# 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

```fsharp
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](https://demystifyfp.gitbook.io/fstoolkit-errorhandling/fstoolkit.errorhandling/result/operators). 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](https://demystifyfp.gitbook.io/fstoolkit-errorhandling/fstoolkit.errorhandling/map3#tweet):

```fsharp
// 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:

```fsharp
// 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:

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