Type of declared attribute.

The 'context type parameter describes where the attribute is expected and the 'payload one what its payload should contain.

declare fully_qualified_name context payload_pattern k declares an attribute. k is used to build the value resulting from parsing the payload.

For instance if a rewriter named "foo" expect the attribute @@default on record field declaration with an expression as payload:

  let default =
    Attribute.declare "foo.default" Attribute.Context.label_declaration
      Ast_pattern.(pstr (pstr_eval __ nil))
      (fun x -> x)

fully_qualified_name is expected to be a dot-separated list of names. When matching, any full suffix will be accepted. So for instance an attribute declared with name "foo.bar.default" will match exactly these attribute names: "default", "bar.default" and "foo.bar.default".

Additionally it is possible to prevent a suffix to be shortened by prefixing it with '@'. So for instance an attribute declared with name "foo.@bar.default" will match exactly these attribute names: "bar.default" and "foo.bar.default".

When matching against a list of attributes on an item, if several matches are possible, the longest one is used. For instance using the attribute "foo.default" declared in the previous example, on this code it will match the @foo.default 0 attribute:

  type t = { x : int [@default 42] [@foo.default 0] }

This is to allow the user to specify a @default attribute for all re-writers that use it but still put a specific one for one specific re-writer.

It is not allowed to declare an attribute with a name that matches a previously-defined one on the same context. For instance trying to declare the same attribute twice will fail.

Same as declare but the callback receives the location of the name of the attribute.

Same as declare but the callback receives the location of the attribute.

Types for attributes without payload.

Same as declare, but the payload is expected to be empty. It is supposed to be used in conjunction with has_flag.

Gets the associated attribute value. Marks the attribute as seen unless mark_as_seen=false. Returns an Error if the attribute is duplicated

See get_res. Raises a located error if the attribute is duplicated

Answers whether the given flag is attached as an attribute. See get_res for the meaning of mark_as_seen.

See has_flag_res. Raises a located error if the attribute is duplicated.

consume_res t x returns the value associated to attribute t on x if present as well as x with t removed.

See consume_res. Raises a located exception in case of error.

remove_seen x attrs removes the set of attributes matched by elements of attrs. Only remove them if they where seen by get or consume.

See remove_seen_res. Raises in case of error.

Code that is voluntarily dropped by a rewriter needs to be given to this object. All attributes inside will be marked as handled.

Raise if there are unused attributes.

Collect all errors due to unused attributes.

Collect all attribute names. To be used in conjunction with check_all_seen.

Check that all attributes collected by collect_unseen_errors have been:

• matched at least once by one of: get, consume or Floating.convert
• seen by check_unused (to allow allowlisted attributed to pass through)

This helps with faulty ppx rewriters that silently drop attributes.

Mark an attribute as seen and handled. This is only to make ppx rewriters that don't use ppxlib works well with the ones that do use it.

Return the list of attributes that have been dropped so far: attributes that haven't been marked and are not present in the given AST. This is used to debug extensions that drop attributes.