Snippets

One of the great resources we can have as developers is snippets and example code to understand how the idiom of a language is spoken and written.

Defining a function:

(defun function-name (param1 param2 &key (keyword1 default-value))
    ;; keywords are optional; optional positional parameters are also available. 
    ;; implicit progn
  )

Defining a method on a type:

(defmethod method-name ((param1 class-name))
   ;; implicit progn
  )

Defining a class

Note that DEFCLASS accessor functions for slots can be SETF’d and serve as both getters and setters for the slot.

:INITARG is the keyword used in MAKE-INSTANCE to denote the value of the initial argument (see below).

:INITFORM is the form used to initialize the slot. Without this, it defaults to nil. I favor using nil, 0, "", or (error "You must set slot <slotname> to a value") as the usual initform set.

Note that (:documentation ...) is the standard documentation mechanism, which can be viewed in the running image with DESCRIBE (at least in SBCL).


;; no superclass, no slots.
(defclass superclass-1 nil nil)

(defclass my-class (superclass-1)
  ((variable
      :accessor accessor-function
      :initarg  :variable
      :initform form-for-initializition.)
   another-variable)
  (:documentation "a class snippet!"))

Creating an instance

(make-instance 'my-class :variable value)

Adding a constructor

This function runs after the instance is initialized.

(defmethod initialize-instance :after ((obj my-class) &key)
  (setf (accessor-function obj) 10))

Defining a constant:

Note that the convention is that constants are surrounded with +

(defconstant +name+ value "docstring")

Defining a global variable:

Note that the convention is that globals are surrounded with *

(defparameter *name* value "docstring")

Creating local variables.

(let ((variable1 value-form)
      (variable2 value-again))
  ;; implicit progn where variable[12] are valid
   )

LOOP

LOOP is a contentious form in Common Lisp: some people love its imperative style, others hate it. Regardless, its really handy! Here are my favorite LOOPs


(loop for i from 0 upto 10
   collect i)

(loop for i from 0 below 10
   collect i)

(loop for i from 0 upto 10
   do
      (side-effect i))

(loop for ele in list
   collect
      (operate-on ele))

(loop for ele across array
   collect
      (operate-on ele))

Lambda functions

The lambda functions is an anonymous function, i.e., unnamed.

Here we map over numeric-list and increment each element in it by 1 with 1+, returning the incremented list.


(mapcar
   #'(lambda (x)
       (1+ x))
   numeric-list)

Macro

Note that Lisp macros should be used with care: they read source code and emit source code.

(defmacro with-resource ((resource) &body body)
   (allocate-resource ,resource)
   (unwind-protect
      (progn
         ,@body)
      (deallocate-resource ,resource)))

See UNWIND-PROTECT for details on this very useful form.

Writing a text file

More complete reading and writing of text files can be done by using the ALEXANDRIA library; these routines are great for starting to customize your own routine.

(defun write-file (filename data)
 (with-open-file (stream
                    filename
                    :direction :output
                    :if-exists :supersede
                    :if-does-not-exist :create)
    (write-sequence
         data
         stream)))

Reading a text file


(defun read-file (filename)
  "Reads `filename` as a sequence of unsigned 8-bit bytes, no
encoding"
  (with-open-file (fin filename
                   :direction :input
                   :if-does-not-exist :error)
    (let ((seq (make-array (file-length fin)
                           :fill-pointer t)))
      (setf (fill-pointer seq)
            (read-sequence seq fin))
      seq)))

Please feel free to contribute your examples or library information to this page! Send in those pull requests and file those bugs!


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