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This is the most comprehensive library for making hash tables I've already seen! And it has wonderful documentation with lots of examples!

make-hash allows to create hash tables in multiple ways, from different kinds of data structures and even using functions for data transformation. For example, you can create a hash by reading rows from the database.

I'll show you only a few examples I especially liked.

First one is creation hash from a sequence while counting each item. Using this, we can easily count how many times each character is used in a text:

POFTHEDAY> (make-hash:make-hash
            :init-format :keybag
            :initial-contents "Alice loves Bob")
#<HASH-TABLE :TEST EQL :COUNT 11 {1008943083}>

POFTHEDAY> (rutils:print-hash-table *)
  #\A 1
  #\l 2
  #\i 1
  #\c 1
  #\e 2
  #\  2
  #\o 2
  #\v 1
  #\s 1
  #\B 1
  #\b 1

In the next example, we'll make a smaller hash table from another one while selecting data by keys:

POFTHEDAY> (let ((full-data
                    '(:foo 1
                      :bar 2
                      :bazz 3
                      :blah 4
                      :minor 5))))
              :init-format :keys
              :init-data full-data
              :initial-contents '(:bar :minor)))
#<HASH-TABLE :TEST EQL :COUNT 2 {10060F6123}>

POFTHEDAY> (rutils:print-hash-table *)
   :BAR 2
   :MINOR 5

And here is how we can build a hash from a data returned by a function. We only need a closure which will return rows of data as values and will return nil at the end.

POFTHEDAY> (defun make-rows-iterator ()
             ;; This list will allow us to simulate
             ;; the data storage:
             (let ((rows '((bob 42)
                           (alice 25)
                           (mike 30)
                           (julia 27))))
               (lambda ()
                 (let ((row (car rows)))
                   (setf rows
                         (cdr rows))
                   (values (first row) ;; This is a key
                           (second row))))))

POFTHEDAY> (make-hash:make-hash
            :init-format :function
            :initial-contents (make-rows-iterator))

POFTHEDAY> (rutils:print-hash-table *)
  BOB 42
  ALICE 25
  MIKE 30
  JULIA 27

make-hash also provides a configurable reader macro:

(install-hash-reader ())  ; default settings and options
#{:a 1 :b 2 :c 3 :d 4}   

(install-hash-reader '(:init-format :pairs)
  :use-dispatch t
  :open-char #\[ :close-char #\])
#['(:a . 1) '(:b . 2) '(:c . 3) '(:d . 4)] 

(install-hash-reader '(:init-format :lists)
  :use-dispatch nil
  :open-char #\{ :close-char #\})
{'(:a 1) '(:b 2) '(:c 3) '(:d 4)}

You will find more examples and instructions on how to define your own initialization formats in the library's documentation:

Let's thank the #poftheday challenge for the chance to discover such cool Common Lisp library!

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