package travesty

  1. Overview
  2. Docs

Module State_transform.MakeSource

Make makes an S (state transformer with fixed state type) from a Basic.

Parameters

Signature

Sourcetype state = B.t

The fixed state type.

include Base.Monad.S
Sourcetype 'a t
Sourceval (>>=) : '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.

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

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

Sourcemodule Monad_infix : sig ... end
Sourceval bind : 'a t -> f:('a -> 'b t) -> 'b t

bind t ~f = t >>= f

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

map t ~f is t >>| f.

Sourceval join : 'a t t -> 'a t

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

Sourceval 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 Caml.ignore. Some monads still do let ignore = ignore_m for historical reasons.

Sourceval all : 'a t list -> 'a list t
Sourceval 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.

Sourcemodule Let_syntax : sig ... end

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

include Monad_exts_types.S with type 'a t := 'a t

S subsumes S_let.

include Monad_exts_types.S_let with type 'a t := 'a t
Sourceval (let+) : 'a t -> ('a -> 'b) -> 'b t

let+ is map.

Sourceval (let*) : 'a t -> ('a -> 'b t) -> 'b t

let* is bind.

Haskell-style operators

Sourceval then_m : _ t -> 'a t -> 'a 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.

Sourceval (>>) : _ t -> 'a t -> 'a t

x >> y is then_m x y.

Sourceval compose_m : ('a -> 'b t) -> ('b -> 'c t) -> 'a -> 'c t

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

Sourceval (>=>) : ('a -> 'b t) -> ('b -> 'c t) -> 'a -> 'c t

x >=> y is compose_m x y.

Guarded monadic computations

Sourceval map_when_m : ?otherwise:('a -> 'a t) -> bool -> 'a -> f:('a -> 'a t) -> 'a t

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

Sourceval when_m : ?otherwise:(unit -> unit t) -> bool -> f:(unit -> unit t) -> unit t

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

Sourceval map_unless_m : ?otherwise:('a -> 'a t) -> bool -> 'a -> f:('a -> 'a t) -> 'a t

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

Sourceval unless_m : ?otherwise:(unit -> unit t) -> bool -> f:(unit -> unit t) -> unit 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

Sourceval tee_m : 'a -> f:('a -> unit t) -> 'a 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) *)
Sourceval tee : 'a -> f:('a -> unit) -> 'a t

tee val ~f behaves as tee_m, 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 ())
include State_transform_types.Generic with type ('a, 's) t := 'a t and type 's state := state with module Inner = B.Inner
include State_transform_types.Generic_builders with type 'a final := 'a with type ('a, 's) t := 'a t with type 's state := state
include State_transform_types.Generic_types with type 'a final := 'a with type ('a, 's) t := 'a t with type 's state := state
Sourceval make : (state -> state * 'a) -> 'a t

make creates a context-sensitive computation that can modify both the current context and the data passing through.

Specialised builders

Sourceval peek : (state -> 'a) -> 'a t

peek creates a context-sensitive computation that can look at the current context, but not modify it.

Sourceval modify : (state -> state) -> Base.unit t

modify creates a context-sensitive computation that can look at and modify the current context.

Sourceval return : 'a -> 'a t

return lifts a value or monad into a stateful computation.

module Inner = B.Inner

Inner is the monad to which we're adding state.

State transformers have the same runner signatures as state monads, but lifted into the inner monad.

include State_transform_types.Generic_runners with type ('a, 's) t := 'a t and type 'a final := 'a Inner.t and type 's state := state
include State_transform_types.Generic_types with type ('a, 's) t := 'a t with type 'a final := 'a Inner.t with type 's state := state
Sourceval run' : 'a t -> state -> (state * 'a) Inner.t

run' unfolds a t into a function from context to final state and result.

Sourceval run : 'a t -> state -> 'a Inner.t

run unfolds a t into a function from context to final result. To get the final context, use run' or call peek at the end of the computation.

include State_transform_types.Fix with type ('a, 's) t := 'a t
Sourceval fix : f:(('a -> 'a t) -> 'a -> 'a t) -> 'a -> 'a t

fix ~f init builds a fixed point on f.

At each step, f is passed a continuation mu and a value a. It may choose to return a recursive application of mu, or some value derived from a.

To begin with, f is applied to mu and init.

module Monadic : State_transform_types.Generic_builders with type 'a state := state and type 'a final := 'a Inner.t and type ('a, 's) t := 'a t

Monadic contains a version of the builder interface that can interact with the inner monad (Inner) this state transformer is overlaying.

include Monad_transform.S_fixed with type 'a t := 'a t and module Inner := Inner
Sourceval lift : 'a Inner.t -> 'a Inner.t t

lift x lifts x from the inner monad to the outer one.