--- language: Forth contributors: - ["Horse M.D.", "http://github.com/HorseMD/"] filename: learnforth.fs --- Forth was created by Charles H. Moore in the 70s. It is an imperative, stack-based language and programming environment, being used in projects such as Open Firmware. It's also used by NASA. Note: This article focuses predominantly on the Gforth implementation of Forth, but most of what is written here should work elsewhere. ```forth \ This is a comment ( This is also a comment but it's only used when defining words ) \ --------------------------------- Precursor ---------------------------------- \ All programming in Forth is done by manipulating the parameter stack (more \ commonly just referred to as "the stack"). 5 2 3 56 76 23 65 \ ok \ Those numbers get added to the stack, from left to right. .s \ <7> 5 2 3 56 76 23 65 ok \ In Forth, everything is either a word or a number. \ ------------------------------ Basic Arithmetic ------------------------------ \ Arithmetic (in fact most words requiring data) works by manipulating data on \ the stack. 5 4 + \ ok \ `.` pops the top result from the stack: . \ 9 ok \ More examples of arithmetic: 6 7 * . \ 42 ok 1360 23 - . \ 1337 ok 12 12 / . \ 1 ok 13 2 mod . \ 1 ok 99 negate . \ -99 ok -99 abs . \ 99 ok 52 23 max . \ 52 ok 52 23 min . \ 23 ok \ ----------------------------- Stack Manipulation ----------------------------- \ Naturally, as we work with the stack, we'll want some useful methods: 3 dup - \ duplicate the top item (1st now equals 2nd): 3 - 3 2 5 swap / \ swap the top with the second element: 5 / 2 6 4 5 rot .s \ rotate the top 3 elements: 4 5 6 4 0 drop 2 / \ remove the top item (don't print to screen): 4 / 2 1 2 3 nip .s \ remove the second item (similar to drop): 1 3 \ ---------------------- More Advanced Stack Manipulation ---------------------- 1 2 3 4 tuck \ duplicate the top item below the second slot: 1 2 4 3 4 ok 1 2 3 4 over \ duplicate the second item to the top: 1 2 3 4 3 ok 1 2 3 4 2 roll \ *move* the item at that position to the top: 1 3 4 2 ok 1 2 3 4 2 pick \ *duplicate* the item at that position to the top: 1 2 3 4 2 ok \ When referring to stack indexes, they are zero-based. \ ------------------------------ Creating Words -------------------------------- \ The `:` word sets Forth into compile mode until it sees the `;` word. : square ( n -- n ) dup * ; \ ok 5 square . \ 25 ok \ We can view what a word does too: see square \ : square dup * ; ok \ -------------------------------- Conditionals -------------------------------- \ -1 == true, 0 == false. 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. `if` `then` . : ?>64 ( n -- n ) dup 64 > if ." Greater than 64!" then ; \ ok 100 ?>64 \ Greater than 64! ok \ Else: : ?>64 ( n -- n ) dup 64 > if ." Greater than 64!" else ." Less than 64!" then ; 100 ?>64 \ Greater than 64! ok 20 ?>64 \ Less than 64! ok \ ------------------------------------ Loops ----------------------------------- \ `?do` is also a compile-only word. : myloop ( -- ) 5 0 ?do cr ." Hello!" loop ; \ ok myloop \ Hello! \ Hello! \ Hello! \ Hello! \ Hello! ok \ `?do` expects two numbers on the stack: the end number (exclusive) and the \ start number (inclusive). \ We can get the value of the index as we loop with `i`: : one-to-12 ( -- ) 12 0 do i . loop ; \ ok one-to-12 \ 0 1 2 3 4 5 6 7 8 9 10 11 ok \ `do` works similarly, except if start and end are exactly the same it will \ loop forever (until arithmetic underflow). : loop-forever 1 1 do i square . loop ; \ ok loop-forever \ 1 4 9 16 25 36 49 64 81 100 ... \ Change the "step" with `+loop`: : threes ( n n -- ) ?do i . 3 +loop ; \ ok 15 0 threes \ 0 3 6 9 12 ok \ Indefinite loops with `begin` `until`: : death ( -- ) begin ." Are we there yet?" 0 until ; \ ok \ ---------------------------- Variables and Memory ---------------------------- \ Use `variable` to declare `age` to be a variable. variable age \ ok \ Then we write 21 to age with the word `!`. 21 age ! \ ok \ Finally we can print our variable using the "read" word `@`, which adds the \ value to the stack, or use `?` that reads and prints it in one go. age @ . \ 21 ok age ? \ 21 ok \ Constants are quite similar, except we don't bother with memory addresses: 100 constant WATER-BOILING-POINT \ ok WATER-BOILING-POINT . \ 100 ok \ ----------------------------------- Arrays ----------------------------------- \ Creating arrays is similar to variables, except we need to allocate more \ memory to them. \ You can use `2 cells allot` to create an array that's 3 cells long: variable mynumbers 2 cells allot \ ok \ Initialize all the values to 0 mynumbers 3 cells erase \ ok \ Alternatively we could use `fill`: mynumbers 3 cells 0 fill \ or we can just skip all the above and initialize with specific values: create mynumbers 64 , 9001 , 1337 , \ ok (the last `,` is important!) \ ...which is equivalent to: \ Manually writing values to each index: 64 mynumbers 0 cells + ! \ ok 9001 mynumbers 1 cells + ! \ ok 1337 mynumbers 2 cells + ! \ ok \ Reading values at certain array indexes: 0 cells mynumbers + ? \ 64 ok 1 cells mynumbers + ? \ 9001 ok \ We can simplify it a little by making a helper word for manipulating arrays: : of-arr ( n n -- n ) cells + ; \ ok mynumbers 2 of-arr ? \ 1337 ok \ Which we can use for writing too: 20 mynumbers 1 of-arr ! \ ok mynumbers 1 of-arr ? \ 20 ok \ ------------------------------ The Return Stack ------------------------------ \ The return stack is used to the hold pointers to things when words are \ executing other words, e.g. loops. \ We've already seen one use of it: `i`, which duplicates the top of the return \ stack. `i` is equivalent to `r@`. : myloop ( -- ) 5 0 do r@ . loop ; \ ok \ As well as reading, we can add to the return stack and remove from it: 5 6 4 >r swap r> .s \ 6 5 4 ok \ NOTE: Because Forth uses the return stack for word pointers, `>r` should \ always be followed by `r>`. \ ------------------------- Floating Point Operations -------------------------- \ Most Forths tend to eschew the use of floating point operations. 8.3e 0.8e f+ f. \ 9.1 ok \ Usually we simply prepend words with 'f' when dealing with floats: variable myfloatingvar \ ok 4.4e myfloatingvar f! \ ok myfloatingvar f@ f. \ 4.4 ok \ --------------------------------- Final Notes -------------------------------- \ Typing a non-existent word will empty the stack. However, there's also a word \ specifically for that: clearstack \ Clear the screen: page \ Loading Forth files: \ s" forthfile.fs" included \ You can list every word that's in Forth's dictionary (but it's a huge list!): \ words \ Exiting Gforth: \ bye ``` ##Ready For More? * [Starting Forth](http://www.forth.com/starting-forth/) * [Simple Forth](http://www.murphywong.net/hello/simple.htm) * [Thinking Forth](http://thinking-forth.sourceforge.net/)