Add basic outline of Forth and explaination of simple concepts.

forth.html.markdown

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+---
+language: forth
+contributors:
+    - ["Horse M.D.", "http://github.com/HorseMD/"]
+filename: learnforth.fs
+---
+
+Forth was created by Charles H. Moore in the 70s.
+
+Note: This article focuses predominantly on the Gforth implementation of Forth, but most
+of what is written here should work elsewhere. 
+
+> If Lisp is the ultimate high level language, Forth is the ultimate low level language.
+
+```forth
+
+\ Forth is an interactive programming language which is comprised of *words*. These are
+\ Forth subroutines which are executed once you press <Cr>, from left to right.
+
+\ ------------------------------ Precursor ------------------------------
+
+\ It's important to know how forth processes instructions. All programming in Forth is
+\ done by manipulating what's known as the parameter stack (more commonly just referred
+\ to as "the stack"). The stack is a typical last-in-first-out (LIFO) stack. Typing:
+
+5 2 3 56 76 23 65
+
+\ Means 5 gets put on the stack first, then 2, then 3, etc all the way to 65, which
+\ is now at the top of the stack. We can see the length and contents of the stack by
+\ passing forth the word `.s`:
+
+.s <7> 5 2 3 56 76 23 65    \ ok
+
+\ Forth's interpreter interprets what you type in one of two ways: as *words* (i.e. the
+\ name of subroutines) or as *numbers*. Words are essentially "symbols that do things".
+
+\ Finally, as the stack is LIFO, we obviously must use postfix notation to manipulate
+\ the stack. This should become clear shortly.
+
+\ ------------------------------ Basic Arithmetic ------------------------------
+
+\ Lets do a simple equation: adding 5 and 4. In infix notation this would be 5 + 4,
+\ but as forth works in postfix (see above about stack manipulation) we input it like so:
+
+5 4 +    \ ok
+
+\ However, this alone yields "ok", yet no answer. Why? The way forth interprets what
+\ we typed is as such: 5 gets added to the top of the stack, and then 4. Finally,
+\ it runs word `+` on the stack (which pops the top and second value, and adds them),
+\ and inserts the result at the top of the stack. Typing the word `.` will yield
+\ the result.
+
+.    \ 9 ok
+
+\ This should illustrate the fundamentals of forth. Lets do a few more arithmetic
+\ tests:
+
+6 7 * .     \ 42 ok
+1360 23 - . \ 1337 ok
+12 12 / .   \ 1 ok
+
+\ And so on.
+
+\ ------------------------------ More Advanced Stack Maniulation ------------------------------
+
+\ Naturally, as we do so much work with the stack, we'll want some useful methods.
+
+drop \ drop (remove) the item at the top of the stack (note the difference between this and `.`)
+dup  \ duplicate the item on top the stack
+rot  \ rotate the top three items (third -> first, first -> second, second -> third)
+swap \ swaps the top item with the second item
+
+\ Examples:
+
+dup *            \ square the top item
+2 5 dup * swap / \ half the top item squared
+6 4 5 rot * -    \ sometimes we just want to reorganize
+4 0 drop 2 /     \ add 4 and 0, remove 0 and divide the top by 2 
+
+\ ------------------------------ Extra Stack Manipulation ------------------------------
+
+tuck   \ acts like dup, except it duplicates the top item into the 3rd* position in the stack
+over   \ duplicate the second item to the top of the stack
+n roll \ where n is a number, *move* the stack item at that position to the top of the stack
+n pick \ where n is a number, *duplicate* the item at that position to the top of the stack
+
+\ 3rd*: when referring to stack indexes, they are zero-based - i.e. the first element is at
+\ position 0, the second element is at position 1, etc... Just like indexing arrays in
+\ most other languages.
+
+\ ------------------------------ Creating Words ------------------------------
+
+\ Quite often one will want to write their own words.
+
+: square ( n -- n ) dup * ;    \ ok
+
+\ Lets break this down. The `:` word says to Forth to enter "compile" mode. After that,
+\ we tell Forth what our word is called - "square". Between the parentheses we have a
+\ comment depicting what this word does to the stack - it takes a number and adds a
+\ number. Finally, we have what the word does, until we reach the `;` word which
+\ says that you've finished your definition, Forth will add this to the dictionary and
+\ switch back into interpret mode.
+
+\ We can check the definition of a word with the `see` word:
+
+see square     \ dup * ; ok
+
+\ ------------------------------ Conditionals ------------------------------
+
+\ TODO
+
+\ ------------------------------ Loops ------------------------------
+
+\ TODO
+
+\ ------------------------------ The Return Stack ------------------------------
+
+\ TODO
+
+\ ------------------------------ Variables and Memory ------------------------------
+
+\ TODO
+
+\ ------------------------------ Final Notes ------------------------------
+
+\ Booleans
+\ Floats
+\ Commenting (types)
+\ bye
+
+```
+
+##Ready For More?
+
+* [Starting Forth](http://www.forth.com/starting-forth/)
+* [Thinking Forth](http://thinking-forth.sourceforge.net/)