Rethrowing functions in Swift - Step-by-Step Execution

Concept Flow - Rethrowing functions

Call rethrowing function

↓

Call passed throwing closure

↓

Closure throws error?

No→Return normally

Yes↓

Rethrow error to caller

↓

Caller handles or propagates error

A rethrowing function calls a closure that can throw an error and only throws if that closure throws, passing the error up to its caller.

Execution Sample

Swift

enum MyError: Error {
    case example
}

func perform(action: () throws -> Void) rethrows {
    try action()
}

try perform {
    throw MyError.example
}

This code defines a rethrowing function that calls a throwing closure and rethrows any error it receives.

Execution Table

Step	Action	Closure Throws?	Function Throws?	Result
1	Call perform with throwing closure	N/A	N/A	Enter perform
2	Inside perform, call action()	Yes	Yes	Error thrown from closure
3	perform rethrows error to caller	N/A	Yes	Error propagates out
4	Caller handles error with try	N/A	N/A	Error caught or propagated

💡 perform stops because closure throws, so perform rethrows and passes error to caller

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
action	closure passed	called and throws error	error rethrown	error propagated

Key Moments - 2 Insights

Why does the perform function need the 'rethrows' keyword?

Because perform only throws if the closure it calls throws. The execution_table row 3 shows perform rethrowing the error it got from the closure.

What happens if the closure does NOT throw an error?

Then perform does not throw either and returns normally, as shown in execution_table row 2 where Closure Throws? would be No and Function Throws? would be No.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the closure throw an error?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Closure Throws?' column in execution_table row 2.

According to variable_tracker, what is the state of 'action' after Step 3?

A Closure called and error rethrown

B Closure called and completed without error

C Closure called and threw error

D Closure not called yet

💡 Hint

Look at variable_tracker row for 'action' after Step 3.

If the closure did not throw, what would change in the execution_table?

A Function Throws? would be Yes at Step 2

B Result would be 'Error thrown from closure' at Step 2

C Closure Throws? would be No at Step 2

D perform would still rethrow error at Step 3

💡 Hint

Refer to key_moments explanation about closure not throwing and execution_table row 2.

Concept Snapshot

Rethrowing functions use 'rethrows' keyword.
They only throw if the closure they call throws.
Use 'try' when calling the closure inside.
Errors from closure are passed up automatically.
Caller handles or propagates the error.
This avoids unnecessary throws if closure doesn't throw.

Full Transcript

This visual trace shows how a Swift rethrowing function works. The function 'perform' takes a closure that can throw an error. When 'perform' calls the closure, if the closure throws, 'perform' rethrows the error to its caller. The execution table shows each step: calling perform, calling the closure, closure throwing, perform rethrowing, and caller handling. The variable tracker shows the closure state changing from passed to called and throwing, then error rethrown. Key moments clarify why 'rethrows' is needed and what happens if the closure does not throw. The quiz tests understanding of when errors are thrown and how states change. The snapshot summarizes the main points about rethrowing functions in Swift.