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226 lines
7.2 KiB
Markdown
226 lines
7.2 KiB
Markdown
---
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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. It is an imperative,
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stack-based language and programming environment, being used in projects
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such as Open Firmware. It's also used by NASA.
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Note: This article focuses predominantly on the Gforth implementation of
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Forth, but most of what is written here should work elsewhere.
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```
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\ This is a comment
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( This is also a comment but it's only used when defining words )
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\ --------------------------------- Precursor ----------------------------------
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\ All programming in Forth is done by manipulating the parameter stack (more
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\ commonly just referred to as "the stack").
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5 2 3 56 76 23 65 \ ok
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\ Those numbers get added to the stack, from left to right.
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.s \ <7> 5 2 3 56 76 23 65 ok
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\ In Forth, everything is either a word or a number.
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\ ------------------------------ Basic Arithmetic ------------------------------
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\ Arithmetic (in fact most words requiring data) works by manipulating data on
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\ the stack.
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5 4 + \ ok
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\ `.` pops the top result from the stack:
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. \ 9 ok
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\ More examples of arithmetic:
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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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13 2 mod . \ 1 ok
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99 negate . \ -99 ok
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-99 abs . \ 99 ok
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52 23 max . \ 52 ok
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52 23 min . \ 23 ok
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\ ----------------------------- Stack Manipulation -----------------------------
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\ Naturally, as we work with the stack, we'll want some useful methods:
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3 dup - \ duplicate the top item (1st now equals 2nd): 3 - 3
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2 5 swap / \ swap the top with the second element: 5 / 2
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6 4 5 rot .s \ rotate the top 3 elements: 4 5 6
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4 0 drop 2 / \ remove the top item (dont print to screen): 4 / 2
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1 2 3 nip .s \ remove the second item (similar to drop): 1 3
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\ ---------------------- More Advanced Stack Manipulation ----------------------
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1 2 3 4 tuck \ duplicate the top item into the second slot: 1 2 4 3 4 ok
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1 2 3 4 over \ duplicate the second item to the top: 1 2 3 4 3 ok
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1 2 3 4 2 roll \ *move* the item at that position to the top: 1 3 4 2 ok
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1 2 3 4 2 pick \ *duplicate* the item at that position to the top: 1 2 3 4 2 ok
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\ When referring to stack indexes, they are zero-based.
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\ ------------------------------ Creating Words --------------------------------
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\ The `:` word sets Forth into compile mode until it sees the `;` word.
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: square ( n -- n ) dup * ; \ ok
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5 square . \ 25 ok
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\ We can view what a word does too:
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see square \ : square dup * ; ok
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\ -------------------------------- Conditionals --------------------------------
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\ -1 == true, 0 == false. However, any non-zero value is usually treated as
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\ being true:
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42 42 = \ -1 ok
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12 53 = \ 0 ok
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\ `if` is a compile-only word. `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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\ Else:
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: ?>64 ( n -- n ) dup 64 > if ." Greater than 64!" else ." Less than 64!" then ;
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100 ?>64 \ Greater than 64! ok
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20 ?>64 \ Less than 64! ok
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\ ------------------------------------ Loops -----------------------------------
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\ `do` is also a compile-only word.
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: myloop ( -- ) 5 0 do cr ." Hello!" loop ; \ ok
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myloop
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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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\ `do` expects two numbers on the stack: the end number and the start number.
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\ We can get the value of the index as we loop with `i`:
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: one-to-12 ( -- ) 12 0 do i . loop ; \ ok
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one-to-12 \ 0 1 2 3 4 5 6 7 8 9 10 11 12 ok
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\ `?do` works similarly, except it will skip the loop if the end and start
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\ numbers are equal.
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: squares ( n -- ) 0 ?do i square . loop ; \ ok
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10 squares \ 0 1 4 9 16 25 36 49 64 81 ok
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\ Change the "step" with `+loop`:
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: threes ( n n -- ) ?do i . 3 +loop ; \ ok
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15 0 threes \ 0 3 6 9 12 ok
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\ Indefinite loops with `begin` <stuff to do> <flag> `until`:
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: death ( -- ) begin ." Are we there yet?" 0 until ; \ ok
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\ ---------------------------- Variables and Memory ----------------------------
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\ Use `variable` to declare `age` to be a variable.
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variable age \ ok
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\ Then we write 21 to age with the word `!`.
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21 age ! \ ok
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\ Finally we can print our variable using the "read" word `@`, which adds the
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\ value to the stack, or use `?` that reads and prints it in one go.
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age @ . \ 21 ok
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age ? \ 21 ok
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\ Constants are quite similar, 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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\ Creating arrays is similar to variables, except we need to allocate more
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\ memory to them.
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\ You can use `2 cells allot` to create an array that's 3 cells long:
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variable mynumbers 2 cells allot \ ok
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\ Initialize all the values to 0
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mynumbers 3 cells erase \ ok
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\ Alternatively we could use `fill`:
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mynumbers 3 cells 0 fill
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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 , \ ok (the last `,` is important!)
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\ ...which is equivalent to:
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\ Manually writing values to each index:
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64 mynumbers 0 cells + ! \ ok
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9001 mynumbers 1 cells + ! \ ok
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1337 mynumbers 2 cells + ! \ ok
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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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\ We can simplify it a little by making a helper word for manipulating arrays:
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: of-arr ( n n -- n ) cells + ; \ ok
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mynumbers 2 of-arr ? \ 1337 ok
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\ Which we can use for writing too:
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20 mynumbers 1 of-arr ! \ ok
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mynumbers 1 of-arr ? \ 20 ok
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\ ------------------------------ The Return Stack ------------------------------
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\ The return stack is used to the hold pointers to things when words are
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\ executing other words, e.g. loops.
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\ We've already seen one use of it: `i`, which duplicates the top of the return
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\ stack. `i` is equivalent to `r@`.
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: myloop ( -- ) 5 0 do r@ . loop ; \ ok
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\ As well as reading, we can add to the return stack and remove from it:
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5 6 4 >r swap r> .s \ 6 5 4 ok
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\ NOTE: Because Forth uses the return stack for word pointers, `>r` should
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\ always be followed by `r>`.
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\ ------------------------- Floating Point Operations --------------------------
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\ Most Forths tend to eschew the use of floating point operations.
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8.3e 0.8e f+ f. \ 9.1 ok
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\ Usually we simply prepend words with 'f' when dealing with floats:
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variable myfloatingvar \ ok
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4.4e myfloatingvar f! \ ok
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myfloatingvar f@ f. \ 4.4 ok
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\ --------------------------------- Final Notes --------------------------------
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\ Typing a non-existent word will empty the stack. However, there's also a word
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\ specifically for that:
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clearstack
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\ Clear the screen:
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page
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\ Loading Forth files:
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\ s" forthfile.fs" included
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\ You can list every word that's in Forth's dictionary (but it's a huge list!):
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\ words
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\ Exiting Gforth:
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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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* [Simple Forth](http://www.murphywong.net/hello/simple.htm)
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* [Thinking Forth](http://thinking-forth.sourceforge.net/)
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