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The deferred analog of Core.Or_error. It is exposed in std.ml as Deferred.Or_error.
The mental model for a function returning an 'a Deferred.Or_error.t is that the function never raises. All error cases are caught and expressed as an Error _ result. This module preserves that property.
Unfortunately, there is no way to enforce this property using the type system, so it is more like a convention, or idiom. A function whose type ends with ... -> 'a
Deferred.Or_error.t and still raises should be considered broken, and be fixed. With that property in mind, Deferred.Or_error.List.iter, for example, does not wrap the execution of the given iter function f inside a monitor. If one of these application raises, the whole function Deferred.Or_error.List.iter will raise as a way to try to alert the developer that the function is broken and needs attention and fixing, rather than silently catching the error and converting it to Or_error.Error.
This behavior is consistent with Core.Or_error's treatment of user-supplied functions.
If you have to deal with a function that does not respect this idiom, you can use Deferred.Or_error.try_with_join to wrap its execution and enforce this property.
The applicative operations match the behavior of the applicative operations in Or_error. This means that all and all_unit are equivalent to combine_errors and combine_errors_unit respectively.
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.
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 try_with :
?extract_exn:bool ->?run:[ `Now | `Schedule ]->?rest:[ `Log | `Raise| `Call of exn -> unit ]->?here:Stdlib.Lexing.position ->?name:string ->(unit ->'aDeferred.t)->'at
try_with f catches exceptions thrown by f and returns them in the Result.t as an Error.t. try_with_join is like try_with, except that f can throw exceptions or return an Error directly, without ending up with a nested error; it is equivalent to try_with f >>| Result.join.
The option extract_exn is passed along to Monitor.try_with ?extract_exn and specifies whether or not the monitor exn wrapper should be skipped (extract_exn:true) or kept (extract_exn:false).
The ~rest argument controls how exceptions are handled after the try_with deferred becomes determined. They may be logged, raised, or passed to a callback.
The ~run argument controls when f gets called. `Now calls f immediately; `Schedule schedules an asynchronous job to run f.
val try_with_join :
?extract_exn:bool ->?run:[ `Now | `Schedule ]->?rest:[ `Log | `Raise| `Call of exn -> unit ]->?here:Stdlib.Lexing.position ->?name:string ->(unit ->'at)->'at
All of the List functions that take a how argument treat it the following way:
val repeat_until_finished :
'state->('state->[ `Repeat of 'state| `Finished of 'result ]t)->'resultt
repeat_until_finished initial_state f works just like Deferred.repeat_until_finished but with the Deferred.Or_error monad. If f returns an Or_error.Error the loop terminates and returns.