# try_call · Rell standard library · Chromia Docs (concept)

> function try_ call(fn: () -> unit ): boolean Safely call a function that may fail (i.e. that may throw an exception). Accepts 0-ary unit-typed function ref

Source: https://docs.chromia.com/pages/rell-stdlib/-rell-system-library/root/try_call/

FUNCTION

# try\_call

```signature
function try_call(fn: () -> unit): boolean
```

Safely call a function that may fail (i.e. that may throw an exception).

Accepts 0-ary unit-typed function references, i.e. references to functions of type `() -> unit`.

Exceptions thrown during the call are caught and logged with a stack trace.

Changes to the database that occur during the call are rolled back when an exception is thrown.

Examples:

Rell

```
function fails(): unit {
    list<integer>()[1]; // out of bounds
}
function succeeds(): unit {}
try_call(fails(*)) // logs an out of bounds exception message and returns false
try_call(succeeds(*)) // logs nothing, returns true
// `fn` can also be written as a lambda:
try_call(() -> { list<integer>()[1]; }) // logs an out of bounds exception message and returns false
try_call(() -> {}) // logs nothing, returns true
```

**Parameters**

* **fn** — the function to call

**Returns**

`true` if call returns without throwing any exceptions, `false` otherwise

Since 0.13.0

---

```signature
function try_call<T>(fn: () -> T): T?
```

Safely call a function that may fail (i.e. that may throw an exception).

Accepts 0-ary function references, i.e. references to functions of type `() -> T`, and returns `T?`,
i.e. the return value of the reference function, or `null`.

Exceptions thrown during the call are caught and logged with a stack trace.

Changes to the database that occur during the call are rolled back when an exception is thrown.

Examples:

Rell

```
function fails(): integer {
    return list<integer>()[1]; // out of bounds
}
function succeeds(): unit { return 0; }
try_call(fails(*)) // logs an out of bounds exception message and returns null
try_call(succeeds(*)) // logs nothing, returns 0
// `fn` can also be written as a lambda:
try_call(() -> list<integer>()[1]) // logs an out of bounds exception message and returns null
try_call(() -> 17) // logs nothing, returns 17
```

**Parameters**

* **fn** — the function to be called

**Returns**

the return value of `fn` if the call returns without throwing any exceptions, `null` otherwise

Since 0.13.0

---

```signature
function try_call<T>(fn: () -> T, default: T): T
```

Safely call a function that may fail (i.e. that may throw an exception).

Accepts 0-ary function references, i.e. references to functions of type `() -> T`, and a default
value to return if the call fails.

Exceptions thrown during the call are caught and logged with a stack trace.

Changes to the database that occur during the call are rolled back when an exception is thrown.

Examples:

Rell

```
function fails(): integer {
    return list<integer>()[1]; // out of bounds
}
function succeeds(): unit { return 0; }
try_call(fails(*), 17) // logs an out of bounds exception message and returns 17
try_call(succeeds(*), 17) // logs nothing, returns 0
// `fn` can also be written as a lambda:
try_call(() -> list<integer>()[1], 17) // logs an out of bounds exception message and returns 17
try_call(() -> 42, 17) // logs nothing, returns 42
```

**Parameters**

* **fn** — the function to be called
* **default** — the default value

**Returns**

the return value of `fn` if the call returns without throwing any exceptions, `default`
otherwise

Since 0.13.0

[View this page on docs.chromia.com](https://docs.chromia.com/pages/rell-stdlib/-rell%20-system%20-library/%5Broot%5D/try_call.html)Generated API reference, mirrored from the live docs