improve readme

README.md

@@ -2,9 +2,13 @@
 
 MadLisp is a [Lisp](https://en.wikipedia.org/wiki/Lisp_%28programming_language%29) interpreter written in PHP. It is inspired by the [Make a Lisp](https://github.com/kanaka/mal) project, but does not follow that convention or syntax strictly.
 
+## Features
+
+The implementation is pretty minimalistic, but there is a good collection of built-in functions. Also [tail call optimization](https://en.wikipedia.org/wiki/Tail_call) is included.
+
 ## Requirements
 
-The project does use syntax of PHP 7.4 so that version or newer is required.
+The project requires PHP 7.4 or newer.
 
 The project does not have any dependencies to external [Composer](https://getcomposer.org/) libraries, but it does use Composer for the autoloader so you need to run **composer install** for that.
 
@@ -29,16 +33,100 @@ Or you can evaluate a file with the -f switch:
 $ php run.php -f file.mad
 ```
 
+## Types
+
+### Numbers
+
+Numeric literals are interpreted as integer or floating point values. For example `1` or `1.0`.
+
+### Strings
+
+Strings are limited by double quotes, for example `"this is a string"`.
+
+### Comments
+
+Comments start with semicolon `;` and end on a newline character.
+
+### Keywords
+
+Special keywords are `true`, `false` and `null` which correspond to same PHP values.
+
+### Sequences
+
+Lists are limited by parenthesis. When they are evaluated, the first item of a list is called as a function with the remaining items as arguments. They can be defined using the built-in `list` function:
+
+```
+> (list 1 2 3)
+(1 2 3)
+```
+
+Vectors are defined using square brackets or the built-in `vector` function:
+
+```
+> [1 2 3]
+[1 2 3]
+
+(vector 4 5 6)
+[4 5 6]
+```
+
+Internally lists and vectors are just PHP arrays, and the only difference is how they are evaluated.
+
+### Hash maps
+
+Hash maps are collections of key-value pairs. Keys are normal strings, not "keywords" starting with colon characters as in many Lisp languages.
+
+Hash maps are defined using curly brackets or using the built-in `hash` function. Odd arguments are treated as keys and even arguments are treated as values. The key-value pair can optionally include colon as a separator to make it more readable, but it is ignored internally.
+
+```
+> (hash "a" 1 "b" 2)
+{"a":1 "b":2}
+
+> {"key":"value"}
+{"key":"value"}
+```
+
+Internally hash maps are just regular associative PHP arrays.
+
+### Symbols
+
+Symbols are words which do not match any other type and are separated by whitespace. They can contain special characters. Examples of symbols are `a`, `name` or `+`.
+
+## Quoting
+
+The special single quote character can be used to quote an expression (skip evaluation).
+
+```
+> '(1 2 3)
+(1 2 3)
+```
+
 ## Special forms
 
+Name  | Example | Example result | Description
+----- | ------- | -------------- | -----------
+and   | `(and 1 0 2)` | `0` | Return the first value that is false, or the last value.
+case  | `(case (= 1 0) 0 (= 1 1) 1)` | `1` | Treat odd arguments as tests and even arguments as values. Evaluate and return the value after the first test that evalutes to true.
+      | `(case (= 1 0) 0 1)` | `1` | You can also give optional last argument to case which is returned if none of the tests evaluated to true.
+def   | `(def addOne (fn (a) (+ a 1)))` | `<function>` | Define a value in the current environment.
+do    | `(do (print 1) 2)` | `12` | Evaluate multiple expressions and return the value of the last.
+env   | `(env +)` | `<function>` | Return a definition from the current environment represented by argument. Without arguments return the current environment as a hash-map.
+eval  | `(eval (quote (+ 1 2)))` | `3` | Evaluate the argument.
+fn    | `(fn (a b) (+ a b))` | `<function>` | Create a function.
+if    |
+let
+load
+or
+quote |
+
 ## Core functions
 
 Name  | Example | Description
 ----- | ------- | -----------
-doc   | (doc +) | Show description of a built-in function.
-read  | (read "(+ 1 2 3)") | Read a string as code and return the expression.
-print | (print "hello world") | Print expression on the screen.
-error | (error "invalid value") | Throw an exception with message as argument.
+doc   | `(doc +)` | Show description of a built-in function.
+read  | `(read "(+ 1 2 3)")` | Read a string as code and return the expression.
+print | `(print "hello world")` | Print expression on the screen.
+error | `(error "invalid value")` | Throw an exception with message as argument.
 
 ## License