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diff --git a/forth.html.markdown b/forth.html.markdown new file mode 100644 index 00000000..f7c0bf34 --- /dev/null +++ b/forth.html.markdown @@ -0,0 +1,225 @@ +--- +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. + +``` +\ 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 (dont 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 into 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` <stuff to do> `then` <rest of program>. +: ?>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 and the start number. + +\ 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 12 ok + +\ `?do` works similarly, except it will skip the loop if the end and start +\ numbers are equal. +: squares ( n -- ) 0 ?do i square . loop ; \ ok +10 squares \ 0 1 4 9 16 25 36 49 64 81 ok + +\ 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` <stuff to do> <flag> `unil`: +: 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 simiar, 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/) |