Invalid conversion from throwing function

The “Invalid conversion from throwing function” error means you are trying to pass a throwing function (one marked with throws) to a context that expects a non-throwing function. Swift requires explicit handling of all throwing calls.

What It Means

Swift functions can be marked with throws to indicate they may produce an error. When you use a throwing function as a value — assigning it to a variable, passing it as a closure argument, or returning it — the receiving type must also be throwing. The compiler prevents you from silently ignoring the error-handling requirement.

Why It Happens

• Passing a throwing closure to a non-throwing parameter — a function expects () -> Void but you pass { try risky() }.
• Assigning a throwing function to a non-throwing variable — let fn: () -> Void = throwingFunction.
• Returning a throwing function from a non-throwing function — your function return type is () -> Void but you return a throwing closure.
• Using try inside a non-throwing closure — closures passed to higher-order functions like map, filter, forEach cannot throw unless the closure parameter is marked throws.
• Combining throwing and non-throwing in a collection — mixing throwing and non-throwing closures in an array or dictionary.

How to Fix It

1. Mark the enclosing function as throws

// ❌ Non-throwing function contains try
func processData() {
    let result = try JSONDecoder().decode(Model.self, from: data)
}

// ✅ Propagate the error
func processData() throws {
    let result = try JSONDecoder().decode(Model.self, from: data)
}

2. Use try? to convert the result to an optional

// ✅ Converts throwing call to optional (nil on error)
if let result = try? JSONDecoder().decode(Model.self, from: data) {
    // use result
}

3. Use try! when you guarantee no error

// ✅ Only when you are certain the call will not fail
let result = try! JSONDecoder().decode(Model.self, from: guaranteedValidData)

4. Handle the error with do-catch

do {
    let result = try JSONDecoder().decode(Model.self, from: data)
} catch {
    print("Decoding failed: \(error)")
}

5. Use rethrows for higher-order functions

If you are writing a function that accepts a throwing closure:

func mapValues<T>(_ transform: (Int) throws -> T) rethrows -> [T] {
    // rethrows means this function only throws if transform throws
}