rfc:nullsafe_operator

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rfc:nullsafe_operator [2020/06/12 18:27] – Add better short circuiting rationale ilijatovilorfc:nullsafe_operator [2020/07/31 08:55] (current) – Close vote ilutov
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   * Date: 2020-06-02   * Date: 2020-06-02
   * Author: Ilija Tovilo, tovilo.ilija@gmail.com   * Author: Ilija Tovilo, tovilo.ilija@gmail.com
-  * Status: Under discussion+  * Status: Implemented
   * Target Version: PHP 8.0   * Target Version: PHP 8.0
   * Implementation: https://github.com/php/php-src/pull/5619   * Implementation: https://github.com/php/php-src/pull/5619
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 ===== Introduction ===== ===== Introduction =====
  
-This RFC proposes a new operator nullsafe operator ''%%?->%%'' with full short circuiting.+This RFC proposes the new nullsafe operator ''%%?->%%'' with full short circuiting.
  
 ===== Proposal ===== ===== Proposal =====
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   * Nullsafe method call (''%%?->%%'')   * Nullsafe method call (''%%?->%%'')
   * Static method call (''%%::%%'')   * Static method call (''%%::%%'')
-  * Assignment (''%%=%%'', ''%%+=%%'', ''%%??=%%'', ''%%= &%%'', etc.) 
-  * Post/pre increment (''%%++%%'', ''%%--%%'') 
  
 The following elements will cause new sub-chains. The following elements will cause new sub-chains.
  
-  * Right hand side of an assignment 
   * Arguments in a function call   * Arguments in a function call
   * The expression in ''%%[]%%'' of an array access   * The expression in ''%%[]%%'' of an array access
 +  * The expression in ''%%{}%%'' when accessing properties (''%%->{}%%'')
  
 Chains are automatically inferred. Only the closest chain will terminate. The following examples will try to illustrate. Chains are automatically inferred. Only the closest chain will terminate. The following examples will try to illustrate.
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 //       --------  chain 2 //       --------  chain 2
 // If $c is null chain 2 is aborted, method d() isn't called, null is passed to `$a->b()` // If $c is null chain 2 is aborted, method d() isn't called, null is passed to `$a->b()`
- 
-   $foo?->bar = $a->b(); 
-// -------------------- chain 1 
-//              ------- chain 2 
-// If $foo is null chain 1 is aborted, `$a->b()` is not evaluated, the assignment is skipped 
- 
-   $foo?->bar++; 
-// ------------ chain 1 
-// If $foo is null, chain 1 is aborted, ++ is skipped 
 </code> </code>
 ==== Rationale ==== ==== Rationale ====
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 $foo?->bar(expensive_function()); $foo?->bar(expensive_function());
 </code> </code>
-The evaluation of ''%%expensive_function()%%'' is undesirable if ''%%$foo%%'' is ''%%null%%'' as its result will simply be discarded. If the function has side effects it could also lead to surpsises.+The evaluation of ''%%expensive_function()%%'' is undesirable if ''%%$foo%%'' is ''%%null%%'' as its result will simply be discarded. If the function has side effects it could also lead to surprises.
  
 **2. You can see which methods/properties return null** **2. You can see which methods/properties return null**
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 Without short circuiting every subsequent method call and property access in the chain will require using the nullsafe operator or you will get a “Call to a member function on null” error. With short circuiting this isn’t necessary which makes it more obvious which methods/properties might return ''%%null%%''. Without short circuiting every subsequent method call and property access in the chain will require using the nullsafe operator or you will get a “Call to a member function on null” error. With short circuiting this isn’t necessary which makes it more obvious which methods/properties might return ''%%null%%''.
  
-**3. Allows for nullsafe operator in write context** +**3. Mixing with other operators**
- +
-<code php> +
-$foo = null; +
-$foo?->bar = 'bar'; +
-var_dump($foo); +
- +
-// Without short circuiting: +
-// Fatal error: Can't use nullsafe result value in write context +
- +
-// With short circuiting: +
-// NULL +
-</code> +
-Without short circuiting the assignment to a nullsafe property would be illegal because it produces an r-value (a value that cannot be assigned to). With short circuiting if a nullsafe operation on the left hand side of the assignment fails the assignment is simply skipped. +
- +
-**4. Mixing with other operators**+
  
 <code php> <code php>
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 ===== Syntax choice ===== ===== Syntax choice =====
  
-The ''%%?%%'' in ''%%?->%%'' denotes the precise place in the code where the short circuiting occurs. It closesly resembles the syntax of every other language that implements a nullsafe operator.+The ''%%?%%'' in ''%%?->%%'' denotes the precise place in the code where the short circuiting occurs. It closely resembles the syntax of every other language that implements a nullsafe operator. 
 + 
 +===== Forbidden usages ===== 
 + 
 +==== Nullsafe operator in write context ==== 
 + 
 +Using the nullsafe operator in write context ist not allowed. 
 + 
 +<code php> 
 +$foo?->bar->baz = 'baz'; 
 +// Can't use nullsafe operator in write context 
 + 
 +foreach ([1, 2, 3] as $foo?->bar->baz) {} 
 +// Can't use nullsafe operator in write context 
 + 
 +unset($foo?->bar->baz); 
 +// Can't use nullsafe operator in write context 
 + 
 +[$foo?->bar->baz] = 'baz'; 
 +// Assignments can only happen to writable values 
 +</code> 
 +It was previously suggested to allow the nullsafe operator in the left hand side of assignments and skip the assignment if the left hand side of the nullsafe operator was ''%%null%%''. However, due to technical difficulties this is not a part of this RFC. It might be addressed in a later RFC. It is also not completely clear whether the right hand side of the assignment should always be evaluated or not. 
 + 
 +==== References ==== 
 + 
 +Taking the reference of a nullsafe chain is not allowed. This is because references require l-values (memory locations, like variables or properties) but the nullsafe operator can sometimes return the r-value ''%%null%%''. 
 + 
 +<code php> 
 +$x = &$foo?->bar; 
 + 
 +// Could loosely be translated to 
 + 
 +if ($foo !== null) { 
 +    $x = &$foo->bar; 
 +} else { 
 +    $x = &null; 
 +    // Only variables should be assigned by reference 
 +} 
 +</code> 
 +For this reason, the following examples are disallowed. 
 + 
 +<code php> 
 +// 1 
 +$x = &$foo?->bar; 
 +// Compiler error: Cannot take reference of a nullsafe chain 
 + 
 +// 2 
 +takes_ref($foo?->bar); 
 +// Error: Cannot pass parameter 1 by reference 
 + 
 +// 3 
 +function &return_by_ref($foo) { 
 +    return $foo?->bar; 
 +    // Compiler error: Cannot take reference of a nullsafe chain 
 +} 
 +</code> 
 +Example 2 is a runtime error because we cannot know at compile time if the given parameter allows passing values by reference.
  
 ===== Backward Incompatible Changes ===== ===== Backward Incompatible Changes =====
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 ===== Vote ===== ===== Vote =====
  
-…+Voting starts 2020-07-17 and ends 2020-07-31.  
 + 
 +<doodle title="Add nullsafe operator to the language?" auth="ilutov" voteType="single" closed="true"> 
 +   * Yes 
 +   * No 
 +</doodle>
rfc/nullsafe_operator.1591986465.txt.gz · Last modified: 2020/06/12 18:27 by ilijatovilo

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