Is it possible to curry method calls in PHP?

Question

I have a SoapClient instance generated for a WSDL file. All except one of the method invocations require the username and the password to be passed id.

Is there any way of currying the method calls so that I can omit the username and password?

Answer 1

As of php 5.3 you can store an anonymous function in a variable. This anonymous function can call the "original" function with some predefined parameters.

function foo($x, $y, $z) {
  echo "$x - $y - $z";
}

$bar = function($z) {
  foo('A', 'B', $z);
};

$bar('C');

edit: You can also use a closure to parametrise the creation of the anonymous function

function foo($x, $y, $z) {
  echo "$x - $y - $z";
}

function fnFoo($x, $y) {
  return function($z) use($x,$y) {
    foo($x, $y, $z);
  };
}

$bar = fnFoo('A', 'B');
$bar('C');

edit2: This also works with objects

class Foo {
  public function bar($x, $y, $z) {
    echo "$x - $y - $z";
  }
}

function fnFoobar($obj, $x, $z) {
  return function ($y) use ($obj,$x,$z) {
    $obj->bar($x, $y, $z);
  };
}

$foo = new Foo;
$bar = fnFoobar($foo, 'A', 'C');
$bar('B');

But the other suggestions using __call() and a wrapper class may be better if you want to "enhance" a complete class.

Answer 2

Here is a class implements automatic currying and partial application:

class lambda
{
    private $f;
    private $args;
    private $count;
    public function __construct($f, $args = [])
    {
        if ($f instanceof lambda) {
            $this->f = $f->f;
            $this->count = $f->count;
            $this->args = array_merge($f->args, $args);
        }
        else {
            $this->f = $f;
            $this->count = count((new ReflectionFunction($f))->getParameters());
            $this->args = $args;
        }
    }

    public function __invoke()
    {
        if (count($this->args) + func_num_args() < $this->count) {
            return new lambda($this, func_get_args());
        }
        else {
            $args = array_merge($this->args, func_get_args());
            $r = call_user_func_array($this->f, array_splice($args, 0, $this->count));
            return is_callable($r) ? call_user_func(new lambda($r, $args)) : $r;
        }
    }
}
function lambda($f)
{
    return new lambda($f);
}

Example:

$add = lambda(function($a, $b) { 
    return $a + $b; 
});
$add1 = $add(1);
echo $add1(2); // 3

Even you can do this:

$int1 = lambda(function($f, $x) {
    return $f($x);
});

$successor = lambda(function($p, $f, $x) {
    return $f($p($f, $x));
}); 

$add = lambda(function($p, $q, $f, $x) {
    return $p($f, $q($f, $x));
}); 

$mul = lambda(function($p, $q, $x) {
    return $p($q($x));
}); 

$exp = lambda(function($m, $n) {
    return $n($m);
});

$int2 = $successor($int1);
$int3 = $add($int1, $int2);
$int6 = $mul($int3, $int2);
$int8 = $exp($int2, $int3);

Answer 3

PHP doesn't have currying per se, but you can do something like that in several ways. In your specific case, something like this may work:

class MySoapClient extends SoapClient {
  ...
  public function __call($meth,$args) {
    if (substr($method,0,5) == 'curry') {
      array_unshift($args,PASSWORD);
      array_unshift($args,USERNAME);
      return call_user_func_array(array($this,substr($meth,5)),$args);
    } else {
      return parent::__call($meth,$args);
    }
  }
}
$soapClient = new MySoapClient();
...
// now the following two are equivalent
$soapClient->currysomeMethod($additionalArg);
$soapClient->someMethod(USERNAME,PASSWORD,$additionalArg);

Although here's a more general solution for currying in PHP >= 5.3:

$curriedMethod = function ($additionalArg) use ($soapClient) { return $soapClient->method(USERNAME,PASSWORD,$additionalArg); }

$result = $curriedMethod('some argument');

Answer 4

I did some related research into this today. This is as close as I could get:

function curryAdd($x)
{
  return function($y = null) use ($x)
  {
    if (is_null($y)) return $x;
    else return curryAdd($x + $y);
  };
}

// echo curryAdd(1)(2)(3)(4);
echo curryAdd(1)
  ->__invoke(2)
  ->__invoke(3)
  ->__invoke(4)
  ->__invoke();

The major problem is PHP will not let you execute a closure directly on a return value (much in the same way PHP will not allow executing a method on an unbound object). However, since closures are an object of type Closure, which have a built-in method __invoke(), the above will work.

Answer 5

Although not a very good solution, you could write a basic wrapper class that used PHPs magic methods (Specifically __call) to call the actual function but append the user name and password to the argument list.

Basic example:

class SC
{
    private $user;
    private $pass;

    public function __construct($user, $pass)
    {
        $this->user = $user;
        $this->pass = $pass;
    }

    public function __call($name, $arguments) 
    {
        $arguments = array_merge(array($this->user, $this->pass), $arguments);  
        call_user_func_array($name, $arguments);
    }
}

Answer 6

You can use partial application and curry functions from Non-standard PHP library.

Answer 7

As mentionned by Ihor, the Non-standard PHP library is interesting. I have already implemented the same method, it is a bit different than Ihor's curried() function

function curryfy($f, $args = []) {
    $reflexion = new ReflectionFunction($f);
    $nbParams = $reflexion->getNumberOfParameters();

    return function (...$arguments) use ($f, $reflexion, $nbParams, $args) {
        if (count($args) + count($arguments) >= $nbParams) {
            return $reflexion->invokeArgs(array_merge($args, $arguments));
        }

        return curryfy($f, array_merge($args, $arguments));
    };
}

Usage :

function display4 ($a, $b, $c, $d) {
    echo "$a, $b, $c, $d\n";
};
$curry4 = curryfy('display4');
display4(1, 2, 3, 4);
$curry4(1)(2)(3)(4);

Answer 8

This answer Is it possible to curry method calls in PHP? doesn't show Currying. That answer shows partial application. A nice tutorial which explains the difference between those concepts can be seen here: http://allthingsphp.blogspot.com/2012/02/currying-vs-partial-application.html

This is Currying:

function sum3($x, $y, $z) {
    return $x + $y + $z;
}

// The curried function    
function curried_sum3($x) {
    return function ($y) use ($x) {
        return function ($z) use ($x, $y) {
            return sum3($x, $y, $z);
        };
    };
}

Invoking the curried function in PHP 7

$result = curried_sum3(1)(2)(3); 
var_dump($result); // int 6

Invoking the curried function in PHP 5

$f1 = curried_sum3(6);
$f2 = $f1(6);
$result = $f2(6);
var_dump($result);

//OUTPUT:
int 18

This is Partial application:

function sum3($x, $y, $z) {
    return $x + $y + $z;
}

function partial_sum3($x) {
    return function($y, $z) use($x) {
        return sum3($x, $y, $z);
    };
}

//create the partial
$f1 = partial_sum3(6);
//execute the partial with the two remaining arguments
$result = $f1(6, 6);

var_dump($result);

//OUTPUT:
int 18