Yield from delegation in PHP - Step-by-Step Execution

Concept Flow - Yield from delegation

Start generator

↓

Yield values from sub-generator

↓

Receive value from caller

↓

Pass value to sub-generator

↓

Sub-generator yields value

↓

Yield value to caller

↓

Repeat until sub-generator ends

↓

End generator

The main generator delegates yielding and receiving values to a sub-generator using 'yield from', passing values back and forth until the sub-generator finishes.

Execution Sample

PHP

<?php
function subGen() {
  yield 1;
  yield 2;
}
function mainGen() {
  yield from subGen();
}
foreach(mainGen() as $val) { echo $val; }

This code delegates yielding values 1 and 2 from subGen() through mainGen() to the caller.

Execution Table

Step	Action	Generator State	Value Yielded	Caller Receives
1	mainGen() starts	mainGen active, subGen not started	none	none
2	yield from subGen() starts subGen	subGen active at first yield	1	1
3	Caller receives 1	subGen paused after yield 1	1	1
4	Caller requests next	subGen resumes	2	2
5	Caller receives 2	subGen paused after yield 2	2	2
6	Caller requests next	subGen ends	none	none
7	mainGen ends	mainGen ends	none	none

💡 subGen finishes yielding all values, so mainGen ends as well

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
mainGen state	not started	active, delegating	active, delegating	ended	ended
subGen state	not started	active at yield 1	active at yield 2	ended	ended
value yielded	none	1	2	none	none

Key Moments - 3 Insights

Why does mainGen yield values even though it has no explicit yield statements?

Because 'yield from' delegates yielding to subGen, so mainGen yields whatever subGen yields (see execution_table steps 2 and 4).

What happens when the sub-generator finishes?

When subGen ends (step 6), mainGen also ends immediately (step 7), stopping the iteration.

How does the caller receive values from subGen through mainGen?

The caller calls mainGen, which delegates to subGen via 'yield from', passing yielded values directly to the caller (steps 3 and 5).

Visual Quiz - 3 Questions

Look at the execution_table, what value does the caller receive at step 3?

A 1

B 2

C none

D subGen object

💡 Hint

Check the 'Value Yielded' and 'Caller Receives' columns at step 3.

At which step does subGen finish yielding all values?

A Step 4

B Step 6

C Step 7

D Step 2

💡 Hint

Look for 'subGen ends' in the 'Generator State' column.

If we add a third yield in subGen, how would the execution_table change?

A mainGen would end earlier.

B Caller would receive no values.

C There would be an extra step with value 3 yielded before subGen ends.

D subGen would never end.

💡 Hint

Think about how 'yield from' passes all yields from subGen to caller.

Concept Snapshot

PHP 'yield from' lets one generator pass values from another generator directly.
Syntax: yield from subGenerator();
It forwards all yields and receives transparently.
Useful to simplify generator delegation.
Stops when sub-generator ends.

Full Transcript

This example shows how PHP's 'yield from' delegates yielding values from a sub-generator to the main generator. The main generator starts, then calls 'yield from' on the sub-generator. The sub-generator yields values 1 and 2, which are passed directly to the caller. When the sub-generator finishes, the main generator ends too. This delegation allows the main generator to yield all values from the sub-generator without writing explicit loops. The execution table traces each step, showing generator states and values passed. The variable tracker shows how generator states and yielded values change over time. Key moments clarify common confusions about delegation and generator ending. The quiz tests understanding of yielded values and generator lifecycle.