152 lines
5.0 KiB
Markdown
152 lines
5.0 KiB
Markdown
# wisp
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A light lisp written in C++
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## Why write a lisp?
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Lisp is one of those niche, beautiful languages that people only really use to either
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1. Write a lisp interpreter
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2. Show off how "code is data!!!"
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_So why add to the list of infinite lisp interpreters?_
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The answer is simple: _**I'm bored out of my mind in quarantine.**_ If you were looking to find out why _this particular_ lisp is special, you're fresh out of luck.
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## Syntax and Special Forms
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Like every other lisp, this language uses s-expressions for code syntax and data syntax. So, for example, the s-expression `(print 5)` is both a valid code snippet, and a valid list containing the items `print` and `5`.
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When the data `(print 5)` is evaluated by the interpreter, it evaluates `print` and `5`, and then applies `print` to `5`.
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Here's the result.
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```lisp
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>>> (print 5)
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5
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=> 5
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```
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That's super cool! But what if we want to define our own functions? _We can use the builtin function `defun`!_
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```lisp
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; define a function `fact` that takes an argument `n`
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(defun fact (n)
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(if (<= n 1)
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1
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(* n (fact (- n 1)))
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))
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```
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Thats awesome! But did you notice anything different about the `defun` function? _It doesn't evaluate its arguments._ If the atom `fact` were evaluated, it would throw an error like so:
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```lisp
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>>> fact
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error: the expression `fact` failed in scope { } with message "atom not defined"
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```
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This is known as a special form, where certain functions "quote" their arguments. We can quote things ourselves too, but the language _automatically_ quotes arguments to special forms itself.
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If you want to "quote" a value yourself, you can do it like this.
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```lisp
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; quote the s-expression (1 2 3) so it's not evaluated
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>>> (print '(1 2 3))
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(1 2 3)
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=> (1 2 3)
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```
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As you can see, quote negates an evaluation. For example, whenever the expression `''a` is evaluated, it becomes `'a`. This can be useful for when you want to write long lists of data or variable names without wanting to evaluate them as code.
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|Special Form|Argument Evaluations|Purpose|
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|`(if cond a b)`|`if` only evaluates its `cond` argument. If `cond` is truthy (non-zero), then `a` is evaluated. Otherwise, `b` is evaluated.|This special form is the main method of control flow.|
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|`(do a b c ...)`|`do` takes a list of s-expressions and evaluates them in the order they were given (in the current scope), and then returns the result of the last s-expression.|This special form allows lambda functions to have multi-step bodies.|
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|`(scope a b c ...)`|`scope` takes a list of s-expressions and evaluates them in the order they were given _in a new scope_, and then returns the result of the last s-expression.|This special form allows the user to evaluate blocks of code in new scopes.|
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|`(defun name params body)`|`defun` evaluates none of its arguments.|This special form allows the user to conveniently define functions.|
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|`(define name value)`|`define` evaluates the `value` argument, which is then assigned to `name` in the current scope.|This special form allows the user to bind atoms to values in a scope.|
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|`(lambda params body)`|`lambda` evaluates none of its arguments.|This special form allows the user to define anonymous functions.|
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|`(quote x)`|`quote` evaluates none of its arguments.|This is equivalent to the `'expr` syntactic sugar.|
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|`(for x list ...)`|`for` evaluates only its list argument.|`for` iterates through the list storing each element in `x`, and then evaluating all of the rest of the values in the `for` body. It then returns the last value evaluated.|
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|`(while cond ...)`|`while` evaluates only its cond argument.|`while` evaluates its condition expression every iteration before running. If it is true, it continues to evaluate every expression in the `while` body. It then returns the last value evaluated.|
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## Examples
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Here are some example math-y functions to wrap your head around.
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```lisp
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; quicksort
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(defun qs (l)
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(if (<= (len l) 1)
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l
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(do
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(define pivot (first l))
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(+
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(qs (filter (lambda (n) (> pivot n)) l))
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(list pivot)
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(qs (tail (filter (lambda (n) (<= pivot n)) l)))
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))
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))
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; decrement a number
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(defun dec (n) (- n 1))
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; increment a number
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(defun inc (n) (+ n 1))
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; not a bool
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(defun not (x) (if x 0 1))
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; negate a number
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(defun neg (n) (- 0 n))
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; is a number positive?
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(defun is-pos? (n) (> n 0))
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; is a number negative?
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(defun is-neg? (n) (< n 0))
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```
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## Usage
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Using and compiling wisp
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#### Dependencies
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Compile with your C++ compiler of choice. This is compatible with all standard versions of C++ since ANSI C++.
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```bash
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$ git clone https://github.com/adam-mcdaniel/wisp
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$ cd wisp
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$ g++ wisp.cpp -o wisp
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```
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#### Using the binary
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Run wisp in interactive mode:
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```bash
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$ ./wisp
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>>> (print "Hello world!")
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Hello world!
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=> "Hello world!"
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```
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Interpret a file:
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```bash
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$ ./wisp -f "examples/hello_world.lisp"
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Hello world!
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```
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Interpret from command line argument:
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```bash
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$ ./wisp -c '(print "Hello world!")'
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Hello world!
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```
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source from :
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https://github.com/adam-mcdaniel/wisp
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