# fold

Namespace: `FsToolkit.ErrorHandling`

## Function Signature

```fsharp
('a -> 'b) -> ('c -> 'b) 
    -> Result<'a, 'c> -> 'b
```

## Examples

### Example 1

`fold` can be used to convert `Result` to another similar type, such as `Choice`:

```fsharp
let choice1 = Ok 42 |> Result.fold Choice1Of2 Choice2Of2
// Choice1Of2 42

let choice2 = Error "An error occurred" |> Result.fold Choice1Of2 Choice2Of2
// Choice2Of2 "An error occurred"
```

### Example 2

In a typical web application, if there is any request validation error, we send `HTTP 400 Bad Request` as response and `HTTP 200 OK` for a successful operation.

Given the following function:

```fsharp
// string -> Result<int, string>
let tryParseInt str =
  match System.Int32.TryParse str with
  | true, x -> Ok x
  | false, _ -> 
    Error (sprintf "unable to parse '%s' to integer" str)
```

And the following fake HTTP response type:

```fsharp
type HttpResponse<'a, 'b> =
  | Ok of 'a
  | BadRequest of 'b
```

Then using `Result.fold`, we can do the following

```fsharp
// HttpRequest -> HttpResponse<int,string>
let handler httpRequest =
  // reading the input from the HTTP request
  let inputStr = httpRequest ... 
  inputStr |> tryParseInt |> Result.fold Ok BadRequest
```


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