Generic function declaration in Kotlin - Step-by-Step Execution

Concept Flow - Generic function declaration

Start

↓

Declare function with <T>

↓

Use T as parameter type

↓

Use T as return type

↓

Call function with specific type

↓

Function executes with that type

↓

End

This flow shows how a generic function is declared with a type placeholder T, used in parameters and return type, then called with a specific type.

Execution Sample

Kotlin

fun <T> identity(value: T): T {
    return value
}

val result = identity(42)

This code declares a generic function identity that returns the same value it receives, then calls it with an Int.

Execution Table

Step	Action	Evaluation	Result
1	Declare generic function identity with type parameter T	T is a placeholder for any type	Function ready to use with any type
2	Call identity with value 42	T is inferred as Int	Function called with value=42 of type Int
3	Inside function: return value	Return 42	Returns 42
4	Assign return to result	result = 42	result holds Int value 42

💡 Function completes after returning the input value

Variable Tracker

Variable	Start	After Call	Final
value	N/A	42	42
result	N/A	N/A	42

Key Moments - 3 Insights

Why do we write <T> before the function name?

The <T> declares a generic type parameter T that can be used inside the function. This is shown in execution_table step 1 where T is a placeholder.

How does Kotlin know what type T is when calling the function?

Kotlin infers T from the argument type at the call site, as shown in step 2 where T is inferred as Int because 42 is an Int.

Can the function return a different type than the input?

No, because the return type is T, the same as the input parameter type, so it returns the same type it receives (step 3).

Visual Quiz - 3 Questions

Look at the execution_table step 2, what type is T inferred as when calling identity(42)?

A Int

B String

C Any

D Double

💡 Hint

Check the 'Evaluation' column in step 2 where T is inferred as Int.

At which step does the function return the input value?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the step where the function returns the value inside the function.

If we call identity("hello"), how would the variable 'value' change in variable_tracker?

A value would be 42

B value would be "hello"

C value would be null

D value would be Int

💡 Hint

Refer to variable_tracker where 'value' holds the input passed to the function.

Concept Snapshot

Generic function declaration syntax:
fun <T> functionName(param: T): T {
    // use T as type
}

- <T> declares a type parameter T
- T can be used as parameter and return type
- Kotlin infers T from the argument when called
- Enables writing flexible, reusable functions

Full Transcript

This visual trace shows how a generic function is declared and used in Kotlin. First, the function identity is declared with a generic type parameter T. This means T can be any type. When calling identity(42), Kotlin infers T as Int because 42 is an Int. Inside the function, the value parameter has type T, so it is Int here. The function returns the same value it received. The result variable receives the returned value 42. The variable tracker shows how 'value' and 'result' change during execution. Key moments clarify why <T> is needed, how Kotlin infers types, and why the return type matches the input type. The quiz questions test understanding of type inference, return step, and variable values when calling with different types.