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@ -22,13 +22,11 @@ of what is written here should work elsewhere.
\ 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
@ -41,20 +39,13 @@ of what is written here should work elsewhere.
\ 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
\ However, this alone yields "ok", yet no answer. Typing the word `.` will yield
\ the result.
. \ 9 ok
\ This should illustrate the fundamentals of forth. Lets do a few more arithmetic
\ tests:
\ This should illustrate how Forth's stack works. Lets do a few more arithmetic tests:
6 7 * . \ 42 ok
1360 23 - . \ 1337 ok
12 12 / . \ 1 ok
@ -84,14 +75,11 @@ 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.
\ When referring to stack indexes, they are zero-based.
\ ------------------------------ 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,
@ -102,39 +90,34 @@ n pick \ where n is a number, *duplicate* the item at that position to the top o
\ 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.
\ The idea 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 <boolean> `if` <stuff to do> `then` <rest of program>.
\ `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` <stuff to do> `then` <rest of program>.
: ?>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!
\ Else:
: ?>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.
\ terminator:
: myloop ( -- ) 5 0 do cr ." Hello!" loop ; \ ok
test
\ Hello!
@ -143,31 +126,21 @@ test
\ 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`.
\ `do` expects two numbers on the stack: the end number and the index number, respectively.
\ Get the value of the index as we loop with `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.
\ Change the "step" with `+loop`:
: threes ( -- ) 15 0 do i . 3 +loop ; \ ok
threes \ 0 3 6 9 12 ok
\ Finally, while loops:
\ Finally, while loops with `begin` <stuff to do> <flag> `unil`:
: death ( -- ) begin ." Are we there yet?" 0 until ;
\ Will print "Are we there yet?" forever. While loops are constructed in the format
\ of `begin` <stuff to do> <flag> `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: