rfc:safe_cast

PHP RFC: Safe Casting Functions

Introduction

Currently, PHP only provides one means of type conversion: explicit casts. These casts never fail or emit errors, making them dangerous to use, as when passed garbage input, they will simply return garbage instead of indicating that something went wrong. This makes it difficult to write robust applications which handle user data. They also prevent any suggestion of strict type hinting for scalar types, because if that were to be added, users would simply use dangerous explicit casts to get around errors and the result would be code that is buggier than it would have been without type hinting at all.

For int and float conversion specifically, ext/filter provides FILTER_VALIDATE_INT and FILTER_VALIDATE_FLOAT. filter_var($foo, FILTER_VALIDATE_INT) and filter_var($foo, FILTER_VALIDATE_FLOAT). However, these are rather unwieldy, encouraging people to use the shorter explicit casts, and suffer from a performance and safety standpoint by their converting values to strings before validating (allowing, for example, booleans, or objects with __toString). Furthermore, their use requires explicit error handling by checking for a FALSE return value. If the programmer forgets to check it, they are no safer than explicit casts.

Proposal

Two families of “safe casting” functions are added to ext/standard, try_* and to_*, for int, float and string. These functions validate their input to ensure data is not lost with the cast (thus the cast can be considered safe), instead of casting blindly. If the input fails to validate, the to_* functions throw a CastException, while the try_* functions return NULL. If validation succeeds, the converted result is returned.

to_int() and try_int() accept only ints, non-NaN integral floats within the range of an integer (PHP_INT_MIN to PHP_INT_MAX), and strings containing decimal integer sequences within the range of an integer. Leading and trailing whitespace is not permitted, nor are leading zeros.

to_float() and try_float() accept only ints, floats, and strings representing floats. Leading and trailing whitespace is not permitted, nor are leading zeros.

to_string() and try_string() accept only strings, objects which cast to strings, ints and floats.

The new class CastException is added to ext/standard, which extends SPL's RuntimeException.

Rationale

The concept was developed in my thoughts, and in discussions with Anthony Ferrara (both in-person at PHPNW14 and online) and others in StackOverflow's PHP chatroom. Here, I list some of my or our rationale for particular decisions.

  • The functions don't accept anything which wouldn't be converted properly by normal unsafe explicit casts (i.e. (int) and intval()), such that their accepted inputs should be a strict subset of inputs converted by unsafe casts. This is why hexadecimal and exponents are not permitted for to_int() and try_int().
  • Whitespace is not allowed because it is expected that most input will lack it, and it can be easily stripped before conversion using trim()
  • Generally, no data is allowed to be lost, with the exception of floats, which are allowed to lose accuracy from their decimal representations. This is because zero data-loss is impossible (or at least highly impractical) for floats, as PHP does not output them in full precision, and many decimal values don't have exact binary representations, and vice-versa. Furthermore, some loss of accuracy is expected when dealing with floats.
  • Leading zeros are not accepted for to_int() and try_int(), because strings containing them are not consistently interpreted (sometimes they are considered octal, sometimes decimal), nor is user intent consistent.
  • There are two sets of functions, one that returns NULL on failure and the other that throws an exception for the following reasons:
    • In some cases, invalid input is an exceptional case, so an exception is desirable, but in other cases, it is not exceptional, so an exception shouldn't be used. Having two functions allows both these scenarios to be covered.
      • It is often repeated that exceptions shouldn't be used for flow control.
    • It is PHP tradition to allow both object-oriented (exceptions) and procedural (error return value) approaches to problems.
    • This placates both people who would like an error return value, and people who would like exceptions.
    • Return values are better for chaining in some circumstances.
    • Checking exceptions is generally slower than checking return values, especially in HHVM).

Examples Table

A sample table of whether values pass or fail generated by this script, on a 64-bit machine:

value try_int try_float try_string
string(6) “foobar” fail fail pass
string(0) “” fail fail pass
string(1) “0” pass pass pass
int(0) pass pass pass
float(0) pass pass pass
string(2) “10” pass pass pass
string(3) “010” fail fail pass
string(3) “+10” pass pass pass
string(3) “-10” pass pass pass
int(10) pass pass pass
float(10) pass pass pass
string(19) “9223372036854775807” pass pass pass
int(9223372036854775807) pass pass pass
string(20) “-9223372036854775808” pass pass pass
int(-9223372036854775808) pass pass pass
string(4) “10.0” fail pass pass
string(5) “75e-5” fail pass pass
string(5) “31e+7” fail pass pass
NULL fail fail fail
bool(true) fail fail fail
bool(false) fail fail fail
object(stdClass)#1 (0) {} fail fail fail
resource(5) of type (stream) fail fail fail
array(0) {} fail fail fail
float(1.5) fail pass pass
string(3) “1.5” fail pass pass
string(5) “10abc” fail fail pass
string(5) “abc10” fail fail pass
string(4) “100 ” fail fail pass
string(4) “ 100” fail fail pass
string(5) “ 100 ” fail fail pass
string(4) “0x10” fail fail pass
float(INF) fail pass pass
float(-INF) fail pass pass
float(NAN) fail pass pass
float(1.844674407371E+19) fail pass pass
float(-1.844674407371E+19) fail pass pass
string(18) “1.844674407371E+19” fail pass pass
string(19) “-1.844674407371E+19” fail pass pass
object(Stringable)#2 (0) {} fail fail pass
object(NotStringable)#3 (0) {} fail fail fail
object(stdClass)#4 (0) {} fail fail fail

Proposed PHP Version(s)

This is proposed for the next major version of PHP, currently PHP 7.

Open Issues

None.

Unaffected PHP Functionality

This does not touch the explicit cast operators ((int), (float), (string) etc.) nor the explicit cast functions (intval(), floatval, strval() etc.).

Future Scope

This might be extended to other types. However, support for the other scalar types has deliberately not been included. For booleans, there is no clear single format to accept, nor a consistent interpretation of particular values depending on the format used. Furthermore, strict boolean conversion is very simple to do so manually. NULL is a type with only one possible value, so there is no point in casting. Resources are special and don't really count as scalars.

Proposed Voting Choices

As this is not a language change and only introduces new functions, only a 50%+1 majority will be required. The vote will be a straight Yes/No vote on accepting the RFC and merging the patch into master.

Vote

Voting opened 2014-11-19 and ended 2014-11-29.

Should the Safe Casting Functions RFC be accepted, and the patch merged into master?
Real name Yes No
ab (ab)  
aharvey (aharvey)  
ajf (ajf)  
derick (derick)  
gwynne (gwynne)  
hywan (hywan)  
jedibc (jedibc)  
laruence (laruence)  
leigh (leigh)  
levim (levim)  
lstrojny (lstrojny)  
marco (marco)  
mbeccati (mbeccati)  
mike (mike)  
patrickallaert (patrickallaert)  
philstu (philstu)  
rasmus (rasmus)  
salathe (salathe)  
seld (seld)  
stas (stas)  
tyrael (tyrael)  
Final result: 5 16
This poll has been closed.

Patches and Tests

I have made a patch and pull request on GitHub against the master branch: https://github.com/php/php-src/pull/874

Theodore Brown has created a userland polyfill with the same behaviour: https://github.com/theodorejb/PolyCast

Implementation

After the project is implemented, this section should contain

  1. the version(s) it was merged to
  2. a link to the git commit(s)
  3. a link to the PHP manual entry for the feature

References

Rejected Features

Keep this updated with features that were discussed on the mail lists.

Changelog

  • v0.1.8 - ext/filter note in Introduction
  • v0.1.7 - Allow positive signs
  • v0.1.6 - Dropped zero round trip data loss principle, added octal and whitespace rationale
  • v0.1.5 - Renamed to_ functions to try_, added to_ functions which throw exception
  • v0.1.4 - Reject leading '+' and '0' for int/float, to_Y($A) !== NULL IFF (X)(Y)$A === $A principle in rationale
  • v0.1.3 - Return NULL, don't include exceptions in vote
  • v0.1.2 - Leading and trailing whitespace is not permitted
  • v0.1.1 - Rationale
  • v0.1 - Created
rfc/safe_cast.txt · Last modified: 2014/11/29 22:20 by ajf