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default

default is used to apply default values, especially when reading data from user-defined inputs. Because of this, default uses a loose definition when determining emptyness by using the empty? function internally. This means values like 0 or "" are considered empty.

default is similar to try, but only returns the fallback value if the value is empty-ish. If an error occurs, default does not fall back to the fallback value, but returns the error instead.

default is a shortcut to writing (if (empty? expr-a) expr-b expr-a)

Examples

  • (default "" "fallback") ➜ "fallback"
  • (default "set" "fallback") ➜ "set"
  • (default (+ "invalid") "fallback") ➜ error

The function is also nice when combined with the bang modifier to apply default values to a variable:

  • (default! $var.foo "default-value")

Forms

(default candidate:any fallback:any) ➜ any

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

default evaluates the candidate expression and returns the evaluated fallback value if the returned candidate value is empty-ish. If the candidate expression returns an error, the error is returned and the fallback expressions is not evaluated.

Context

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