47 lines
2.1 KiB
Ruby
47 lines
2.1 KiB
Ruby
module Mom
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# SlotLoad is an abstract base class for moving data into a slot
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# A Slot is basically an instance variable, but it must be of known type
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#
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# The value loaded can be a constant (SlotConstant) or come from another Slot (SlotMove)
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#
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# The Slot is the left hand side, the right hand side being determined by the subclass.
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# The only known object (*) for the left side is the current message, which is a bit like
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# the oo version of a PC (program Counter)
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# (* off course all class objects are global, and so they are allowed too)
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#
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# A maybe not immediately obvious corrolar of this design is the total absence of
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# general purpose instance variable accessors. Ie only inside an object's functions
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# can a method access instance variables, because only inside the method is the type
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# guaranteed.
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# From the outside a send is neccessary, both for get and set, (which goes through the method
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# resolution and guarantees the correct method for a type), in other words perfect data hiding.
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#
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# @left: is an array of symbols, that specifies the first the object, and then the Slot.
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# The first element is either a known type name (Capitalized symbol of the class name) ,
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# or the symbol :message
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# And subsequent symbols must be instance variables on the previous type.
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# Examples: [:message , :self] or [:Space : :next_message]
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#
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# @right: depends on the derived Class
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#
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class SlotLoad < Instruction
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attr_reader :left , :right
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def initialize(left , right)
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@left , @right = left , right
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end
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end
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# A SlotConstant moves a constant into a know Slot.
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# Eg when you write a = 5 , the 5 becomes a constant, and so the right side
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# the a is an instance variable on the current frame, and the frame is an instance
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# of the current message, so the effect is something like message.frame.a = 5
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# @left: See SlotLoad, and array of symbols
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# @right: A Constant from parse, ie an instance of classes in basc_value, like TrueStatement
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class SlotConstant < SlotLoad
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end
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class SlotMove < SlotLoad
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end
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end
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