Data Inheritance

[tripleo1]

http://c3d.github.io/xl/#data-inheritance

I don't understand the difference between the two examples presented.

Also, what is data inheritance (or am I looking too deep)?

[c3d]

The first example is the interface for the type:

type colored_text like text with
    foreground : color
    background : color

What this means is that when you use the colored_text type, you are allowed to use two fields called foreground and background, each having the color type. Therefore, the following code is valid:

CT : colored_text
if CT.foreground = CT.background then print "Will be hard to read"

This is irrespective of how the fields foreground and background are actually implemented (i.e. getter/setters or data fields or whatever).

The second example is an implementation of that interface using data inheritance, which is very much like inheritance in C++ where you have the original data with the new data added after it:

type colored_text is text with
   background : color
   foreground : color

What this means is that the colored_text type is implemented as a text base value followed by two background and foreground data fields.

As the next section explains, another possible implementation could have a totally different internal structure:

type colored_text is matching colored_text(fg : rgb_color; bg: rgb_color; text_data: byte_stream)

Of course, if you decide that this is what the implementation looks like, then you need to explicitly fulfill the interface contract. In particular, since you need to be able to convert to text to fulfill the like text part of the interface, you need to provide a conversion function, for example (assuming there is an explicit conversion from text_data to text):

T: colored_text as text is text(T.text_data)

That means you can now use a colored_text like a text:

foo T:text is print "foo ", T
bar C: colored_text is foo C // OK: can pass C using implicit conversion above