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Abstract type: Difference between revisions
→{{header|REBOL}}
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print [crlf "A rectangle:"]
r: make rectangle [size: 32x5] r/draw</lang>
=={{header|Red}}==
<lang Red>Red [
Title: "Abstract Type"
Original-Author: oofoe
]
; The "shape" class is an abstract class -- it defines the "pen"
; property and "line" method, but "size" and "draw" are undefined and
; unimplemented.
shape: make object! [
pen: "X"
size: none
line: func [count][loop count [prin self/pen] prin newline]
draw: does [none]
]
; The "box" class inherits from "shape" and provides the missing
; information for drawing boxes.
box: make shape [
size: 10
draw: does [loop self/size [line self/size]]
]
; "rectangle" also inherits from "shape", but handles the
; implementation very differently.
rectangle: make shape [
size: 20x10
draw: does [loop self/size/y [line self/size/x]]
]
; Unlike some languages discussed, REBOL has absolutely no qualms
; about instantiating an "abstract" class -- that's how I created the
; derived classes of "rectangle" and "box", after all.
print "An abstract shape (nothing):"
s: make shape [] s/draw ; Nothing happens.
print [newline "A box:"]
b: make box [pen: "O" size: 5] b/draw
print [newline "A rectangle:"]
r: make rectangle [size: 32x5] r/draw</lang>
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