If / else

If/else can be used for control flow as below.

(let abs {x}
  (if (< x 0)
    (- 0 x)
    x))

if let

Use if let when you want to test one pattern and fall back to an else branch. The syntax is:

(if let [<pattern> <value>]
  <then-branch>
  <else-branch>)

Example:

(let selector_or_default {initialised subject}
  (if let [(Some selector) (:selector initialised)]
    selector
    (process/select (process/new_selector) subject)))

if let is shorthand for a match with one explicit arm and _ fallback:

(if let [(Some x) maybe]
  x
  0)

;; equivalent to

(match maybe
  (Some x) ~> x
  _ ~> 0)