vimeo/psalm

Psalm does not infer type in array_reduce()

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#6,370 opened on Aug 27, 2021

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 (6 comments) (1 reaction) (0 assignees)PHP (668 forks)batch import
Help wantedenhancement

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Description

Given the following code:

/**
 * @param array<array{tags: string}> $a
 */
function test(array $a): string
{
    return array_reduce($a, function (string $carry, array $value) {
        return $carry . $value['tags'];
    }, '');
}

Psalm is unable to infer a type for $value in the callback function given to array_reduce():

https://psalm.dev/r/b9e02bcaaa

In this example, it should infer the type as array{tags: string}.

Can this be fixed?

Contributor guide

Research direction
Study the Psalm source code for array reduce handling, specifically the type inference for the callback parameter. Trace the logic that should infer the array element type from the iterable's generic type.
Tech stack
php
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
PHPPsalm
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60