Namespace: FsToolkit.ErrorHandling

Function Signature:

('a -> Result<'b,'c>) -> 'a option -> Result<'b option, 'c>

Note that traverse is the same as map >> sequence. See also Option.sequenceResult.

See also Scott Wlaschin's Understanding traverse and sequence.


Example 1

If we have a value of type string option and want to call the tryParseInt function that we defined in the Result.map2 example, we can achieve it using the traverseResult function as below:

Some "42" |> Option.traverseResult tryParseInt
// Ok (Some 42)
None |> Option.traverseResult tryParseInt
// Ok None
Some "foo" |> Option.traverseResult tryParseInt
// Error "unable to parse 'foo' to integer"

Example 2

The CreatePostRequest type that we defined in Result.map3 example contains a Location option. The corresponding DTO objects would look like this:

type LocationDto = {
Latitude : float
Longitude : float
type CreatePostRequestDto = {
Tweet : string
Location : LocationDto option

Let's assume that we have this function to convert a LocationDto to a Location:

// LocationDto -> Result<Location, string>
let locationFromDto (dto : LocationDto) = result {
let! lat = Latitude.TryCreate dto.Latitude
let! lng = Longitude.TryCreate dto.Longitude
return {Location.Latitude = lat; Longitude = lng}

Then in order to create a similar function to convert a CreatePostRequestDto to a CreatePostRequest, we can make use of traverseResult as below:

let createPostRequestFromDto (dto : CreatePostRequestDto) = result {
// Parse the location DTO option to a Location option,
// returning an error if it's Some and invalid
let! location =
dto.Location |> Option.traverseResult locationDtoToLocation
let! tweet = Tweet.TryCreate dto.Tweet
return {
Tweet = tweet
Location = location

See also the example 2 of ResultOption.map2.