apply

Namespace: FsToolkit.ErrorHandling

apply combines two Result values and returns a new Result value. If both Result values are Ok, it applies the function from the first Result to the value from the second Result, producing a new Result type. If either Result is an Error, the apply function propagates the error by returning the corresponding Error value.

Function Signature

Result<('okInput -> 'okOutput), 'error> -> Result<'okInput, 'error> 
    -> Result<'okOutput, 'error>

Examples

Example 1

Note: The apply function is most useful in its infix operator form <*> when using it over a multi-parameter function. Examples of this is shown in the documentation for this operator. The example below is a bit artificial since map is arguably better.

Assume that we have a function to find the remaining characters of a Tweet:

// Tweet -> int
let remainingCharacters (tweet : Tweet) =
  280 - tweet.Value.Length

If we want a function that takes a plain string instead, we can achieve it using the apply function:

// string -> Result<int,string>
let remainingCharactersStr (tweetStr : string) =
  Tweet.TryCreate tweet
  |> Result.apply (Ok remainingCharacters)

But as mentioned, using the map function is simpler in this case:

// string -> Result<int,string>
let remainingCharactersStr (tweetStr : string) =
  Tweet.TryCreate tweet
  |> Result.map remainingCharacters