> For the complete documentation index, see [llms.txt](https://funkcia.lukemorales.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://funkcia.lukemorales.io/data-types/option/error-propagation.md).
# Error Propagation
Funkcia offers a concise and convenient way to write your code in a more imperative style that utilizes the native scope provided by the generator syntax. This syntax is more linear and resembles normal synchronous code.
Drawing primarily from Rust's `?` operator for error propagation, and inspired by Gleam's [`use` expressions](https://gleam.run/news/v0.25-introducing-use-expressions/), [neverthrow's `safeTry`](https://github.com/supermacro/neverthrow?tab=readme-ov-file#safetry), and [Effect's `gen`](https://effect.website/docs/getting-started/using-generators/#_top) functions, the following functions provide a clean way to handle sequential operations while maintaining proper error handling and type safety.
#### use
Evaluates a generator early returning when an `Option.None` is propagated or returning the `Option` returned by the generator.
* Each `yield*` automatically unwraps the `Option` value or propagates `None`.
* If any operation returns `Option.None`, the entire generator exits early.
import { Option } from 'funkcia';
declare const safeParseInt: (string: string, radix?: number) => Option<number>;
// ┌─── Option<number>
// ▼
const option = Option.use(function* () {
const x: number = yield* safeParseInt('10');
const y: number = yield* safeParseInt('invalid'); // breaks the circuit, returning Option.None
return Option.some(x + y); // doesn't run
});
// Output: None
#### fn
Returns a function that evaluates a generator when called with the declared arguments, early returning when an `Option.None` is propagated or returning the `Option` returned by the generator.
* Each `yield*` automatically unwraps the `Option` value or propagates `None`.
* If any operation returns `Option.None`, the entire generator exits early.
{% code overflow="wrap" %}
```typescript
import { Option } from 'funkcia';
declare const safeParseInt: (string: string, radix?: number) => Option;
// ┌─── (a: string, b: string) => Option
// ▼
const sumParsedIntegers = Option.fn(function* (a: string, b: string) {
const x: number = yield* safeParseInt(a);
const y: number = yield* safeParseInt(b);
return Option.some(x + y);
});
const option = sumParsedIntegers('10', '20');
// Output: Some(30)
```
{% endcode %}
### Understanding the use method
The `use` method provides a way to write sequential operations that might fail, similar to Rust's `?` operator. It lets you write code that looks synchronous while safely handling potential failures.
It essentially creates a "safe context" where you can work with values as if they were guaranteed to exist, while maintaining all the safety guarantees of `Option`. If anything fails, the failure propagates automatically. Like an electronic relay that controls current flow, relay controls computation flow: `Option.Some` continues, `Option.None` breaks the circuit.
Here's a practical example:
import { Option } from 'funkcia';
declare function findUser(id: string): Option<User>;
declare function getUserPermissions(user: User): Option<Permissions>;
declare function checkAccess(permissions: Permissions, resource: string): Option<Access>;
const access = Option.use(function* () {
// First, try to find the user
const user = yield* findUser('user_123');
// If user is found (`findUser` returns `Option.Some(User)`, get their permissions
const permissions = yield* getUserPermissions(user);
// If all steps succeed, we can use the accumulated context to check access to specific resource
return checkAccess(permissions, 'api-key');
});
The equivalent code without `use` would be much more nested:
```typescript
import { Option } from 'funkcia';
declare function findUser(id: string): Option;
declare function getUserPermissions(user: User): Option;
declare function checkAccess(permissions: Permissions, resource: string): Option;
const access = findUser('user_123')
.andThen(user =>
getUserPermissions(user)
.andThen(permissions =>
checkAccess(permissions, 'api-key')
)
);
```
Or with intermediate variables:
{% code fullWidth="false" %}
```typescript
import { Option } from 'funkcia';
declare function findUser(id: string): Option;
declare function getUserPermissions(user: User): Option;
declare function checkAccess(permissions: Permissions, resource: string): Option;
const maybeUser = findUser('user_123');
const maybePermissions = maybeUser.andThen(getUserPermissions);
const access = maybePermissions.andThen(permissions => {
return checkAccess(permissions, 'api-key');
});
```
{% endcode %}
[^1]: user: User
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