cl-hash-util

2017-08-30

cl-hash-util

cl-hash-util is a very basic library for dealing with CL's hash tables. The idea was spawned through working with enough JSON APIs and config files, causing a lot of headaches in the process. For instance, to get a value deep within a hash, you have to do:

(gethash "city" (gethash "location" (gethash "user" obj)))

I find the inside-out approach unintuitive, regardless of how many lisp nerds are going to yell at me about how it's correct.

With cl-hash-util, you can write:

(hash-get obj '("user" "location" "city"))

hash-get can also deal with getting elements out of lists and arrays:

(hash-get obj '("user" "friends" 0 "name"))

which normally would have to be written as such:

(gethash "name" (elt (gethash "friends" (gethash "user" obj)) 0))

...uuuugly.

cl-hash-util also provides an easy way to build hash tables on the fly. Where you'd normally have to do something like:

(let ((myhash (make-hash-table :test #'equal)))
  (setf (gethash "name" myhash) "andrew")
  (setf (gethash "location" myhash) "santa cruz")
  myhash)

You can now do:

;; functional version
(hash-create '(("name" "andrew") ("location" "santa cruz")))

;; convenience macro `hash`
(hash ("name" "andrew") ("location" "santa cruz"))

You can also do nested hashes:

(hash ("name" "andrew")
      ("location" (hash ("city" "santa cruz")
                        ("state" "CA"))))

This saves a lot of typing =].

With-keys

With-keys is the hash table equivalent of with-slots.

(defvar ht (hash ("name" "andrew") ("location" "santa cruz")))
(with-keys
   ("name" (loc "location"))
    ht
    (setf loc (string-upcase loc))
  (format nil "Hi, ~a in ~a!" name loc))
"Hi, andrew in SANTA CRUZ!"

The first parameter is a list of keys. With-keys will reference the keys in the hash table provided as the second parameter. With-keys will attempt to convert each key into a symbol in the current package, binding the hash table value to it during body execution. String keys are upcased before conversion to symbols.

If you don't want with-keys to guess at a symbol, supply a list - (symbol key) - in place of the key, as in (loc "location") above.

Collecting-hash-table

A collection macro that builds and outputs a hash table. To add to the hash table, call the collect function with a key and a value from within the scope of the collecting-hash-table macro. The value will be inserted or combined with existing values according to the specified accumulation mode.

This code collects words into bins based on their length:

(collecting-hash-table (:mode :append)
  (dotimes (i 10)
    (let ((word (format nil "~r" i)))
      (collect (length word) word)))

Result: <hash table: 5 => ("three" "seven" "eight") 3 => ("one" "two" "six") 4 => ("zero" "four" "five" "nine")>

The mode can be set in the parameters section of collecting-hash-table with the :mode keyword. The :mode keyword can also be passed to individual collect calls.

Keyword parameters:

:test - Test function parameter passed to make-hash-table when creating a new hash table

:existing - Pass an existing hash table to the macro for modification. Using this option at the same time as :test will result in an error.

:mode - Set the default mode for the collect function.

Modes

:replace - Acts the same as (setf (gethash key ht) value), replacing any existing value with the new one.

:keep - Only inserts the value if the key did not previously exist.

:tally - Ignores the input value, instead adding 1 to the key value.

:sum - Adds the input value to the key value. Input should be numeric.

:append - Appends the value to the list that is presumed to be under the key. If the key doesn't yet exist, places the value in a new list.

:push - Like append, but sticks things on the other end.

:concatenate - Assumes that both new and existing values are lists, storing the concatenation of them under the key.

Obviously, not all modes are compatible with each other. Collecting-hash-table makes no attempt to save you from intermingling them.

Custom modes may be created by supplying a function instead of a mode descriptor. This function will be applied in a reduce-like fashion: when a value already exists under a key, the existing value and the new value will be passed to the supplied function. Its return value will be stored under the key.

If more flexibility is needed, then a list of two functions can be supplied. The first function should accept two parameters: first, the existing value; second the new value. It will be called when a key already exists. The second function should take one parameter. It is called when a key does not exist yet. In both cases the key value is set to the function return value.

Conversion functions

Included is a suite of functions for converting between hash tables, alists and plists: alist->plist, plist->alist, alist->hash, plist->hash, hash->alist, and hash->plist.

The alist->hash and plist->hash take the same :existing and :mode keywords that collecting-hash-table takes.

Author
Andrew Danger Lyon <orthecreedence@gmail.com>
License
MIT