4.03)

Some extension nodes are understood by the compiler itself:

type t = .. type t += X of int | Y of string let x = [%extension_constructor X] let y = [%extension_constructor Y]
# x <> y;;
- : bool = true