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8.13  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  [when expr]  
 

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::= [mutablefield-name :  poly-typexpr  {attribute}  
 
constr-decl::= (constr-name ∣  ()) [ of constr-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::= exception constr-decl  
  exception constr-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 [reclet-binding  { and let-binding }  
  external value-name :  typexpr =  external-declaration  { item-attribute }  
  type-definition  
  exception-definition  { item-attribute }  
  class-definition  
  classtype-definition  
  module module-name  { ( module-name :  module-type ) }  [ : module-type ]  =  module-expr  { item-attribute }  
  module type modtype-name =  module-type  { item-attribute }  
  open module-path  { item-attribute }  
  include module-expr  { item-attribute }  
  module rec module-name :  module-type =   module-expr  { item-attribute }   { and module-name :  module-type =  module-expr   { item-attribute } }  
 
specification::= val value-name :  typexpr  { item-attribute }  
  external value-name :  typexpr =  external-declaration  { item-attribute }  
  type-definition  
  exception constr-decl  { item-attribute }  
  class-specification  
  classtype-definition  
  module module-name :  module-type  { item-attribute }  
  module module-name  { ( module-name :  module-type ) } :  module-type  { item-attribute }  
  module type modtype-name  { item-attribute }  
  module type modtype-name =  module-type  { item-attribute }  
  open module-path  { item-attribute }  
  include module-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)

8.13.1  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

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