gistfile1.txt

#require "core";;
#require "ppx_fields_conv";;
#use "z.ml";;
type t = { tag : bool; }
val tag : t -> bool = <fun>
module Fields :                                                                               
  sig                                                                                         
    val names : string list
    val tag : ([< `Read | `Set_and_create ], t, bool) Field.t_with_perm
    val make_creator :
      tag:(([< `Read | `Set_and_create ], t, bool) Field.t_with_perm ->
           'a -> ('b -> bool) * 'c) ->
      'a -> ('b -> t) * 'c
    val create : tag:bool -> t
    val map :
      tag:(([< `Read | `Set_and_create ], t, bool) Field.t_with_perm -> bool) ->
      t
    val iter :
      tag:(([< `Read | `Set_and_create ], t, bool) Field.t_with_perm -> unit) ->
      unit
    val fold :
      init:'a ->
      tag:('a ->
           ([< `Read | `Set_and_create ], t, bool) Field.t_with_perm -> 'b) ->
      'b
    val map_poly : ([< `Read | `Set_and_create ], t, 'a) Field.user -> 'a list
    val for_all :
      tag:(([< `Read | `Set_and_create ], t, bool) Field.t_with_perm -> bool) ->
      bool
    val exists :
      tag:(([< `Read | `Set_and_create ], t, bool) Field.t_with_perm -> bool) ->
      bool
    val to_list :
      tag:(([< `Read | `Set_and_create ], t, bool) Field.t_with_perm -> 'a) ->
      'a list
    module Direct :
      sig
        val iter :
          t ->
          tag:(([< `Read | `Set_and_create ], t, bool) Field.t_with_perm ->
               t -> bool -> 'a) ->
          'a
        val fold :
          t ->
          init:'a ->
          tag:('a ->
               ([< `Read | `Set_and_create ], t, bool) Field.t_with_perm ->
               t -> bool -> 'b) ->
          'b
        val for_all :
          t ->
          tag:(([< `Read | `Set_and_create ], t, bool) Field.t_with_perm ->
               t -> bool -> bool) ->
          bool
        val exists :
          t ->
          tag:(([< `Read | `Set_and_create ], t, bool) Field.t_with_perm ->
               t -> bool -> bool) ->
          bool
        val to_list :
          t ->
          tag:(([< `Read | `Set_and_create ], t, bool) Field.t_with_perm ->
               t -> bool -> 'a) ->
          'a list
        val map :
          t ->
          tag:(([< `Read | `Set_and_create ], t, bool) Field.t_with_perm ->
               t -> bool -> bool) ->
          t
        val set_all_mutable_fields : 'a -> unit
      end
  end
type foo = Bar of int | Quux of t



let x = Quux { tag = true };;
  val x : foo = Quux {tag = true}
tag x
  Error: This expression has type foo but an expression was expected of type t

z.ml

open Core.Std

type t = { tag : bool } [@@deriving fields]

type foo = Bar of int
         | Quux of t