Custom Classes proposal

[x87]

Class Syntax

Here is the proposal to add a new syntax for defining custom classes in scripts. They may eventually become a replacement for static classes in the classes.db.

Let's take a look at an example:

class Actor
  putInCar(car)
     036A: put_actor self in_car car
  end
end

var
  0@: Actor
end

0@.putInCar($car)

Here we defined a class named Actor that has one method putInCar in. putInCar method has one parameter car. Then we declared a variable 0@ of the type Actor and finally called method putInCar on that class instance.

Important Notes

• class declaration is lazy. it means no code is produced until it gets actually used in the code. if we were to remove 0@.putInCar($car) no code was produced in this script.
• each method receives an implicit variable self that represents the class instance the method is called on. in this case it is the variable 0@
• in this example 0@.putInCar($car) gets compiled into 036A: put_actor 0@ in_car $car

Design Choices

• when there is a method call statement the compiler takes the method body and replaces the statement with the entire body. it means the method could have many instructions and possibly even labels and they will be put in place of the call statement. in this sense class method are more templates than functions as in the other languages.

• this however does not prevent local variables from being altered inside the method body. it means there is no separate scope created when the class method is called and if you, for example, change the variable 0@ in the class method it will be a side-effect for the caller:

class Actor
  putInCar(car)
     036A: put_actor self in_car car
     0@ = 2
  end
end

0@ = 1
0@.putInCar($car)
// 0@ is now equal to 2

There are a few possible choices to this dilemma:

1. leave it as is, considering this a liberate language design decision (when you invoke subroutines with gosub they could alter local scope as well)
2. forbid using local variables syntax inside methods. only use arguments passed into the method. this could be inconvenient if you need to keep a local state.
3. add extra prologue and epilogue before and after each method call that will store and restore all local variables. this adds extra overhead in runtime
4. use CLEO's scm_function that basically does # 3. this adds unnecessary dependency on CLEO.
   • however if preserving a local scope is important, one could consider using a scm function inside a method body:

class Actor
  putInCar(car)
     0AB1: call_scm_func @local 0

     :local
     036A: put_actor self in_car car
     0@ = 2
     0AB2: ret 0
  end
end

0@ = 1
0@.putInCar($car)
// 0@ is still 1

[x87]

Conditional Methods

method could be conditional:

class Actor
  inAnyCar()
     00DF:   actor self in_any_car 
  end
end

var
  0@: Actor
end
if
  0@.InAnyCar()
then

gets compiled as

if
   00DF:   actor 0@ in_any_car
then

Design Choices

If the method is a template that simply gets inserted in-place of a call statement (see Design choices in the first post) you can not put anything except conditional opcodes in a method to be able to use it in the conditional expression. i.e. this would not be allowed:

class Math
  isZero(x)
     if
       x == 0
     then
        0485:  return_true
     else
        059A:  return_false
     end
  end
end

if
  Math.isZero(1) // false
then

[x87]

Returning Results

It's unclear how to work with returning a value from the methods, i.e consider:

class Actor
  pos()
     00A0: store_actor self position_to x y z
  end

how to represent x, y, z in the code?

Design Choices

1. pass variables into the method as parameters

  pos(x, y, z)
     00A0: store_actor self position_to x y z
  end

0@.pos(1@, 2@, 3@) // 00A0: store_actor 0@ position_to 1@ 2@ 3@

always having to pass a variable to store a result into can be tedious for simple cases like:

  sqr(x, result)
     0A90: result  = x * x
  end

one would want to simply have

class Math
  sqr(x)
     0A90: result  = x * x
  end
end

0@ = Math.sqr(2)

2. add an implicit variable result then will represent the left side of the call expression:

  pos()
     00A0: store_actor self position_to result[0] result[1] result[2] // variant 1
     00A0: store_actor self position_to result.0 result.1 result.2 // variant 2
  end

then

( 1@, 2@, 3@ ) = 0@.pos() // 00A0: store_actor 0@ position_to 1@ 2@ 3@

this adds a lot of unusual syntax into the language.

3. combine both approaches, i.e. have an implicit result for a single value and pass variables into the method for multiple values:

class Math
  sqr(x)
     0A90: result  = x * x
  end

   pi(int, frac)
      int = 3
      frac = .14
   end
end

0@ = Math.sqr(2) // 0@ = 4
Math.pi(1@, 2@) // 1@ = 3, 2@ = .14

[wmysterio]

Powerful! The question is: will the compiler pick up the declared classes if they are described in another file and were included via the {$i} directive. There are plans to write such classes (I already have everything ready for this) and I would like to save the developer from copying everything every time.

I also wanted to ask: will it be possible to add constants to a class? Like this

class Draw
    const MaxWidth = 640.0
end
//
0@ > Draw.MaxWidth

[x87]

@wmysterio
yes, the compiler definitely should be able to read class declarations from imported files.

static class variables could be a nice feature! Will keep that in mind.

[x87]

Class instances should support inline declaration similar to primitive types:

int x = 0
Player p = Player.Create(x, 10.0, 20.0, -100.0)