Computation Expression

Result Computation Expression

Namespace: FsToolkit.ErrorHandling

Examples:

Example 1

The example from Result.map3 can be solved using the result computation expression as below:

// Result<int, string>
let addResult = result {
let! x = tryParseInt "35"
let! y = tryParseInt "5"
let! z = tryParseInt "2"
return add x y z
}

Example 2

The example from Result.map3 can be solved using the result computation expression as below:

// Result<CreatePostRequest,string>
let createPostRequestResult = result {
let! lat = Latitude.TryCreate 13.067439
let! lng = Longitude.TryCreate 80.237617
let! tweet = Tweet.TryCreate "Hello, World!"
return createPostRequest userId lat lng tweet
}

Example 3

Given the following functions:

tryGetUser : string -> User option
isPwdValid : string -> User -> bool
authorize : User -> Result<unit, AuthError>
createAuthToken : User -> AuthToken

Here's how a simple login use-case can be written (using some helpers from the Result module):

type LoginError = InvalidUser | InvalidPwd | Unauthorized of AuthError
let login (username : string) (password : string) : Result<AuthToken, LoginError> =
result {
// requireSome unwraps a Some value or gives the specified error if None
let! user = username |> tryGetUser |> Result.requireSome InvalidUser
// requireTrue gives the specified error if false
do! user |> isPwdValid password |> Result.requireTrue InvalidPwd
// Error value is wrapped/transformed (Unauthorized has signature AuthError -> LoginError)
do! user |> authorize |> Result.mapError Unauthorized
return user |> createAuthToken
}