package ppx_core

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".

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.

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

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.

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

Replace all attribute names by a String.copy and collect them. To be used in conjuction with check_all_seen.

Check that all attributes collected by freshen_and_collect have been:

• matched at least once by one of: get, consume or Floating.convert
• seen by check_unused (to allow white-listed 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 ppx_core works well with ppx_driver.

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.