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| -rw-r--r-- | forth.html.markdown | 51 |
1 files changed, 12 insertions, 39 deletions
diff --git a/forth.html.markdown b/forth.html.markdown index 06c2c6dc..7b805057 100644 --- a/forth.html.markdown +++ b/forth.html.markdown @@ -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: |