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go-functional

Languages: PT, EN

What is it?

It is a small library in Go that contains the implementation of common monads in functional programming that makes use of generics support.

How to install

go get github.com/MisterKaiou/go-functional

Why?

The current way of returning a value or an error, with a ([value], error) tuple, is not very expandable. Since you would have to resort to the classic if err != nil { do() } to handle the error, which quickly gets tiresome.

Of course, in functional languages this would hardly be a problem as the entire language and its APIs are created with monads in mind. We don't have this on Go's API, but even if you do it only in your code, it's a drastic change that will require, at most, getting used to reading result.Map(** parameters **), which in my opinion , it's not much to ask considering how clean the code can get without the countless lines of if err != nil.

Behavior

Result

  • The value of Ok for an error Result is nil;
  • The value of Ok for a successful Result is the value it holds;
  • The error value for an error Result is the error that was used to create it;
  • The error value for a successful Result is nil.

Option

  • The value of Some for an Option representing nothing is nil;
  • The value of Some for an Option that represents something is the value it holds;
  • The value of None for an Option representing nothing is a pointer to a newly created instance of None
  • The value of None for an Option that represents something is nil

You don't have access to the internal values, and this is by design, you really shouldn't do this or the purpose of these types would be in vain. Unwrap is a convenience and I still discourage its use.


Types

In order not to prolong the description too much, examples of how to handle error or None cases will be shown only once. Since, the same behavior happens in all types and functions.


Result[Of any]

Result[Of any] is a struct that represents a result that contains Of or error. Since we have an interface to represent errors within the language itself, I chose to use it.


Examples:


String: Result implements the stringer interface, and the value returned when calling the String method depends on its internal value.

  • If Ok: String will return fmt.String applied to the internal value. Ex:
str := result.Ok(42).String() // Same as fmt.String(42)
print(str) // Prints: "42"
  • If it is Error: String will return error.Error() in the internal value. Ex:
str := result.Error[int](errors.New("error")).String() // Same as err.Error()
print(str) // Prints: error

Map: Given a Result, it accepts a function that takes its value and returns another value. Finally, the returned value is stored inside a new Result

Ok:

res := result.Ok(42) // *Result[int]
mappedRes := result.Map(res, func(val int) bool { return val == 42 }) // *Result[bool]

printf("The value of mappedRes is: %s", mappedRes)
// Prints: The value of mappedRes is: true"

Erro:

res := result.Error[int](errors.New("error")) // *Result[int]
mappedRes := result.Map(res, func(val int) bool { return val == 0 }) //*Result[bool]

printf("The value of mappedRes is: %s", mappedRes) 
// Prints: The value of mappedRes is: error"

MapError: Given a Result, it accepts a function that receives its error and returns another error. Finally, the returned value is stored in a new Result.

res := Error[int](errors.New("something failed"))

mapped := MapError(res, func(err error) error { return errors.New(fmt.Sprint("oh no ", err)) })
println(mapped.String()) // Prints: oh no something failed

Bind: Given a Result, it accepts a function that takes its value and returns another Result.

res := result.Ok(42) // *Result[int]
bRes := result.Bind(res, func(val int) bool { return result.Ok(val == 42) }) // *Result[bool]

printf("The value of bRes is: %s", bRes)
// Prints: The value of bRes is: true"

Match: Given a Result, it accepts two functions that return a value of the same type, but the first one receives the result contained in it and the second one receives the error.

res := result.Ok(42)

matchedRes := result.Match(res,
	func(ok int) string { return "All fine around here" },
	func(err error) string { return err.Error() })


printf("The value of matchedRes is: %s", matchedRes)
// Prints: The value of matchedRes is: All fine around here"

Unwrap: It is a Result method. Returns the value contained in it or calls panic.

Note: Prefer to use Match instead of Unwrap

res := result.Ok(69).Unwrap() // res = 69
// Ou
res := result.Error[bool](errors.New("error")).Unwrap() // Panic

Option[Of any]

Option[Of any] is a struct that represents either something or nothing.


Examples:


String: Option implements the stringer interface, and the value when calling the String method depends on its internal value.

  • If Some: String will return the value of fmt.String applied to the internal value. Ex:
str := option.Some(42).String() // Same as fmt.String(42)
print(str) // Prints: "42"
  • If None: String will return "None". Ex:
str := option.None[int]().String()
print(str) // Prints: None

Map: Given an Option, it accepts a function that takes its value and returns another value. Finally, the returned value is stored inside a new Option

Ok:

opt := option.Some(42) // *Option[int]
mappedOpt := option.Map(opt, func(val int) bool { return val == 42 }) // *Option[bool]

printf("The value of mappedOpt is: %s", mappedOpt)
// Prints: The value of mappedOpt is: true"

Erro:

opt := option.None[int]() // *Option[int]
mappedOpt := option.Map(opt, func(val int) bool { return val == 0 }) //*Option[bool]

printf("The value of mappedOpt is: %s", mappedOpt) 
// Prints: The value of mappedOpt is: None"

Bind: Given an Option, it accepts a function that takes its value and returns another Option.

opt := option.Some(42) // *Option[int]
bRes := option.Bind(opt, func(val int) bool { return option.Some(val == 42) }) // *Option[bool]

printf("The value of bRes is: %s", bRes)
// Prints: The value of bRes is: true"

Match: Given an Option, accepts two functions that return a value of the same type. The first receives the result contained in it and the second does not receive parameters.

opt := option.Some(42)

matchedOpt := option.Match(opt,
	func(ok int) string { return "Something here" },
	func() string { return "Nothing here" })

printf("The value of matchedOpt is: %s", matchedOpt)
// Prints: The value of matchedOpt is: Something here"

Unwrap: It is a Option method. Returns the value contained in it or calls panic.

Note: Prefer to use Match instead of Unwrap

res := option.Some(69).Unwrap() // res = 69
// Ou
res := option.None[bool]().Unwrap() // Panic

More info on the docs!


Real Example

The code snippet below was taken from a real application of mine using this library together with gin:

func (h *Handler) Login() response.HttpResult {
	nonce := h.NonceGenerator()
	stateStr := r.Bind(nonce, func(it auth.Nonce) *r.Result[string] {
		saved := h.NonceStore.Save(it)
		return r.Map(saved, func(_ unit.Unit) string {
			return it.String()
		})
	})
	authUrl := r.Bind(stateStr, auth.GenerateAuthenticationURL)
	return response.Redirect(authUrl)
}

Apart from the abstractions I created, like response.HttpResult and response.Redirect, notice that every call to Bind or Map would be if err != nil { return/panic }. A method that just creates the OAuth login URL would be way longer than these 9 lines if we weren't using this code style.

Adding the r alias to the import helps make reading less tiring.

Future plans

Add other monads like:

  • IO
  • Either
  • State (The need for this in Go is debatable as we don't have immutability unless we consider Pass by Value something close to that)

Use cases for these are kind of rare, so it's not included in the library as it is now. But if it is a requirement for at least someone, it will be implemented.