try.md

try

try is used to catch errors in expressions. It will evaluate an expression and return its return value, unless the expression errors out, then a fallback value is returned.

try is similar to default, but only returns the fallback value if an error occurs, compared to default which tests for empty-ishness.

Examples

• (try (+ 1 2) "fallback") ➜ 3 (no error occurred in +)
• (try (+ 1 "invalid") "fallback") ➜ "fallback"
• (try (+ 1 "invalid") (+ "also invalid")) ➜ error

Forms

(try candidate:expression) ➜ any

This is equivalent to (try candidate null).

(try candidate:expression fallback:expression) ➜ any

• candidate is an arbitrary expression.
• fallback is an arbitrary expression.

try evaluates the candidate expression and returns its return value upon success. However when the candidate return an error, the fallback expression is evaluated and its return value (or error) are returned.

Context

try executes candidate and fallback in their own scopes, so variables from either expression are not visible in the other and neither leak outside of try.