Psalm Error: List Incorrectly Defined As Non-Empty

Introduction

In the realm of PHP development, static analysis tools like Psalm play a crucial role in identifying potential issues and ensuring code quality. One such issue arises when a list is erroneously defined as a non-empty list, leading to unexpected behavior and potential runtime errors. This article delves into a specific scenario where Psalm fails to recognize an empty list, causing a violation of the non-empty list invariant. We will explore the code example, analyze the problem, and discuss the implications of this error.

Understanding the Non-Empty List Invariant

Before diving into the specifics of the error, it's essential to understand the concept of a non-empty list invariant. In programming, an invariant is a condition that must always be true at a particular point in the code. For a non-empty list, the invariant is that the list must contain at least one element. This invariant is crucial for certain operations that rely on the presence of elements in the list, such as accessing the first element or iterating over the list.

Psalm, as a static analysis tool, aims to enforce these invariants by analyzing the code and identifying potential violations. However, as we will see in the example below, there are cases where Psalm may fail to correctly identify an empty list, leading to a violation of the non-empty list invariant.

The Code Example: A Violation of the Non-Empty List Invariant

The following code snippet demonstrates a scenario where Psalm erroneously defines a list as non-empty, leading to a violation of the invariant:

<?php

/** @param non-empty-list<int> $a */
function foo(array $a): void {
    echo $a[0];
}

$d = [];
$d[] = 0;
if (time() & 1) {
    $d[] = 2;
}
array_pop($d);
foo($d); // No error!

In this code, the function foo is defined to accept a non-empty list of integers as its argument. The code then initializes an empty array $d, adds the element 0 to it, and conditionally adds the element 2 based on the result of a bitwise operation on the current timestamp. Finally, it removes the last element from the array using array_pop and calls the foo function with the resulting array.

The problem here is that after the array_pop call, the array $d might be empty if the conditional statement did not add the element 2. However, Psalm does not recognize this possibility and incorrectly assumes that $d is always a non-empty list, leading to no error being reported when foo($d) is called. This is a clear violation of the non-empty list invariant, as the foo function attempts to access the first element of the array ($a[0]), which will result in an error if the array is empty.

Analyzing the Issue

The root cause of this issue lies in Psalm's inability to accurately track the state of the array $d across the conditional statement and the array_pop call. Psalm's static analysis relies on inferring the types and states of variables based on the code's logic. In this case, Psalm seems to be overlooking the possibility that the conditional statement might not be executed, or that the array_pop call might empty the array.

This highlights a limitation of static analysis tools: they cannot always perfectly predict the runtime behavior of the code, especially when dealing with dynamic conditions or complex control flow. While Psalm is generally effective at catching errors, it is not foolproof and can sometimes miss potential issues.

Implications of the Error

The erroneous definition of a list as non-empty can have significant implications for the reliability and correctness of the code. In this specific example, the violation of the non-empty list invariant can lead to a runtime error when the foo function is called with an empty array. This error can crash the application or lead to unexpected behavior.

More broadly, this type of error can undermine the benefits of using static analysis tools. If developers cannot rely on Psalm to accurately identify potential issues, they may lose confidence in the tool and be less likely to use it. This can lead to a decrease in code quality and an increase in the risk of runtime errors.

Addressing the Issue

To address this issue, it's crucial to take a multi-faceted approach. First, developers should be aware of the limitations of static analysis tools and not rely on them as a sole means of ensuring code quality. Manual code review, unit testing, and integration testing are all essential components of a comprehensive testing strategy.

Second, it's important to report such issues to the developers of Psalm. By providing feedback and submitting bug reports, developers can help improve the accuracy and reliability of the tool. The Psalm team is actively working to address these kinds of issues and appreciates user feedback.

Potential Solutions

In the specific case of the code example, there are several ways to prevent the error. One approach is to explicitly check if the array is empty before calling the foo function:

<?php

/** @param non-empty-list<int> $a */
function foo(array $a): void {
    echo $a[0];
}

$d = [];
$d[] = 0;
if (time() & 1) {
    $d[] = 2;
}
array_pop($d);
if (!empty($d)) {
    foo($d);
}

This check ensures that the foo function is only called if the array $d is not empty, preventing the potential runtime error. Another approach is to modify the code to ensure that the array is always non-empty, for example, by adding a default element if necessary.

Alternative Analysis with PHPStan

It's worth noting that while Psalm failed to identify the issue in the original code, another popular static analysis tool, PHPStan, also did not report an error. This further emphasizes the challenges of static analysis and the importance of using a combination of tools and techniques to ensure code quality. The link provided in the original report (https://phpstan.org/r/4710c606-9675-49a3-9b2a-bd11a4e56fc3) demonstrates this behavior.

This highlights the need for developers to be vigilant and not solely rely on static analysis tools to catch all potential errors. It also underscores the value of using multiple static analysis tools in conjunction, as they may have different strengths and weaknesses and catch different types of errors.

Conclusion

The case of the erroneously defined non-empty list in Psalm highlights the challenges and limitations of static analysis. While static analysis tools like Psalm are valuable for identifying potential issues, they are not foolproof and can sometimes miss errors. Developers should be aware of these limitations and use a combination of techniques, including manual code review, unit testing, and integration testing, to ensure code quality.

It's also crucial to report any issues found to the developers of static analysis tools so they can improve their accuracy and reliability. By working together, developers and tool creators can build better software and reduce the risk of runtime errors.

Remember to consult the official Psalm documentation for the most up-to-date information and best practices. You can find it here.