Side Effects

Also Known As

TL;DR

Explanation

Side effects include: modifying global or class-level state, writing to files or databases, sending emails, mutating arguments passed by reference, and throwing exceptions. They are necessary for useful programs but should be isolated to well-defined boundaries and minimised in business logic. Unexpected side effects are a major source of bugs. The command-query separation principle (CQS) formalises this: methods that return values should not modify state, and methods that modify state should not return values.

Common Misconception

Why It Matters

Common Mistakes

• Functions that modify global state, static properties, or their arguments without signalling it in the signature.
• Constructors with side effects — database writes, HTTP calls, or file I/O in __construct().
• Not distinguishing between acceptable necessary side effects (persisting data) and hidden accidental ones.
• Functions that return different values for the same input due to hidden state — breaks caching and testing.

Code Examples

// Hidden side effects — callers can't predict what changes
function calculateTotal(array \$cart): float {
    \$this->lastCalculated = time();          // hidden mutation
    \$this->analytics->track('cart_viewed');  // hidden I/O
    \$total = array_sum(array_column(\$cart, 'price'));
    \$this->cache->set('cart_total', \$total); // hidden write
    return \$total;
}

// Pure calculation — no side effects
function calculateTotal(array \$cart): float {
    return array_sum(array_column(\$cart, 'price'));
}

// Side effects handled explicitly by the caller
\$total = calculateTotal(\$cart);
\$this->analytics->track('cart_viewed');
\$this->cache->set('cart_total:' . \$cartId, \$total);

References

• ↗ https://en.wikipedia.org/wiki/Side_effect_(computer_science)

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