package containers

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Monadic Traverse

This allows the traversal of a 'a t list where _ t is also a monad.

For example, a 'a option list will be traversed by extracting a value from each option, returning Some [x1;…;x_n]; but if one of the option is None then the whole result is None.

Another example is with result: ('a, 'err) result list can be transformed into ('a list, 'err) result by returning the first error, or Ok [x1; …; xn] if all the elements were successful.

This describes the behavior of sequence_m; map_m is a combination of map and sequence_m; map_m_par is like map_m but useful for some pseudo monads like Lwt.


module M : MONAD


val sequence_m : 'a M.t t -> 'a t M.t

Traverse the list of monadic values using M.(>>=), re-combining elements with (::). See the documentation of the Traverse functor itself.

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

Fold a function with a monadic effect through a list.

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

Combination of map and sequence_m.

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

map_m_par f (x :: l) is like map_m but f x and f l are evaluated "in parallel" before combining their result (for instance in Lwt).

Basically, when encoutering x :: tl, this computes f x and map_m_par f tl, and only then is M.(>>=) used to combine the two results into a new list.


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