When you throw an exception in PHP, you effectively perform a GOTO: your position in the program's execution jumps to the appropriate exception handler, and execution continues. This is fine, but it obviously means that your original function has a side-effect: calling it will fundamentally alter the program flow. What we need is a functional way of accomplishing the same thing.
Enter, the Either monad. Either has two constructors, Left and Right. These work very similarly to Maybe's Just and Nothing: map, ap, and chain work as you'd expect on the Right instance, but are effectively no-ops on the Left. However, the difference is that Left, unlike Nothing, holds a value.
This means that, typically, your left branch will hold the 'exception', and the right will hold the value. If an exception happens, all future computations are ignored, and the exception can be handled in a pure way. For example:
<?php
use PhpFp\Either\Constructor\{Left, Right};
$login = function ($username, $password)
{
if ($username != 'foo') {
return new Left(
'Invalid username'
);
}
if ($password != 'bar') {
return new Left(
'Incorrect password'
);
}
return new Right(['hello' => 'world']);
}
$prop = function ($k)
{
return function ($xs) use ($k)
{
return isset ($xs[$k])
? new Right($xs[$k])
: new Left('No such key.');
};
};
$id = function ($x)
{
return $x;
};
// Some examples...
$badUsername = $login('fur', 'bar')->chain($prop('id'));
$badPassword = $login('foo', 'bear')->chain($prop('id'));
$badKey = $login('foo', 'bar')->chain($prop('brian'));
$good = $login('foo', 'bar')->chain($prop('id'));
assert($badUsername->either($id, $id) === 'Invalid username');
assert($badPassword->either($id, $id) === 'Incorrect password');
assert($badKey->either($id, $id) === 'No such key.');
assert($good->either($id, $id) === 'world');As the above shows, a failure is carried through the computation and all further operations (with the only (for now) exception of bimap below), and must be handled by either, the function for retrieving the inner value.
Of course, exceptions are the usual analogy, but Either is a more general type, and is helpful in most computations with two potential values. What if a user can input via a file or stdin? We could use Either String File, map over the instance with a File -> String function, then extract the value once we know they're both acceptable.
In the following type signatures, constructors and static functions are written as one would see in pure languages such as Haskell. The others contain a pipe, where the type before the pipe represents the type of the current Either instance, and the type after the pipe represents the function.
This is the applicative constructor for the Either monad. It returns the given value wrapped in a Right instance:
<?php
use PhpFp\Either\Either;
$id = function ($x) { return $x; };
assert(Either::of('test')->either($id, $id) == 'test');Standard constructor for Left instances.
<?php
use PhpFp\Either\Either;
use PhpFp\Either\Constructor\Left;
$either = Either::left('test');
assert($either instanceof Either);
assert($either instanceof Left);Standard constructor for Right instances. Typically you should call Either::of instead.
<?php
use PhpFp\Either\Either;
use PhpFp\Either\Constructor\Right;
$either = Either::right('test');
assert($either instanceof Either);
assert($either instanceof Right);Sometimes, you will have a piece of exception-throwing code that you wish to wrap in an Either, and this function can help. If an exception occurs, it will be wrapped and returned in a Left. Otherwise, the returned value will be wrapped in a Right:
<?php
use PhpFp\Either\Either;
$id = function ($x) { return $x; };
$f = function () { throw new \Exception; };
$g = function () { return 'hello'; };
assert(Either::tryCatch($f)->either($id, $id) instanceof \Exception);
assert(Either::tryCatch($g)->either($id, $id) === 'hello');Apply an Either-wrapped argument to an Either-wrapped function, where a Left function will behave as identity.
<?php
use PhpFp\Either\Constructor\{Left, Right};
$id = function ($x) { return $x; };
$addTwo = Either::of(
function ($x)
{
return $x + 2;
}
);
$a = new Right(5);
$b = new Left(4);
assert($addTwo->ap($a)->either($id , $id) === 7);
assert($addTwo->ap($b)->either($id, $id) === 4);Sometimes, it can be useful to define computations to be performed on the Left values, and this is the way to do so. For this function, you supply left and right transformations, and the appropriate one will be used:
<?php
use PhpFp\Either\Constructor\{Left, Right};
$addOne = function ($x) { return $x + 1; };
$subOne = function ($x) { return $x - 1; };
$id = function ($x) { return $x; };
assert (Either::right(2)->bimap($addOne, $subOne)->either($id, $id) === 1);
assert (Either::left(2)->bimap($addOne, $subOne)->either($id, $id) === 3);The standard monadic binding function (Haskell's >>=). This is for mapping with a function that returns an Either value: instead of using map and getting Either e (Either e a), you get Either e a and the two levels are "flattened". The introduction has a good example, but here's a smaller one:
<?php
use PhpFp\Either\Constructor\{Left, Right};
$f = function ($x)
{
return Either::of($x * 2);
}
$id = function ($x) { return $x; };
assert(Either::right(8)->chain($f)->either($id, $id) === 16);
assert(Either::left(8)->chain($f)->either($id, $id) === 8);This is the standard functor map, which transforms the inner value. As with the other Either operations, remember that this has no impact on a Left value, which can only be transformed with bimap:
<?php
use PhpFp\Either\Constructor\{Left, Right};
$f = function ($x) { return $x - 5; };
$id = function ($x) { return $x; };
assert(Either::right(8)->map($f)->either($id, $id) === 3);
assert(Either::left(8)->map($f)->either($id, $id) === 8);This is the function that should be used to get the value out of the Either monad. Strictly, if you're being well-behaved and watching your types, the two supplied functions, while potentially accepting differently-typed inputs for Left and Right, should return values of the same type:
<?php
use PhpFp\Either\Constructor\{Left, Right};
$left = function ($x) { return (int) $x; };
$right = function ($x) { $x; };
assert(Either::left('7')->either($left, $right) === 7);
assert(Either::right(2)->either($left, $right) === 2);Similarly to the others, I'm aware of at least a couple of typeclasses that could be added to this implementation, so feel free to submit issues or PRs if you'd like to see others included.
However, the much more pressing concern is with the documentation: if something isn't crystal clear, please leave an issue or submit a suggested fix in order to make this as clear and descriptive as possible!