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package travesty
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Extend creates extensions for a Monad.S.

Parameters

module M : Base.Monad.S

Signature

Haskell-style operators

val then_m : _ M.t -> 'a M.t -> 'a M.t

then_m x y sequentially composes the actions x and y as with >>=, but, rather than using the returned value of x, it instead just returns y.

val (>>) : _ M.t -> 'a M.t -> 'a M.t

x >> y is then_m x y.

val compose_m : ('a -> 'b M.t) -> ('b -> 'c M.t) -> 'a -> 'c M.t

compose_m f g is the Kleisli composition of f and g.

val (>=>) : ('a -> 'b M.t) -> ('b -> 'c M.t) -> 'a -> 'c M.t

x >=> y is k_compose x y.

Guarded monadic computations

val map_when_m : ?otherwise:('a -> 'a M.t) -> bool -> 'a -> f:('a -> 'a M.t) -> 'a M.t

map_when_m ?otherwise condition ~f a is f a when condition is true, and otherwise a (by default, return) otherwise.

val when_m : ?otherwise:(unit -> unit M.t) -> bool -> f:(unit -> unit M.t) -> unit M.t

when_m ?otherwise condition ~f is f () when condition is true, and otherwise () (by default, return) otherwise.

val map_unless_m : ?otherwise:('a -> 'a M.t) -> bool -> 'a -> f:('a -> 'a M.t) -> 'a M.t

map_unless_m ?otherwise condition ~f a is f a when condition is false, and otherwise a (by default, return) otherwise.

val unless_m : ?otherwise:(unit -> unit M.t) -> bool -> f:(unit -> unit M.t) -> unit M.t

unless_m ?otherwise condition ~f is f () when condition is false, and otherwise () (by default, return) otherwise.

Executing monadic effects in the middle of pipelines

val tee_m : 'a -> f:('a -> unit M.t) -> 'a M.t

tee_m val ~f executes f val for its monadic action, then returns val.

Example (using an extended Or_error):

let fail_if_negative x =
  On_error.when_m (Int.is_negative x)
    ~f:(fun () -> Or_error.error_string "value is negative!")
in
Or_error.(
  42 |> tee_m ~f:fail_if_negative >>| (fun x -> x * x)
) (* Ok (1764) *)
val tee : 'a -> f:('a -> unit) -> 'a M.t

tee val ~f behaves as tee, but takes a non-monadic f.

Example (using an extended Or_error):

let print_if_negative x =
  if Int.negative x then Stdio.print_string "value is negative!"
in
Or_error.(
  try_get_value ()
  >>= tee ~f:print_if_negative
  >>= try_use_value ()
)