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Design Alternatives

This document explains why we chose Universal Catch over other error handling designs.

Rejected Designs

Why not use Result types like Rust?

function Extract(text: string) -> Result<Resume, Error> { ... }

let result = Extract(text)
match result {
  Ok(resume) => ...
  Err(e) => ...
}

Rejected because:

  • All or Nothing: Result types work best when used consistently across the codebase. Mixing Result-returning functions with throwing functions creates friction at every boundary.
  • Viral Complexity: Changing a return type to Result forces all callers to update their signatures to handle or propagate it.

Why not use classic try/catch blocks?

// Imperative
function ProcessBatch(urls: string[]) -> Resume[] {
  // 1. Hoisting Tax: Declare variable with nullable type
  let aggregator: MetricsAggregator | null = null

  // 2. Indentation Tax: Wrap initialization
  try {
    aggregator = MetricsAggregator.new()
  } catch {
    log.warn("Failed to initialize aggregator")
  }

  let results = []

  for (url in urls) {
    let resume = ExtractResume(url)

    // 3. Safety Tax: Check for null on every use
    if (aggregator != null) {
      aggregator.record(resume)
    }
    results.append(resume)
  }
  return results
}

// Declarative
function Extract(text: string) -> Resume | null {
  try {
    // Confusing: Are we "trying" to define the client?
    client "gpt-4o"
    prompt #"Extract resume from {{ text }}"#
  } catch {
    e: TimeoutError => null
  }
}

Rejected because:

  • Indentation Tax: Wrapping in try re-indents every line, breaking git blame and inflating diffs.
  • Hoisting Tax: Variables declared inside try are not accessible in catch or after the block, forcing declarations to be moved outside.
  • Declarative Incompatibility: try implies sequential execution. Wrapping declarative properties like client and prompt in an imperative block creates a semantic mismatch.

Why not make try an expression (like Kotlin)?

let resume = try {
  Extract(text)
} catch {
  e => null
}

Rejected because:

  • Partial Solution: Solves the variable hoisting issue but doesn't work for declarative code. You can't wrap client and prompt declarations in a try expression.

Why not use function modifiers?

function Extract(text: string) -> Resume try {
  client "gpt-4"
  prompt #"..."#
} catch {
  e => null
}

Rejected because:

  • Syntactic Irregularity: Introduces a special grammar rule that doesn't compose with other constructs.

Why not use wrapper functions?

function Extract(text: string) -> Resume | null {
  try {
    return _ExtractInternal(text)
  } catch {
    return null
  }
}

Rejected because:

  • Boilerplate: Doubles the function count for simple error handling.
  • Tooling Degradation: Breaks the link between the prompt definition and the execution context (e.g., "Prompt Preview" or "Run Function" features).
  • Cognitive Load: Developers must manage and recall two versions of every function.

Why not use checked exceptions like Java?

function Extract(text: string) -> Resume throws TimeoutError, ParseError { ... }

Rejected because:

  • Virality: Adding a new error type to a low-level function forces signature updates to every caller in the stack. In practice, teams declare throws Error everywhere to avoid maintenance, defeating the purpose.

Why pattern matching syntax in catch?

vs catch (e) { if/instanceof }:

// Traditional: bind one variable, discriminate inside
try { Extract(text) }
catch (e) {
  if (e instanceof TimeoutError) { retry() }
  else if (e instanceof ParseError) { return null }
  else { throw e }
}

// Pattern matching: discrimination in the syntax
Extract(text) catch {
  e: TimeoutError => retry()
  e: ParseError => null
}

vs chained catch blocks (Java):

try { Extract(text) }
catch (e: TimeoutError) { retry() }
catch (e: ParseError) { return null }

With chained blocks, catching "everything else" means catch (e) { ... }, which catches panics too. No way to let bugs propagate.

Why untyped patterns exclude Panics:

In match, an untyped pattern matches everything. In catch, an untyped pattern like e matches all Errors but not Panics. Bugs crash loudly by default. To catch panics: p: Panic => ....

Why implicit re-throw:

Unhandled cases propagate automatically. Start by handling one error type, add more as you harden. No else { throw e } boilerplate.

Trade-off: log + rethrow is slightly more verbose:

// Traditional catch (e) - concise for log + rethrow
catch (e) {
  log(e)
  throw e
}

// Pattern matching - requires a block
catch {
  e => {
    log(e)
    throw e
  }
}

But without pattern matching, you can't distinguish Errors from Panics:

// Traditional: catches EVERYTHING, including bugs
catch (e) {
  return defaultValue  // Swallows IndexOutOfBounds, AssertionError...
}

// Pattern matching: only catches recoverable errors
catch {
  e => defaultValue
  // IndexOutOfBounds, AssertionError crash immediately - not caught
}

// To handle panics too, be explicit:
catch {
  p: Panic => {
    log.fatal(p)
    throw p
  }
  e => defaultValue
}

The slight verbosity is the cost of having untyped patterns mean "all errors except bugs."