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fallback

Provide a default value to use when a module call fails.

Quick Reference​

ParameterTypeDescription
fallbackExpressionDefault value returned on failure

Syntax:

result = Module(input) with fallback: defaultValue

Syntax​

result = Module(args) with fallback: <expression>

Type: Expression (must match module's return type)

Description​

The fallback option specifies a value to return when a module call fails, instead of propagating the error. When combined with retry, the fallback is used only after all retry attempts are exhausted.

The fallback expression must have the same type as the module's return type. This is validated at compile time.

Lazy evaluation

The fallback expression is only evaluated if the module fails. If the module succeeds, the fallback is never computed. This allows expensive fallback expressions without performance penalty on success.

Use for graceful degradation

Fallback is ideal for providing degraded but functional behavior (e.g., cached data, default config). For error visibility, combine with on_error: log to ensure failures are still recorded.

Examples​

Simple Fallback Value​

config = GetConfig(key) with fallback: "default"

If GetConfig fails, return "default" instead.

Fallback with Retry​

price = GetPrice(symbol) with retry: 3, fallback: 0.0

Try up to 4 times, then return 0.0 if all attempts fail.

Record Fallback​

user = LoadUser(id) with fallback: { name: "Unknown", id: 0 }

Return a default user record on failure.

Fallback with Another Call​

primary = PrimaryService(req) with fallback: BackupService(req)

Fall back to a secondary service on failure.

Conditional Fallback​

in useCache: Boolean

data = FetchData(id) with fallback: when useCache then cachedData else emptyData

Use conditional logic in fallback expressions.

Behavior​

  1. Execute the module (with retries if configured)
  2. If successful, return the result
  3. If all attempts fail:
    • Evaluate the fallback expression
    • Return the fallback value

Note: The fallback expression is only evaluated if needed (lazy evaluation).

Type Checking​

The fallback expression is type-checked against the module's return type:

# Valid - fallback matches return type (Int)
count = CountItems(list) with fallback: 0

# Invalid - type mismatch (String vs Int)
count = CountItems(list) with fallback: "none"  # Compile error!
  • retry - Retry before using fallback
  • timeout - Timeout before using fallback
  • on_error - Alternative error handling

Best Practices​

  • Provide meaningful default values
  • Consider the downstream impact of fallback values
  • Use fallback for graceful degradation, not error suppression
  • Log errors separately when using fallback (consider on_error: log)