2. Data Types and Immutability

Activity 2.01: Creating a Simple In-Memory Database

Solution:

1. First, create the helper functions. You can get the Hash Map by executing the read-db function with no arguments, and write to the database by executing the write-db function with a Hash Map as an argument:

   user=>
   (def memory-db (atom {}))
   #'user/memory-db
   (defn read-db [] @memory-db)
   #'user/read-db
   user=> (defn write-db [new-db] (reset! memory-db new-db))
   #'user/write-db

2. Start by creating the create-table function. This function should take one parameter: the table name. It should add a new key (the table name) at the root of our Hash Map database, and the value should be another Hash Map containing two entries – an empty vector at the data key and an empty Hash Map at the indexes key:

   user=>
   (defn create-table
     [table-name]
     (let [db (read-db)]
       (write-db (assoc db table-name {:data [] :indexes {}}))...