# Generic structs and aliases (https://docs-dmpho5eos-ton-core-docs.vercel.app/llms/tolk/types/generics/content.md)



Generic structs and type aliases exist only at the type level and incur no runtime cost.

```tolk
struct Container<T> {
    element: T?
}

struct Nothing

type Wrapper<T> = Nothing | Container<T>
```

Example:

```tolk
fun checkElement<T>(c: Container<T>) {
    return c.element != null;
}

fun demo() {
    var c: Container<int32> = { element: null };

    var n: Wrapper<int> = Nothing {};
    var i: Wrapper<int32> = Container { element: 0 };
}
```

## Type arguments [#type-arguments]

When referencing a generic `Container`, `T` must typically be provided:

```tolk
fun getItem(c: Container)        // error
fun getItem(c: Container<int>)   // ok
fun getItem<T>(c: Container<T>)  // ok
```

For generic functions, type arguments can be inferred from the call site:

```tolk
fun doSmth<T>(value: Container<T>) {
    // ...
}

fun demo() {
    doSmth({ element: 123 });         // T = int
    doSmth({ element: cellOrNull });  // T = cell?
}
```

When calling a method on a generic type, type arguments may be omitted if they can be inferred:

```tolk
type Maybe<T> = MaybeJust<T> | MaybeNothing

fun Maybe<T>.none(): Maybe<T> {
    return MaybeNothing {}
}

fun demo() {
    // `Maybe.none()` is enough — identical to `Maybe<address>.none()`
    var a_none: Maybe<address> = Maybe.none();
}
```

## Default type arguments [#default-type-arguments]

A type parameter can have a default type: `<T = xxx>`. Such type parameters may be omitted at use sites.

```tolk
struct StrangeUnion<T1, T2 = null> {
    item: T1 | T2
}

fun demo() {
    var m1: StrangeUnion<int> = { item: 10 };
    var m2: StrangeUnion<int, bool> = { item: 20 };
    // m1.item is `int?`
    // m2.item is `int | bool`
}
```

If all type parameters have defaults, they can be omitted:

```tolk
struct ComplexMsg<TBody = int32> {
    // ...
}

fun demo(m: ComplexMsg) {
    // m is `ComplexMsg<int32>`
}
```

Type arguments cannot reference one another; expressions such as `T2 = T1` are invalid.

## Type aliases [#type-aliases]

Generic parameters are supported not only by structs, but also by type aliases. The following example defines a generic type alias `Response<TResult, TError>`:

```tolk
struct Ok<TResult> { result: TResult }
struct Err<TError> { err: TError }

type Response<R, E> = Ok<R> | Err<E>

fun loadNextRef(mutate s: slice): Response<cell, int32> {
    return s.remainingRefsCount()
        ? Ok { result: s.loadRef() }
        : Err { err: ERR_NO_MORE_REFS }
}

fun demo() {
    match (val r = loadNextRef(mutate parsedSlice)) {
        Ok => { r.result }    // cell
        Err => { r.err }      // int32
    }
}
```

## Methods [#methods]

Methods for generics are declared exactly as for regular structures. In this form, the compiler treats `T`, which is unknown symbol, as a type parameter:

```tolk
fun Container<T>.getElement(self) {
    return self.element
}
```

Without `self` parameter, a [method](/llms/languages/tolk/syntax/functions-methods/content.md) will be static.