Yield from delegation in PHP

Introduction

Yield from delegation helps you easily get values from one generator inside another without extra loops.

You want to combine results from multiple generators into one sequence.

You have a generator that calls another generator and want to pass values through.

You want cleaner code when working with nested generators.

You want to pause and resume multiple sequences smoothly.

Syntax

PHP

yield from <generator_expression>;

This passes all yielded values from the inner generator to the outer one.

It simplifies code by avoiding manual loops to yield each value.

Examples

This example shows how yield from passes values from gen1 inside gen2.

PHP

<?php
function gen1() {
    yield 1;
    yield 2;
}

function gen2() {
    yield from gen1();
    yield 3;
}

foreach (gen2() as $value) {
    echo $value . " ";
}

This example combines two generators using yield from to produce letters then numbers.

PHP

<?php
function letters() {
    yield 'a';
    yield 'b';
}

function numbers() {
    yield 1;
    yield 2;
}

function combined() {
    yield from letters();
    yield from numbers();
}

foreach (combined() as $item) {
    echo $item . " ";
}

Sample Program

This program shows how yield from lets outerGenerator include all values from innerGenerator easily.

PHP

<?php
function innerGenerator() {
    yield 'apple';
    yield 'banana';
}

function outerGenerator() {
    yield 'start';
    yield from innerGenerator();
    yield 'end';
}

foreach (outerGenerator() as $fruit) {
    echo $fruit . "\n";
}

Important Notes

yield from can delegate to any iterable, not just generators.

It helps keep generator code clean and readable.

Summary

Yield from delegation passes values from one generator to another easily.

It avoids writing loops to yield each value manually.

Use it to combine or chain generators simply.

Practice

1.

What does the yield from statement do in PHP generators?

easy

A. It delegates yielding values from another generator or iterable.

B. It stops the generator immediately.

C. It converts a generator into an array.

D. It throws an exception inside the generator.

Solution

1. Step 1: Understand the purpose of yield from

   yield from is used to delegate yielding values from another generator or iterable, so you don't have to write a loop to yield each value manually.

2. Step 2: Compare with other options

   Stopping a generator, converting it to an array, or throwing exceptions are unrelated to yield from.

3. Final Answer:

   It delegates yielding values from another generator or iterable. -> Option A

4. Quick Check:

   yield from delegates values [OK]

Hint: Remember: yield from passes values from another generator [OK]

Common Mistakes:

• Thinking yield from stops the generator
• Confusing yield from with array conversion
• Assuming it throws exceptions

2.

Which of the following is the correct syntax to delegate yielding from another generator $gen inside a generator function?

function gen() {
    // your code here
}

easy

A. yield $gen;

B. yield to $gen;

C. yield($gen);

D. yield from $gen;

Solution

1. Step 1: Recall correct yield from syntax

   The correct syntax to delegate yielding from another generator is yield from $gen;.

2. Step 2: Check other options for syntax errors

   yield $gen; yields the generator object itself, not its values. yield($gen); is invalid syntax. yield to $gen; is not valid PHP syntax.

3. Final Answer:

   yield from $gen; -> Option D

4. Quick Check:

   Correct syntax uses yield from [OK]

Hint: Use exact phrase yield from to delegate generators [OK]

Common Mistakes:

• Using just yield $gen; instead of yield from
• Adding parentheses incorrectly
• Writing invalid keywords like 'yield to'

3.

What will be the output of the following PHP code?

function gen1() {
    yield 1;
    yield 2;
}

function gen2() {
    yield from gen1();
    yield 3;
}

foreach (gen2() as $value) {
    echo $value . ' ';
}

medium

A. 1 3 2

B. 3 1 2

C. 1 2 3

D. 1 2

Solution

1. Step 1: Understand yield from gen1() in gen2()

   This delegates yielding values 1 and 2 from gen1() inside gen2().

2. Step 2: Trace the output of the foreach loop

   The loop yields 1, then 2 from gen1(), then yields 3 from gen2(). So output is "1 2 3 "

3. Final Answer:

   1 2 3 -> Option C

4. Quick Check:

   Delegation yields 1,2 then 3 [OK]

Hint: Remember yield from outputs all delegated values first [OK]

Common Mistakes:

• Assuming order is changed
• Forgetting the last yield after delegation
• Thinking yield from yields the generator object

4.

Identify the error in this PHP generator code:

function genA() {
    yield 1;
    yield 2;
}

function genB() {
    yield genA();
    yield 3;
}

foreach (genB() as $val) {
    echo $val . ' ';
}

medium

A. It yields the generator object instead of its values.

B. Missing semicolon after yield statements.

C. Cannot yield inside a function.

D. The foreach loop is invalid syntax.

Solution

1. Step 1: Analyze yield genA(); in genB()

   This yields the generator object genA() itself, not the values it produces.

2. Step 2: Understand correct delegation with yield from

   To yield values from genA(), use yield from genA(); instead of yield genA();.

3. Final Answer:

   It yields the generator object instead of its values. -> Option A

4. Quick Check:

   Use yield from to delegate values [OK]

Hint: Use yield from to yield values, not the generator object [OK]

Common Mistakes:

• Using yield instead of yield from for delegation
• Assuming yielding a generator yields its values
• Ignoring the need to delegate properly

5.

Given two generators genX() yielding numbers 1 to 3 and genY() yielding numbers 4 to 6, how can you create a generator genZ() that yields all numbers from 1 to 6 using yield from?

function genX() {
    yield 1;
    yield 2;
    yield 3;
}

function genY() {
    yield 4;
    yield 5;
    yield 6;
}

function genZ() {
    // Fill in code here
}

hard

A. yield genX(); yield genY();

B. yield from genX(); yield from genY();

C. yield genX() + genY();

D. yield from genY(); yield from genX();

Solution

1. Step 1: Use yield from to delegate both generators in order

   To yield all numbers from 1 to 6 in sequence, delegate genX() first, then genY() using yield from.

2. Step 2: Check order and syntax

   yield from genX(); yield from genY(); yields 1,2,3 then 4,5,6. yield from genY(); yield from genX(); reverses order. Options B and C yield generator objects or invalid expressions.

3. Final Answer:

   yield from genX(); yield from genY(); -> Option B

4. Quick Check:

   Delegate generators in order with yield from [OK]

Hint: Chain generators with multiple yield from statements [OK]

Common Mistakes:

• Yielding generators instead of delegating
• Reversing the order of delegation
• Trying to add generators directly