Operators

Validation Infix Operators

Namespace: FsToolkit.ErrorHandling.Operator.Validation

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 -> Result<Latitude, string list>
  static member TryCreate (lat : float) =
    // ...

type Longitude = private Longitude of float with
  // float -> Result<Longitude, string list>
  static member TryCreate (lng : float) =
    // ...

type Tweet = private Tweet of string with
  // string -> 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 Validation Operators module to perform the validation of the incoming request and capture all the errors as shown below:

open FsToolkit.ErrorHandling.Operator.Validation

// float -> float -> string -> Result<CreatePostRequest, string list>
let validateCreatePostRequest lat lng tweet = 
  createPostRequest
  <!> Latitude.TryCreate lat
  <*> Longitude.TryCreate lng
  <*> Tweet.TryCreate tweet

By using the Validation 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 (Result<'a, string list>). If these functions instead returned Result<'a, string> (only a single error), we can use <*^> and <!^> to get the same result:

open FsToolkit.ErrorHandling.Operator.Validation

// float -> float -> string -> Result<CreatePostRequest, string list>
let validateCreatePostRequest lat lng tweet = 
  createPostRequest
  <!^> Latitude.TryCreate lat
  <*^> Longitude.TryCreate lng
  <*^> Tweet.TryCreate tweet