predicate-specifications.md

Predicate functions

A predicate is a function that returns a true or false value and their names end with ? by convention.

(odd? 1)

(string? "am i a string")

(int? 2.3)

(int? 2.3)

Hint::clojure.core predicate functions

There are 80+ predicate functions in clojure.core

Using predicate functions as specifications

Predicate functions can be used as un-named specifications to test values conform.

Include the clojure.spec.alpha namespace to access the spec functions.

(require '[clojure.spec.alpha :as spec])

(spec/conform int? 42)

(spec/conform seq? (range 4))

Custom predicate functions

Define custom predicate functions with defn or fn or the short form #()

Using an anonymous function

(spec/conform (fn [value] (= value 42)) 42)

When the expression is quite terse, then the short form of an anonymous function is typically used. The % represents the value passed as an argument.

(spec/conform #(= % 42) 42)