Lisp

Introduction

This is a quick tour on lisp. I decided to look at this because of the build a compiler youtube by lens_r which made me feel I am missing something. This has been rabbit hole which I have fallen done

Concepts

This is my view and maybe wrong but to me the first thing I noticed was the huge amount of parentheses which are involved. It made the language look really ugly and made me feel don't do this. I think I have touched this in the past because of a gun to my head. That said, like people, you probably need to understand before having a view, and even then, not for you is a better view and it is negative. So here is the half day I spent

Comments

;;;; Describe Program 4 semi colons
;;; Comments

;; Indented Comments

#|| Multiline
My Comments
||#

Format to terminal

(format t "Hello World ~%")

Print

(print "Hello World")

Variables

To declare

(defvar *my-variable* "Hello World" )

It is common practice for local variable to have asterixes

We can declare variables for functions too, in this case read-data now is the built-in read function.

(defvar *read-data* (read))

Functions

We can of course define functions. So putting the above together we are prompted for a value and it prints it to the terminal. Not the ~% is the linefeed in list and ~a is a formatter like printf

(defvar *read-data* (read))

(defun read-and-print (name)
  (format t "Read and Print value is ~a ~%" name))

(read-and-print *read-data*)

Changing values of variables

We use setf to change the values of existing variables

(defvar *read-data* (read))
;; You can change a value using setf
(setf *read-data* "Hello World 1")
(read-and-print *read-data*)
(setf *read-data* "Hello World 2")
(read-and-print *read-data*)

Formatting

This is like printf

(format t "PI to 5 characters is ~5f ~%" 3.14159)
(format t "PI to 4 decimal places is ~,4f ~%" 3.14159)

Cells

A Cell, I think is a set of parentheses and we can embed them inside each other. So

(format t "Text with this ~d ~%" (+ 5 4))
; Add a cell within a cell
(format t "Text with this ~d ~%" (+ 5 (+ 4 3)))
;; and again
(format t "Text with this ~d ~%" (+ 5 (+ 4 (+ 100 5))))

Maths Stuff

Just some examples. This is a whirlwind tour not a tutorial

(format t "Modulus of 10/3 is ~d ~%" (mod 10 3))
(format t "Remainder of 10/3 is ~d ~%" (rem 10 3))
;; All work the same way e.g. sqrt
(format t "Square root of 9 is ~d ~%" (sqrt 9))
;; and floor
(format t "Floor of 9.9 is ~d ~%" (floor 9.9))

The blasted quote

I had to read about this because of my stuffed up brain. In lisp there is some referred to as a quote. And they kept mentioning shorthand but they were incredible unclear what for. and it what the function quote. What they meant is ` means quote ()

(format t "Quote is ~a ~%" (+ 1  2)) ; Quote is 3
(format t "Quote is ~a ~%" (quote (+ 1 2))) ; Quote is (+ 1 2) 
(format t "Quote is ~a ~%" `(+ 1 2)) ; Quote is (+ 1 2)

Equality

Equality in Lisp has three approaches

• For symbols we use eq
• For numbers we use =
• For strings we use equal

;; Symbols
(format t "Are 'hello' and 'hello' equal? ~a ~%" (eq 'hello 'hello))

;; Numbers
(format t "Are 1 and 1 equal? ~a ~%" (= 1 1)) ; T
(format t "Are 1 and 2 equal? ~a ~%" (= 1 2)) ; NIL

;; Strings
(format t "Are 'hello' and 'hello' equal? ~a ~%" (equal "hello" "hello"))

We can compare things with the same value but presented differently with equalp

;; equalp is used to compare numbers
(format t "Are 1 and 1.0 equal? ~a ~%" (equalp 1 1.0)) ;T
(format t "Are 1.01 and 1.010 equal? ~a ~%" (equalp 1.01 1.010)) ;T
(format t "Are 1.010 and 1.01 equal? ~a ~%" (equalp 1.010 1.01)) ;T

;; for strings for case insensitive
(format t "Are 'hello' and 'HELLO' equal? ~a ~%" (equalp "hello" "HELLO")) ;T