## 8.14 Extension nodes

- 8.14.1 Built-in 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 8.13.

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

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

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

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

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

Examples:

    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 ] ];

Quoted strings {|...|} are particularly interesting for payloads that embed foreign syntax fragments. Those fragments can be interpreted by a preprocessor and turned into OCaml code without requiring escaping quotes. 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.

### 8.14.1 Built-in extension nodes

(Introduced in OCaml 4.03)

Some extension nodes are understood by the compiler itself:

- “ocaml.extension_constructor” or “extension_constructor” take as payload a constructor from an extensible variant type (see 8.15) and return its extension constructor slot.

     type t = ..
     type t += X of int | Y of string
     let x = [%extension_constructor X]
     let y = [%extension_constructor Y]

      x <> y;;
    - : bool = true