Flattening and Direct semantics By example: Inheritance Featherweight Jigsaw (FJig) (nice and clean) surface syntax Generalized inheritance.

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Slide 2 Flattening and Direct semantics By example: Inheritance Featherweight Jigsaw (FJig) (nice and clean) surface syntax Generalized inheritance Ill gloss over its (ugly but equivalent) core syntax Conclusions Slide 3 Slide 4 Please think how youd explain inheritance to a muggle 1 ( 1 here, indicates someone whos never heard of OO) Odds are youre thinking of flattening semantics Slide 5 class C1 { void m1() {} } class C2 extends C1 { void m2() {} } Its as we wrote C2 like this class C2 { void m1() {} void m2() {} } What does this mean? Slide 6 Extended Language Giving semantics of some feature by translating it away Programs translated in (equivalent) programs that dont use the feature Language class C2 extends C1 { void m2() {} } class C2 { void m1() {} void m2() {} } Extension flattened into the vanilla language Lets see pros and cons Slide 7 class C1 { void m1() {} } class C2 extends C1 { void m2() {} } class C3 extends C2 { void m3() {} } class C1 { void m1() {} } class C2 { void m1() {} void m2() {} } class C3 { void m1() {} void m2() {} void m3() {} } Slide 8 class C1 { void m1() {} } class C2 { void m1() {} void m2() {} } class C3 { void m1() {} void m2() {} void m3() {} } Lookup as easy as it can get No changes in the definition (the program changes, not lookup) For instance, lookup(C2, )= (inheritance has been flattened out) Slide 9 class C1 { void m1() {} } class C2 extends C1 { void m2() {} } class C3 extends C2 { void m3() {} } class C1 { void m1() {} } class C2 { void m1() {} void m2() {} } class C3 { void m1() {} void m2() {} void m3() {} } the binary depends on Slide 10 Intuitive: good tool for teaching/understanding Been used to give the semantics of mixins, traits, but Poor implementation choice Lot of compiling dependencies (unlike Java, like C++) Code bloating Slide 11 Slide 12 Please think how youd implement inheritance Odds are youre thinking of direct semantics Slide 13 Extended Language class C2 extends C1 { void m2() {} } Classes can be defined in new ways Since programs are not translated, good old method lookup cant work anymore Language class C1 { void m1() {} } We can compute lookup(C, m)= For instance, lookup(C1, m1)= lookup(C2, m1)=??? Slide 14 Needs to know what extends means For instance, lookup(C3, m2) starts from C3 Dont find m2, so proceeds with C2 And finds m2 class C1 { void m1() {} } class C2 extends C1 { void m2() {} } class C3 extends C2 { void m3() {} } Slide 15 FlatteningDirect IntuitiveYESLess than the other Good old method lookup YESNO Is efficient?NOYES Slide 16 Slide 17 class C ClassExpression ClassExpression ::= BasicClass | merge ClassExpression1, ClassExpression2 | rename N to N in ClassExpression | hide N in ClassExpression | others BasicClass ::= { constr/fields/methods } Slide 18 One constructor; takes any kind of parameters Internal representation hidden from clients A series of members: fields and methods Members can be: abstract virtual frozen local no definition someone is expected to provide it there is a definition, but it can be later replaced there is a definition, that current clients will see forever (even if replaced) there is a definition, that no other object will ever see Slide 19 Nice per se More than that, can encode: standard inheritance mixins traits its a general framework for sw composition Wouldnt be wonderful to have an intuitive (flattening) semantics and a (rather complex but) efficient (direct) one? Actually, no it wouldnt unless theyre equivalent! And they are! Slide 20 class A { abstract void m(); frozen int answer() { return 41; } local int loc() { return 1; } } class B { frozen int answer() { return 42; } virtual void m() { loc(); } local int loc() { return answer(); } } class C merge (rename answer to wrongAnswer in A), B whats new C().answer()? Im cheating using an extra-sugared syntax primitive types and void Slide 21 class C merge (rename answer to wrongAnswer in A), B class C merge (rename answer to wrongAnswer in { abstract void m(); frozen int answer() { return 41; } local int loc() { return 1; } } ), B Slide 22 class C merge { abstract void m(); frozen int wrongAnswer() { return 41; } local int loc() { return 1; } }, B Slide 23 class C merge { abstract void m(); frozen int wrongAnswer() { return 41; } local int loc() { return 1; } }, { frozen int answer() { return 42; } virtual void m() { loc(); } local int loc() { return answer(); } } Slide 24 class C merge { abstract void m(); frozen int wrongAnswer() { return 41; } local int loc() { return 1; } }, { frozen int answer() { return 42; } virtual void m() { loc(); } local int loc() { return answer(); } } Slide 25 class C { // equivalent flattened definition frozen int wrongAnswer() { return 41; } local int loc() { return 1; } frozen int answer() { return 42; } virtual void m() { loc(); } local int loc() { return answer(); } } now we can easily see what new C().answer() is and, quite obviously, its 42 Slide 26 class A { abstract void m(); frozen int answer() { return 41; } local int loc() { return 1; } } class C merge (rename answer to wrongAnswer in A), B again, what does new C().answer() means? what is lookup(C, answer)? class B { frozen int answer() { return 42; } virtual void m() { loc(); } local int loc() { return answer(); } } Slide 27 class A { abstract void m(); frozen int answer() { return 41; } local int loc() { return 1; } } class C merge (rename answer to wrongAnswer in A), B class B { frozen int answer() { return 42;} virtual void m() { loc(); } local int loc() { return answer(); } } lookup(C, answer) = lookup(merge , answer) = = lookup((rename), answer) and non-deterministically = lookup(B, answer) Slide 28 lookup(rename FromN to ToN in ClassExpression, N)= failure if N=FromN lookup(ClassExpression, FromN) if N=ToN lookup(ClassExpression, N) otherwise In the example: lookup((rename answer to wrongAnswer in A), answer) fails, so lookup(C, answer) = 2 choices = lookup(B, answer) Slide 29 lookup(B, answer) = lookup( ClassExpression, answer) where ClassExpression is the class expression of B, which is a base (flatten) class. So, we can conclude. Believe it or not, Ive just scratched the surface: direct semantics is no picnic Slide 30 Flatten semantics: easy, intuitive but performs poorly Direct semantics can be (rather) complex but its a good choice for implementation FJig: general language for software composition encompasses inheritance, mixin, traits. Given both semantics and proved equivalent Implemented interpreter as master-thesis: http://www.disi.unige.it/person/LagorioG/FJig/ Exploring the equivalence for feature requiring static types (e.g. overloading and static binding) Investigating smart implementation techniques Slide 31