FS-1004-result-type.md

F# RFC FS-1004 - Result type

The design suggestion Result type has been implemented in F# 4.1. This RFC covers the detailed proposal for this suggestion.

Discussion thread

• Approved in principle
• Details: under discussion
• Implementation: Completed

Introduction

In order to be able to write code that you can easily consume that does not throw an exception it's useful to have a fsharp type who contains the result on success or the error on failure

type Result<'TSuccess,'TError> = 
     | Success of 'TSuccess 
     | Error of 'TError

The type should be added in FSharp.Core library

Naming

Type name	Success case	Failure case
Result	Ok	Error

Testing considerations

The standard range of testing considerations for F# library types apply.

Proposal for additional functions

Uncontentious:

bind : ('T -> Result<'U, 'TError>) -> Result<'T, 'TError> -> Result<'U, 'TError>
map : ('T -> 'U) -> Result<'T, 'TError> -> Result<'U, 'TError>
mapError : ('TError -> 'U) -> Result<'T, 'TError> -> Result<'T, 'U>

A result builder should be a separate RFC which covers adding builders for both Option and Result.

Suggested:

attempt : (unit -> 'a) -> Result<'a,exn>