FsToolkit.ErrorHandling provides the standard infix operators for map (<!>), apply (<*>), and bind (>>=) to work with Result<'a, 'b list>.
There are also variants of the map and apply operators (<!^> and <*^>) that accept Result<'a, 'b> (non-list) as the right-hand argument.
Examples
Example 1
Assume that we have following types and functions:
type Latitude = private Latitude of float with
// float -> Async<Result<Latitude, string list>>
static member TryCreate (lat : float) =
// ...
type Longitude = private Longitude of float with
// float -> Async<Result<Longitude, string list>>
static member TryCreate (lng : float) =
// ...
type Tweet = private Tweet of string with
// string -> Async<Result<Tweet, string list>>
static member TryCreate (tweet : string) =
// ...
// Latitude -> Longitude -> Tweet -> CreatePostRequest
let createPostRequest lat long tweet =
// ...
We can make use of the standard operators in the AsyncValidation Operators module to perform the asyncValidation of the incoming request and capture all the errors as shown below:
open FsToolkit.ErrorHandling.Operator.AsyncValidation
// float -> float -> string -> Async<Result<CreatePostRequest, string list>>
let validateCreatePostRequest lat lng tweet =
createPostRequest
<!> Latitude.TryCreate lat
<*> Longitude.TryCreate lng
<*> Tweet.TryCreate tweet
By using the AsyncValidation operators instead of the Result operators, we collect all the errors:
validateCreatePostRequest 300. 400. ""
// Error
["300.0 is a invalid latitude value"
"400.0 is a invalid longitude value"
"Tweet shouldn't be empty"]
Example 2
In the above example, all the TryCreate functions return a string list as the error type (Async<Result<'a, string list>>). If these functions instead returned Async<Result<'a, string>> (only a single error), we can use <*^> and <!^> to get the same result:
open FsToolkit.ErrorHandling.Operator.AsyncValidation
// float -> float -> string -> Async<Result<CreatePostRequest, string list>>
let validateCreatePostRequest lat lng tweet =
createPostRequest
<!^> Latitude.TryCreate lat
<*^> Longitude.TryCreate lng
<*^> Tweet.TryCreate tweet
