Janet 1.0.0-dev Documentation


Janet uses the get function to retrieve values from inside data structures. In many cases, however, you do not need the get function at all. Janet supports destructuring, which means both the def and var special forms can extract values from inside structures themselves.

# Before, we might do
(def my-array @[:mary :had :a :little :lamb])
(def lamb (get my-array 4))
(print lamb) # Prints :lamb

# Now, with destructuring,
(def [_ _ _ _ lamb] my-array)
(print lamb) # Again, prints :lamb

# Destructuring works with tables as well
(def person @{:name "Bob Dylan" :age 77})
  {:name person-name
   :age person-age} person)

To update a mutable data structure, use the put function. It takes 3 arguments, the data structure, the key, and the value, and returns the data structure. The allowed types keys and values depend on what data structure is passed in.

(put @[] 100 :a)
(put @{} :key "value")
(put @"" 100 92)

Note that for Arrays and Buffers, putting an index that is outside the length of the data structure will extend the data structure and fill it with nils in the case of the Array, or 0s in the case of the Buffer.

The last generic function for all data structures is the length function. This returns the number of values in a data structure (the number of keys in a dictionary type).