inspired-by-fsharp/README.md

# Inspired by F#

This project contains PHP utility classes whose functionality is inspired by their F# counterparts.

## What It Provides

This early-stage library currently provides two classes, both of which are designed to wrap values and indicate the state of the action that produced them. `Option<T>` represents a variable that may or may not have a value. `Result<TOK, TError>` represents the result of an action; the "ok" and "error" states both provide a value.

|                                                    | `Option<T>`<br>Replaces `null` checks                                          | `Result<TOK, TError>`<br>Replaces exception-based error handling            |
|----------------------------------------------------|--------------------------------------------------------------------------------|-----------------------------------------------------------------------------|
| **Creating**                                       | `::Some(T)` for Some<br>`::None()` for None<br>`::of($value)` _None if `null`_ | `::OK(TOK)` for OK<br>`::Error(TError)` for Error                           |
| **Querying**                                       | `->isSome(): bool`<br>`->isNone(): bool`                                       | `->isOK(): bool`<br>`->isError(): bool`                                     |
| **Reading**<br>_throws if called on missing value_ | `->get(): T`                                                                   | `->getOK(): TOK`<br>`->getError(): TError`                                  |
| **Transforming**<br>_still `Option` or `Result`_   | `->map(callable(T): U): U`                                                     | `->map(callable(TOK): U): U`<br>`->mapError(callable(TError): U): U`        |
| **Iterating**                                      | `->iter(callable(T): void): void`                                              | `->iter(callable(TOK): void): void`                                         |
| **Inspecting**<br>_returns the original instance_  | `->tap(callable(Option<T>): void): void`                                       | `->tap(callable(Result<TOK, TError>): void): void`                          |
| **Continued Processing**                           | `->bind(callable(T): Option<TBound>): Option<TBound>`                          | `->bind(callable(TOK): Result<TBoundOK, TError>): Result<TBoundOK, TError>` |

In addition to this, `Option<T>` provides:
- `->getOrDefault(T)` will return the Some value if it exists or the given default if the option is None.
- `->getOrCall(callable(): mixed)` will call the given function if the option is None. That function may return a value, or may be `void` or `never`.
- `->filter(callable(T): bool)` will compare a Some value against the callable, and if it returns `true`, will remain Some; if it returns `false`, the value will become None.
- `->is(T, $strict = true)` will return `true` if the option is Some and the value matches. Strict equality (the default) uses `===` for the comparison; if strict is set to `false`, the comparison will use `==` instead.
- `->unwrap()` will return `null` for None options and the value for Some options.

`Result<TOK, TError>` also provides:
- `toOption()` will transform an OK result to a Some option, and an Error result to a None option.

Finally, we would be remiss to not acknowledge some really cool prior art in this area - the [PhpOption](https://github.com/schmittjoh/php-option) project. `Option::of` recognizes their options and converts them properly, and `Option<T>` instances have a `->toPhpOption()` method that will convert these back into PhpOption's `Some<T>` and `None` instances. There is also a [ResultType](https://github.com/GrahamCampbell/Result-Type) project from the same team, though this project's result does not (yet) have any conversion methods for it.

## The Inspiration

[F#](https://fsharp.org/) is an ML-style language that runs under .NET. It has most of the functional programming paradigms, but as it runs on what was designed as an object-oriented runtime - and can use and interoperate with all the .NET libraries - it is a pragmatic approach to functional programming. (Many of its decade+ old features have been implemented into recent versions of C#.)

This library, too, makes some pragmatic choices about structure. In F#, for example, an optional value could be obtained like...

```fsharp
let value =
    Option.ofObj myVar
    |> Option.map (fun it -> it.Replace("howd", "part"))
    |> Option.defaultValue "There was no string"
```

If `myVar` were `null`, this `value` would have "There was no string"; if `myVar` had "howdy", `value` would have "party". Each `Option` call takes the option as its last parameter, and `|>` is the pipeline operator; it provides the previous value as the last parameter to the next operation. A prior version of this library had static functions to mimic this, which resulted in something like...

```php
$value = Option::defaultValue('There was no string',
    Option::map(fn($it) => str_replace('howd', 'part', $it),
        Option::of($myVar)));
```

...which reads right-to-left (or bottom-to-top, the way it is formatted there). By implementing these as instance methods, the PHP code looks much cleaner.

```php
$value = Option::of($myVar)
    ->map(fn($it) => str_replace('howd', 'part', $it))
    ->getOrDefault('There was no string');
```

If PHP gets a pipeline operator, we'll revisit lots of stuff here (in a non-breaking way, of course).

## Ideas

This library currently has the features which its author needs. To suggest others, reach out to Daniel on the Fediverse at @daniel@fedi.summershome.org or on Twitter at @Bit_Badger.