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Arg contains convenience functions that extract constants from AST fragments, to be used when parsing options or [\@attributes] attached to types, fields or constructors.
The ~name argument is used in error messages and should receive the name of the deriving plugin, e.g. "show".
enum values expr extracts a polymorphic variant constant from expr, or returns Result.Error "one of: `a, `b, ..." if expr does not contain a polymorphic variant constructor included in values.
list f expr extracts a list constant from expr and maps every element through f, or returns Result.Error "list:..." where ... is the error returned by f, or returns Result.Error "list" if expr does not contain a list.
val get_attr :
deriver:string ->'aconv->Parsetree.attribute option->'a option
get_attr ~deriver conv attr extracts the expression from attr and converts it with conv, raising Location.Error if attr is not a structure with a single expression or conv fails; or returns None if attr is None. The name of the deriving plugin should be passed as deriver; it is used in error messages.
Example usage:
let deriver = "index"
(* ... *)
let kind =
match Ppx_deriving.attr ~deriver "kind" pcd_attributes |>
Ppx_deriving.Arg.(get_attr ~deriver (enum ["flat"; "nested"])) with
| Some "flat" -> `flat | Some "nested" -> `nested | None -> `default
in ..
val get_flag : deriver:string ->Parsetree.attribute option-> bool
get_flag ~deriver attr returns true if attr is an empty attribute or false if it is absent, raising Location.Error if attr is not a structure. The name of the deriving plugin should be passed as deriver; it is used in error messages.
val get_expr : deriver:string ->'aconv->Parsetree.expression ->'a
get_expr ~deriver conv exp converts expression exp with conv, raising Location.Error if conv fails. The name of the deriving plugin should be passed as deriver; it is used in error messages.