--- 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 , 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. \ ------------------------------ 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 \ ------------------------------ More Advanced 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 ------------------------------ \ Booleans: \ In forth, -1 is used to represent truth, and 0 is used to represent false. \ The idea behind this is that -1 is 11111111 in binary, whereas 0 is obviously 0 in binary. \ However, any non-zero value is usually treated as being true: 42 42 = / -1 ok 12 53 = / 0 ok \ `if` is a *compile-only word*. This means that it can *only* be used when we're compiling a word. \ when creating conditionals, the format is `if` `then` . : ?>64 ( n -- n ) DUP 64 > if ." Greater than 64!" then ; \ ok 100 ?>64 \ Greater than 64! ok \ This unimaginative example displays "Greater than 64!" when the number on the stack is greater \ than 64. However, it does nothing when the test is false. Let's fix that with the `else` word! : ?>64 ( n -- n ) DUP 64 > if ." Greater than 64!" else ." Less than 64!" then ; \ ok 100 ?>64 \ Greater than 64! ok 20 ?>64 \ Less than 64! ok \ As you can see, conditionals behave more or less like they do in most programming languages. \ ------------------------------ Loops ------------------------------ \ `do` is like `if` in that it is also a compile-only word, though it uses `loop` as its \ terminator. : myloop ( -- ) 5 0 do cr ." Hello!" loop ; \ ok test \ Hello! \ Hello! \ Hello! \ Hello! \ Hello! ok \ `do` expects two numbers before it: the end number and the index number, respectively. \ (cr means carraige-return, essentially it a newline). This is equivalent to a for-loop \ in other languages, with a definite number of times to loop. \ So what if we want to get the value of the index as we loop? We use `i`. : one-to-15 ( -- ) 15 0 do i . loop ; \ ok one-to-15 \ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ok : squares ( -- ) 10 0 do i DUP * . loop ; \ ok squares \ 0 1 4 9 16 25 36 49 64 81 ok \ Thidly, we can also change how large the step is between each loop iteration with `+loop`. \ `+loop` reads the number on the top of the stack for how far to move each iteration. : threes ( -- ) 15 0 do i . 3 +loop ; \ ok threes \ 0 3 6 9 12 ok \ Finally, while loops: : death ( -- ) begin ." Are we there yet?" 0 until ; \ Will print "Are we there yet?" forever. While loops are constructed in the format \ of `begin` `until`. The loop will run until flag is a \ truthy value (not 0). \ ------------------------------ Variables and Memory ------------------------------ \ Sometimes we'll be in a situation where we want more permanent variables: \ First, we use `variable` to declare `age` to be a variable. variable age \ Then we write 21 to age with the word `!`. 21 age ! \ Finally we can print our variable using the "read" word '@', which adds the value \ to the stack, or use a handy word called `?` that reads and prints it in one go. age @ . \ 12 ok age ? \ 12 ok \ What's happening here is that `age` stores the memory address, and we use `!` \ and `@` to manipulate it. \ Constants are quite simiar, except we don't bother with memory addresses: 100 constant WATER-BOILING-POINT \ ok WATER-BOILING-POINT . \ 100 ok \ Arrays! \ Set up an array of length 3: variable mynumbers 2 cells allot \ Initialize all the values to 0 mynumbers 3 cells erase \ (alternatively we could do `0 fill` instead of `erase`, but as we're setting \ them to 0 we just use `erase`). \ or we can just skip all the above and initialize with specific values: create mynumbers 64 , 9001 , 1337 , \ the last `,` is important! \ ...which is equivalent to: \ [64, 9001, 1337] 64 mynumbers 0 cells + ! 9001 mynumbers 1 cells + ! 1337 mynumbers 2 cells + ! \ Reading values at certain array indexes: 0 cells mynumbers + ? \ 64 ok 1 cells mynumbers + ? \ 9001 ok 2 cells mynumbers + ? \ 1337 ok \ Of course, you'll probably want to define your own words to manipulate arrays: : ?mynumbers ( n -- n ) cells mynumbers + ; \ ok 64 mynumbers 2 cells + ! \ ok 2 ?mynumbers ? \ 64 ok \ ------------------------------ The Return Stack ------------------------------ \ TODO \ ------------------------------ Final Notes ------------------------------ \ Floats \ Commenting (types) \ bye ``` ##Ready For More? * [Starting Forth](http://www.forth.com/starting-forth/) * [Thinking Forth](http://thinking-forth.sourceforge.net/)