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8.12 Attributes

(Introduced in OCaml 4.02, infix notations for constructs other than expressions added in 4.03)

Attributes are “decorations” of the syntax tree which are mostly ignored by the type-checker but can be used by external tools. An attribute is made of an identifier and a payload, which can be a structure, a type expression (prefixed with :), a signature (prefixed with :) or a pattern (prefixed with ?) optionally followed by a when clause:

attr-id::= lowercase-ident
   capitalized-ident
   attr-id.  attr-id
 
attr-payload::= [ module-items ]  
   :typexpr
   : [ specification ]  
   ?pattern  [whenexpr]  
 

The first form of attributes is attached with a postfix notation on “algebraic” categories:

attribute::= [@attr-id  attr-payload]
 
expr::= ...  
 expr  attribute
 
typexpr::= ...  
 typexpr  attribute
 
pattern::= ...  
 pattern  attribute
 
module-expr::= ...  
 module-expr  attribute
 
module-type::= ...  
 module-type  attribute
 
class-expr::= ...  
 class-expr  attribute
 
class-type::= ...  
 class-type  attribute
 

This form of attributes can also be inserted after the `tag-name in polymorphic variant type expressions (tag-spec-first, tag-spec, tag-spec-full) or after the method-name in method-type.

The same syntactic form is also used to attach attributes to labels and constructors in type declarations:

field-decl::= [mutable] field-name:  poly-typexpr  {attribute}  
 
constr-decl::= (constr-name ∣  ()) [ ofconstr-args ]  {attribute}  
 

Note: when a label declaration is followed by a semi-colon, attributes can also be put after the semi-colon (in which case they are merged to those specified before).

The second form of attributes are attached to “blocks” such as type declarations, class fields, etc:

item-attribute::= [@@attr-id  attr-payload]
 
typedef::= ...  
 typedef  item-attribute
 
exception-definition::= exceptionconstr-decl
 exceptionconstr-name=  constr
 
module-items::= [;;] ( definition ∣  expr  { item-attribute } )  { [;;] definition ∣  ;;expr  { item-attribute } }  [;;]  
 
class-binding::= ...  
 class-binding  item-attribute
 
class-spec::= ...  
 class-spec  item-attribute
 
classtype-def::= ...  
 classtype-def  item-attribute
 
definition::= let [rec] let-binding  { andlet-binding }  
 externalvalue-name:  typexpr=  external-declaration  { item-attribute }  
 type-definition
 exception-definition  { item-attribute }  
 class-definition
 classtype-definition
 modulemodule-name  { (module-name:  module-type) }  [ :module-type ]  =  module-expr  { item-attribute }  
 moduletypemodtype-name=  module-type  { item-attribute }  
 openmodule-path  { item-attribute }  
 includemodule-expr  { item-attribute }  
 modulerecmodule-name:  module-type=   module-expr  { item-attribute }   { andmodule-name:  module-type=  module-expr   { item-attribute } }  
 
specification::= valvalue-name:  typexpr  { item-attribute }  
 externalvalue-name:  typexpr=  external-declaration  { item-attribute }  
 type-definition
 exceptionconstr-decl  { item-attribute }  
 class-specification
 classtype-definition
 modulemodule-name:  module-type  { item-attribute }  
 modulemodule-name  { (module-name:  module-type) } :  module-type  { item-attribute }  
 moduletypemodtype-name  { item-attribute }  
 moduletypemodtype-name=  module-type  { item-attribute }  
 openmodule-path  { item-attribute }  
 includemodule-type  { item-attribute }  
 
class-field-spec::= ...  
 class-field-spec  item-attribute
 
class-field::= ...  
 class-field  item-attribute
 

A third form of attributes appears as stand-alone structure or signature items in the module or class sub-languages. They are not attached to any specific node in the syntax tree:

floating-attribute::= [@@@attr-id  attr-payload]
 
definition::= ...  
 floating-attribute
 
specification::= ...  
 floating-attribute
 
class-field-spec::= ...  
 floating-attribute
 
class-field::= ...  
 floating-attribute
 

(Note: contrary to what the grammar above describes, item-attributes cannot be attached to these floating attributes in class-field-spec and class-field.)

It is also possible to specify attributes using an infix syntax. For instance:

let[@foo] x = 2 in x + 1          === (let x = 2 [@@foo] in x + 1)
begin[@foo][@bar x] ... end       === (begin ... end)[@foo][@bar x]
module[@foo] M = ...              === module M = ... [@@foo]
type[@foo] t = T                  === type t = T [@@foo]
method[@foo] m = ...              === method m = ... [@@foo]

For let, the attributes are applied to each bindings:

let[@foo] x = 2 and y = 3 in x + y === (let x = 2 [@@foo] and y = 3 in x + y)
let[@foo] x = 2
and[@bar] y = 3 in x + y           === (let x = 2 [@@foo] and y = 3 [@@bar] in x + y)

8.12.1 Built-in attributes

Some attributes are understood by the type-checker:

module X = struct [@@@warning "+9"] (* locally enable warning 9 in this structure *)end [@@deprecated "Please use module 'Y' instead."] let x = begin[@warning "+9"] […] end type t = A | B [@@deprecated "Please use type 's' instead."]
let fires_warning_22 x = assert (x >= 0) [@ppwarning "TODO: remove this later"]
Warning 22: TODO: remove this later
let rec is_a_tail_call = function | [] -> () | _ :: q -> (is_a_tail_call[@tailcall]) q let rec not_a_tail_call = function | [] -> [] | x :: q -> x :: (not_a_tail_call[@tailcall]) q
Warning 51: expected tailcall
let f x = x [@@inline] let () = (f[@inlined]) ()
type fragile = | Int of int [@warn_on_literal_pattern] | String of string [@warn_on_literal_pattern]
let fragile_match_1 = function | Int 0 -> () | _ -> ()
Warning 52: Code should not depend on the actual values of this constructor's arguments. They are only for information and may change in future versions. (See manual section 9.5) val fragile_match_1 : fragile -> unit = <fun>
let fragile_match_2 = function | String "constant" -> () | _ -> ()
Warning 52: Code should not depend on the actual values of this constructor's arguments. They are only for information and may change in future versions. (See manual section 9.5) val fragile_match_2 : fragile -> unit = <fun>
module Immediate: sig type t [@@immediate] val x: t ref end = struct type t = A | B let x = ref A end
module Int_or_int64 : sig type t [@@immediate64] val zero : t val one : t val add : t -> t -> t end = struct include Sys.Immediate64.Make(Int)(Int64) module type S = sig val zero : t val one : t val add : t -> t -> t end let impl : (module S) = match repr with | Immediate -> (module Int : S) | Non_immediate -> (module Int64 : S) include (val impl : S) end

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