package base

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Module type
Class type

The option type indicates whether a meaningful value is present. It is frequently used to represent success or failure, using None for failure. To be more descriptive about why a function failed, see the Or_error module.

Usage example from a utop session follows. Hash table lookups use the option type to indicate success or failure when looking up a key.

  # let h = Hashtbl.of_alist (module String) [ ("Bar", "Value") ];;
  val h : (string, string) Hashtbl.t = <abstr>;;
  - : (string, string) Hashtbl.t = <abstr>
  # Hashtbl.find h "Foo";;
  - : string option = None
  # Hashtbl.find h "Bar";;
  - : string option = Some "Value"

Type and Interfaces

type 'a t = 'a option =
  1. | None
  2. | Some of 'a
val compare : ('a -> 'a -> int) -> 'a t -> 'a t -> int
val globalize : ('a -> 'a) -> 'a t -> 'a t
val hash_fold_t : (Hash.state -> 'a -> Hash.state) -> Hash.state -> 'a t -> Hash.state
val t_sexp_grammar : 'a Sexplib0.Sexp_grammar.t -> 'a t Sexplib0.Sexp_grammar.t
include Equal.S1 with type 'a t := 'a t
val equal : 'a Equal.equal -> 'a t Equal.equal
include Invariant.S1 with type 'a t := 'a t
val invariant : ('a -> unit) -> 'a t -> unit
include Sexpable.S1 with type 'a t := 'a t
val t_of_sexp : (Sexplib0.Sexp.t -> 'a) -> Sexplib0.Sexp.t -> 'a t
val sexp_of_t : ('a -> Sexplib0.Sexp.t) -> 'a t -> Sexplib0.Sexp.t

Applicative interface

Options form an applicative, where:

  • return x = Some x
  • None <*> x = None
  • Some f <*> None = None
  • Some f <*> Some x = Some (f x)
include Applicative.S_local with type 'a t := 'a t
val both : 'a t -> 'b t -> ('a * 'b) t
val (<*>) : ('a -> 'b) t -> 'a t -> 'b t

same as apply

val (<*) : 'a t -> unit t -> 'a t
val (*>) : unit t -> 'a t -> 'a t
val apply : ('a -> 'b) t -> 'a t -> 'b t
val map2 : 'a t -> 'b t -> f:('a -> 'b -> 'c) -> 'c t
val map3 : 'a t -> 'b t -> 'c t -> f:('a -> 'b -> 'c -> 'd) -> 'd t
module Applicative_infix : sig ... end

Monadic interface

Options form a monad, where:

  • return x = Some x
  • (None >>= f) = None
  • (Some x >>= f) = f x
include Monad.S_local with type 'a t := 'a t
val (>>=) : 'a t -> ('a -> 'b t) -> 'b t

t >>= f returns a computation that sequences the computations represented by two monad elements. The resulting computation first does t to yield a value v, and then runs the computation returned by f v.

val (>>|) : 'a t -> ('a -> 'b) -> 'b t

t >>| f is t >>= (fun a -> return (f a)).

module Monad_infix : sig ... end
val bind : 'a t -> f:('a -> 'b t) -> 'b t

bind t ~f = t >>= f

val return : 'a -> 'a t

return v returns the (trivial) computation that returns v.

val map : 'a t -> f:('a -> 'b) -> 'b t

map t ~f is t >>| f.

val join : 'a t t -> 'a t

join t is t >>= (fun t' -> t').

val ignore_m : 'a t -> unit t

ignore_m t is map t ~f:(fun _ -> ()). ignore_m used to be called ignore, but we decided that was a bad name, because it shadowed the widely used Stdlib.ignore. Some monads still do let ignore = ignore_m for historical reasons.

val all : 'a t list -> 'a list t
val all_unit : unit t list -> unit t

Like all, but ensures that every monadic value in the list produces a unit value, all of which are discarded rather than being collected into a list.

module Let_syntax : sig ... end

These are convenient to have in scope when programming with a monad:

Extracting Underlying Values

val value : 'a t -> default:'a -> 'a

Extracts the underlying value if present, otherwise returns default.

val value_exn : ?here:Base__.Import.Stdlib.Lexing.position -> ?error:Error.t -> ?message:string -> 'a t -> 'a

Extracts the underlying value, or raises if there is no value present. The error raised can be augmented using the ~here, ~error, and ~message optional arguments.

val value_map : 'a t -> default:'b -> f:('a -> 'b) -> 'b

Extracts the underlying value and applies f to it if present, otherwise returns default.

val value_or_thunk : 'a t -> default:(unit -> 'a) -> 'a

Extracts the underlying value if present, otherwise executes and returns the result of default. default is only executed if the underlying value is absent.

val fold : 'a t -> init:'acc -> f:('acc -> 'a -> 'acc) -> 'acc

On None, returns init. On Some x, returns f init x.

val mem : 'a t -> 'a -> equal:('a -> 'a -> bool) -> bool

Checks whether the provided element is there, using equal.

val length : 'a t -> int
val iter : 'a t -> f:('a -> unit) -> unit
val exists : 'a t -> f:('a -> bool) -> bool

On None, returns false. On Some x, returns f x.

val for_all : 'a t -> f:('a -> bool) -> bool

On None, returns true. On Some x, returns f x.

val find : 'a t -> f:('a -> bool) -> 'a option

find t ~f returns t if t = Some x and f x = true; otherwise, find returns None.

val find_map : 'a t -> f:('a -> 'b option) -> 'b option

On None, returns None. On Some x, returns f x.

val to_list : 'a t -> 'a list
val to_array : 'a t -> 'a array
val call : 'a -> f:('a -> unit) t -> unit

call x f runs an optional function ~f on the argument.

val merge : 'a t -> 'a t -> f:('a -> 'a -> 'a) -> 'a t

merge a b ~f merges together the values from a and b using f. If both a and b are None, returns None. If only one is Some, returns that one, and if both are Some, returns Some of the result of applying f to the contents of a and b.

val filter : 'a t -> f:('a -> bool) -> 'a t


val try_with : (unit -> 'a) -> 'a t

try_with f returns Some x if f returns x and None if f raises an exception. See Result.try_with if you'd like to know which exception.

val try_with_join : (unit -> 'a t) -> 'a t

try_with_join f returns the optional value returned by f if it exits normally, and None if f raises an exception.

val some : 'a -> 'a t

Wraps the Some constructor as a function.

val first_some : 'a t -> 'a t -> 'a t

first_some t1 t2 returns t1 if it has an underlying value, or t2 otherwise.

val some_if : bool -> 'a -> 'a t

some_if b x converts a value x to Some x if b, and None otherwise.


val is_none : 'a t -> bool

is_none t returns true iff t = None.

val is_some : 'a t -> bool

is_some t returns true iff t = Some x.


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