Let's Build Objects in GForth

2016-01-02 04:56:00 UTC

Although FORTH is considered by many to be a procedural language, its flexibility has allowed it to adapt other programming styles as well. If you are running GForth (HINT: sudo apt-get install gforth), you already have access to a great object oriented programming (OOP) library called oop.fs.

The documentation is well written, but might be a little bit wordy if you just want to play around.

Below is an example of OOP in GForth that will cover 80% of the use cases:

  • Instance CRUD
  • Instance level variable CRUD
  • Method definitions
  • Method calls
  • Inheritance

\ include oof.fs for GFORTH OOP. There's also mini-oof.fs, which is
\ only 12 lines long and gives you (most of) the same functionality
\ in a smaller package

include oof.fs

\ Let's define a class called "square". It inherits from the object
\ class, which  is the mother of all classes in oof.fs.
\ Syntax: <parent> class <child>
object class square
    \ Give it an instance method called "area"
    method area

    \ Give instances of square 2 cells of memory.
    \ On my system, that means 128 bytes.
    1 cells var length
    1 cells var width

  \ Now that class "square" is defined, we can implement it.

    \ Constructor expects 2 integers on the stack when called: width
    \ and length.
    : init ( n-width n-length -- self )
      \ Take the first item on the stack (length) and store that into
      \ the  instance variable "length".
      length !
      \ Same thing for width
      width ! ;

    : area ( self -- n-area )
      \ Grab length and push onto stack
      length @
      \ Grab width and push onto stack
      width  @
      \ Multiply length by width
      * ;

\ Create a new instance of square called "my-square"
20 30 square : my-square

\ Call the method "my-square" on square instance "my-square"...
\ ... Print the return value to screen.
my-square area . cr

\ Destroy the square object (for example purposes)
my-square dispose

\ gracefully exit.

I hope that helps. If you have any questions, you can always ask the friendly folks at /r/forth.