Alternative Error Handling Ideas

This document explores "wild" alternatives to the Scoped Catch (Header) proposal, specifically addressing the feedback about Control Flow Inversion ("handle first, compute after") and Variable Access.

Idea 1: The "On Error" Statement (Imperative Registration)

Instead of a static block at the top, on_error is an imperative statement that registers a handler for the current scope from that line forward.

Syntax

function Process(id: string) {
    // 1. Initial handler
    on_error {
        _: NotFound => return null
    }

    let user = db.getUser(id)

    // 2. Register a new handler that HAS ACCESS to 'user'
    on_error {
        e => {
            log.error("Failed processing user", { user: user, error: e })
            throw e
        }
    }

    // This risky operation is covered by the second handler
    risky_operation(user)
}

Pros

• Solves the "Data" Problem: You can define handlers after you have the data they need.
• Linear Flow: You read top-to-bottom.
• Additive: You can drop an on_error line anywhere without re-indenting.

Cons

• "Inscrutable Chain": If you have multiple on_error statements, it can be hard to track which one is active. (Mitigation: on_error could stack or replace? "Defer" semantics?)

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Idea 2: Postfix catch (Expression Level)

Attach error handling directly to the expression that might fail. This is similar to Ruby's rescue modifier or Perl's or die.

Syntax

function Process(text: string) {
    // Simple fallback
    let data = extract(text) catch { return null }

    // Complex handling with block
    let result = risky_call() catch {
        e: Timeout => retry()
        e: AuthError => raise e
    }
}

Pros

• Extremely Local: The handler is right next to the failure.
• No Indentation: Happy path stays on the left.
• Familiar: Similar to .catch() promises in JS.

Cons

• Verbosity: If you have 10 lines of risky code, you need 10 catch clauses (or wrap them in a block, re-introducing nesting).

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Idea 3: Trailing rescue (Implicit Try)

Allow a rescue / catch block at the end of a function or scope. The entire scope is implicitly treated as the "try" block.

Syntax

function Example() {
    let x = 1
    do_thing()

    return x
} catch {
    // Handles errors from the ENTIRE function body above
    e: Error => {
        log(x) // ⚠️ Scoping issue: is 'x' available?
        return 0
    }
}

Pros

• Natural Control Flow: "Do this. If that failed, do this."
• Clean Happy Path: The main logic is unindented at the top.

Cons

• Variable Scoping: Accessing variables defined in the body is tricky (they might not be initialized).
• Distance: The handler might be far away from the error source.

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Idea 4: Guard Clauses / let else

Focus on "ensuring success" rather than "catching failure". Inspired by Swift guard and Rust let else.

Syntax

function Example() {
    // "Ensure this succeeds, otherwise run this block"
    guard let data = risky_call() else {
        return null
    }

    // 'data' is safe to use here
    process(data)
}

Pros

• Happy Path Focus: Emphasizes the successful data flow.
• Early Return: Encourages handling errors immediately and returning.

Cons

• Limited Pattern Matching: Usually only handles "success vs failure", harder to match specific error types (Timeout vs Auth) without more syntax.

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Idea 5: Decorators / Attributes

Move error handling completely out of the function body, into metadata.

Syntax

@[Catch(TimeoutError, return: null)]
@[Catch(AuthError, strategy: "retry", attempts: 3)]
function Example() {
    risky_call()
}

Pros

• Zero Clutter: The function body is pure logic.
• Reusable Policies: Can define standard error policies.

Cons

• Rigid: Hard to do custom logic (logging specific variables, complex recovery).
• "Magic": Hides control flow.

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Idea 6: The "Recover" Expression

A dedicated block for attempting code and recovering.

Syntax

function Example() {
    let val = attempt {
        risky_step_1()
        risky_step_2()
    } recover {
        _: Timeout => null
        e => throw e
    }
}

Pros

• Explicit Scope: Clear what is covered.
• Expression-oriented: Returns a value.

Cons

• Indentation: Back to the try/catch nesting problem.