Usage

Basic variant

type color =
  | Red
  | Green
  | Blue
[@@deriving toString, fromString]

let () =
  assert (colorToString Red = "Red");
  assert (colorFromString "Green" = Some Green);
  assert (colorFromString "purple" = None);
  assert (colorFromStringExn "Blue" = Blue)

Polymorphic variant

type direction = [ `North | `South | `East | `West ]
[@@deriving toString, fromString]

let () =
  assert (directionToString `North = "North");
  assert (directionFromString "South" = Some `South)

Polymorphic variant inheritance is supported when the inherited type has the same deriver:

type base =
  [ `Foo [@name "foo"]
  | `Bar [@name "bar"]
  ]
[@@deriving to_string, of_string]

type extended =
  [ base
  | `Baz [@name "baz"]
  ]
[@@deriving to_string, of_string]

let () =
  assert (extended_to_string `Foo = "foo");
  assert (extended_of_string "baz" = Some `Baz)

Overriding the string representation

Use [@as "..."] or [@name "..."] to control the string value used for a constructor:

type status =
  | Active [@as "active"]
  | Inactive [@name "inactive"]
  | Pending
[@@deriving toString, fromString]

let () =
  assert (statusToString Active = "active");
  assert (statusToString Pending = "Pending");
  assert (statusFromString "inactive" = Some Inactive);
  assert (statusFromString "Active" = None)

Both attributes behave identically. If both are present, [@as] takes precedence.

OCaml style

type my_type =
  | Foo
  | Bar [@as "bar"]
[@@deriving to_string, of_string]

let () =
  assert (my_type_to_string Foo = "Foo");
  assert (my_type_of_string "bar" = Some Bar);
  assert (my_type_of_string_exn "bar" = Bar)

Payload constructors

Payload constructors are rejected by default because the deriver cannot infer a lossless string conversion for their arguments.

Use [@no_args] when only the constructor tag matters. The generated to_string/toString function ignores the payload, while of_string/fromString skips that constructor because it cannot reconstruct the payload.

type event =
  | Started
  | Unknown of string [@no_args]
[@@deriving to_string, of_string]

let () =
  assert (event_to_string (Unknown "x") = "Unknown");
  assert (event_of_string "Unknown" = None)

Use [@no_string_exn] when stringifying the constructor should fail loudly:

type event =
  | Started
  | Unknown of string [@no_string_exn]
[@@deriving to_string, of_string]

event_to_string (Unknown "x") raises Failure, and event_of_string skips Unknown.

For polymorphic variants only, [@no_string] also narrows the return type of of_string/fromString so the marked constructor is excluded from parser results:

type event =
  [ `Started
  | `Unknown of string [@no_string]
  ]
[@@deriving to_string, of_string]

let parsed : [ `Started ] option = event_of_string "Started"

Limitations

The derivers reject:

• Records
• Abstract types without a polymorphic variant manifest
• Open types
• Payload constructors unless they use [@no_args], [@no_string_exn], or polymorphic-variant-only [@no_string]
• Non-type polymorphic variant inheritance

Building

dune build
dune test