public static function fmap(callable $f, $a)
{
assert($a instanceof Instance, 'Second argument must be Writer');
return bind($a, function ($inner) use($f) {
return Monad\Writer::ret($f($inner));
});
}
public function testRun()
{
$ret = runWriter(tell("a")->bind(function ($_) {
return Monad\Writer::ret(1);
}));
$this->assertEquals([1, "a"], $ret);
$ret = runWriter(tell("a")->bind(function ($_) {
return ret(1);
// Any#cast => Writer
}));
$this->assertEquals([1, "a"], $ret);
$ret = runWriter(tell("a")->bind(function ($_) {
return tell("b");
})->bind(function ($_) {
return ret(2);
}));
$this->assertEquals([2, "ab"], $ret);
$ret = runWriter(tell("a")->bind(function ($_) {
return tell("b");
})->bind(function ($_) {
return ret(2);
})->bind(function ($a) {
return ret(3);
}));
$this->assertEquals([3, "ab"], $ret);
$ret = runWriter(ret(2)->bind(function ($_) {
return tell("a");
})->bind(function ($_) {
return ret(2);
}));
$this->assertEquals([2, "a"], $ret);
}