2014-11-12 18:22:02 +00:00
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---
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language: forth
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contributors:
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- ["Horse M.D.", "http://github.com/HorseMD/"]
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filename: learnforth.fs
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---
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Forth was created by Charles H. Moore in the 70s.
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Note: This article focuses predominantly on the Gforth implementation of Forth, but most
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of what is written here should work elsewhere.
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> If Lisp is the ultimate high level language, Forth is the ultimate low level language.
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```forth
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\ Forth is an interactive programming language which is comprised of *words*. These are
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\ Forth subroutines which are executed once you press <Cr>, from left to right.
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\ ------------------------------ Precursor ------------------------------
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\ It's important to know how forth processes instructions. All programming in Forth is
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\ done by manipulating what's known as the parameter stack (more commonly just referred
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\ to as "the stack"). The stack is a typical last-in-first-out (LIFO) stack. Typing:
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5 2 3 56 76 23 65
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\ Means 5 gets put on the stack first, then 2, then 3, etc all the way to 65, which
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\ is now at the top of the stack. We can see the length and contents of the stack by
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\ passing forth the word `.s`:
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.s <7> 5 2 3 56 76 23 65 \ ok
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\ Forth's interpreter interprets what you type in one of two ways: as *words* (i.e. the
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\ name of subroutines) or as *numbers*. Words are essentially "symbols that do things".
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\ Finally, as the stack is LIFO, we obviously must use postfix notation to manipulate
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\ the stack. This should become clear shortly.
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\ ------------------------------ Basic Arithmetic ------------------------------
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\ Lets do a simple equation: adding 5 and 4. In infix notation this would be 5 + 4,
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\ but as forth works in postfix (see above about stack manipulation) we input it like so:
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5 4 + \ ok
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2014-11-13 00:02:44 +00:00
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\ However, this alone yields "ok", yet no answer. Typing the word `.` will yield
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\ the result.
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. \ 9 ok
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2014-11-13 00:02:44 +00:00
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\ This should illustrate how Forth's stack works. Lets do a few more arithmetic tests:
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6 7 * . \ 42 ok
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1360 23 - . \ 1337 ok
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12 12 / . \ 1 ok
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\ And so on.
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2014-11-12 21:56:13 +00:00
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\ ------------------------------ Stack Maniulation ------------------------------
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\ Naturally, as we do so much work with the stack, we'll want some useful methods.
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drop \ drop (remove) the item at the top of the stack (note the difference between this and `.`)
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dup \ duplicate the item on top the stack
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rot \ rotate the top three items (third -> first, first -> second, second -> third)
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swap \ swaps the top item with the second item
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\ Examples:
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dup * \ square the top item
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2 5 dup * swap / \ half the top item squared
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6 4 5 rot * - \ sometimes we just want to reorganize
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4 0 drop 2 / \ add 4 and 0, remove 0 and divide the top by 2
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2014-11-12 21:56:13 +00:00
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\ ------------------------------ More Advanced Stack Manipulation ------------------------------
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tuck \ acts like dup, except it duplicates the top item into the 3rd* position in the stack
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over \ duplicate the second item to the top of the stack
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n roll \ where n is a number, *move* the stack item at that position to the top of the stack
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n pick \ where n is a number, *duplicate* the item at that position to the top of the stack
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2014-11-13 00:02:44 +00:00
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\ When referring to stack indexes, they are zero-based.
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\ ------------------------------ Creating Words ------------------------------
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\ Quite often one will want to write their own words.
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: square ( n -- n ) dup * ; \ ok
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\ Lets break this down. The `:` word says to Forth to enter "compile" mode. After that,
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\ we tell Forth what our word is called - "square". Between the parentheses we have a
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\ comment depicting what this word does to the stack - it takes a number and adds a
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\ number. Finally, we have what the word does, until we reach the `;` word which
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\ says that you've finished your definition, Forth will add this to the dictionary and
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\ switch back into interpret mode.
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\ We can check the definition of a word with the `see` word:
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see square \ dup * ; ok
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\ ------------------------------ Conditionals ------------------------------
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2014-11-12 21:56:13 +00:00
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\ Booleans:
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\ In forth, -1 is used to represent truth, and 0 is used to represent false.
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\ The idea is that -1 is 11111111 in binary, whereas 0 is obviously 0 in binary.
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\ However, any non-zero value is usually treated as being true:
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42 42 = / -1 ok
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12 53 = / 0 ok
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2014-11-13 00:02:44 +00:00
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\ `if` is a *compile-only word*. This means that it can only be used when we're compiling a word.
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\ when creating conditionals, the format is `if` <stuff to do> `then` <rest of program>.
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: ?>64 ( n -- n ) DUP 64 > if ." Greater than 64!" then ; \ ok
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100 ?>64 \ Greater than 64! ok
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2014-11-13 00:02:44 +00:00
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\ Else:
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: ?>64 ( n -- n ) DUP 64 > if ." Greater than 64!" else ." Less than 64!" then ; \ ok
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100 ?>64 \ Greater than 64! ok
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20 ?>64 \ Less than 64! ok
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2014-11-12 18:22:02 +00:00
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\ ------------------------------ Loops ------------------------------
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2014-11-12 22:33:04 +00:00
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\ `do` is like `if` in that it is also a compile-only word, though it uses `loop` as its
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\ terminator:
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: myloop ( -- ) 5 0 do cr ." Hello!" loop ; \ ok
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test
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\ Hello!
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\ Hello!
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\ Hello!
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\ Hello!
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\ Hello! ok
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2014-11-13 00:02:44 +00:00
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\ `do` expects two numbers on the stack: the end number and the index number, respectively.
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2014-11-13 00:02:44 +00:00
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\ Get the value of the index as we loop with `i`:
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: one-to-15 ( -- ) 15 0 do i . loop ; \ ok
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one-to-15 \ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ok
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: squares ( -- ) 10 0 do i DUP * . loop ; \ ok
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squares \ 0 1 4 9 16 25 36 49 64 81 ok
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2014-11-13 00:02:44 +00:00
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\ Change the "step" with `+loop`:
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: threes ( -- ) 15 0 do i . 3 +loop ; \ ok
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threes \ 0 3 6 9 12 ok
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2014-11-13 00:02:44 +00:00
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\ Finally, while loops with `begin` <stuff to do> <flag> `unil`:
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: death ( -- ) begin ." Are we there yet?" 0 until ;
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2014-11-12 23:55:50 +00:00
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\ ------------------------------ Variables and Memory ------------------------------
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2014-11-12 23:55:50 +00:00
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\ Sometimes we'll be in a situation where we want more permanent variables:
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\ First, we use `variable` to declare `age` to be a variable.
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variable age
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2014-11-12 23:55:50 +00:00
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\ Then we write 21 to age with the word `!`.
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21 age !
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\ Finally we can print our variable using the "read" word '@', which adds the value
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\ to the stack, or use a handy word called `?` that reads and prints it in one go.
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age @ . \ 12 ok
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age ? \ 12 ok
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\ What's happening here is that `age` stores the memory address, and we use `!`
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\ and `@` to manipulate it.
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\ Constants are quite simiar, except we don't bother with memory addresses:
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100 constant WATER-BOILING-POINT \ ok
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WATER-BOILING-POINT . \ 100 ok
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\ Arrays!
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\ Set up an array of length 3:
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variable mynumbers 2 cells allot
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\ Initialize all the values to 0
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mynumbers 3 cells erase
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\ (alternatively we could do `0 fill` instead of `erase`, but as we're setting
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\ them to 0 we just use `erase`).
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\ or we can just skip all the above and initialize with specific values:
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create mynumbers 64 , 9001 , 1337 , \ the last `,` is important!
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\ ...which is equivalent to:
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\ [64, 9001, 1337]
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64 mynumbers 0 cells + !
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9001 mynumbers 1 cells + !
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1337 mynumbers 2 cells + !
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\ Reading values at certain array indexes:
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0 cells mynumbers + ? \ 64 ok
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1 cells mynumbers + ? \ 9001 ok
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2 cells mynumbers + ? \ 1337 ok
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\ Of course, you'll probably want to define your own words to manipulate arrays:
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: ?mynumbers ( n -- n ) cells mynumbers + ; \ ok
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64 mynumbers 2 cells + ! \ ok
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2 ?mynumbers ? \ 64 ok
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\ ------------------------------ The Return Stack ------------------------------
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\ TODO
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\ ------------------------------ Final Notes ------------------------------
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\ Floats
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\ Commenting (types)
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\ bye
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```
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##Ready For More?
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* [Starting Forth](http://www.forth.com/starting-forth/)
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* [Thinking Forth](http://thinking-forth.sourceforge.net/)
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