1Types in Modula-3Amer DiwanWhy bother with Modula-3?• A small and clean, yet usable object-oriented language• Language design goal: the entire language definition should fit in 50 pages• Buzzword compliant--statically and strongly typed, objects, exceptions, threads, garbage collection, modules, generics, ...2Outline• Modula-3’s notion of types– Type equality in Modula-3– Subtyping rules– Assignment rules (based on subtyping rules)– Information hiding in Modula-3Type equality• Modula-3 uses structural equivalence• But structural equality can be “over-ridden” if needed3Subtyping of pointer types• NULL <: REF T <: REFANYSubtyping of fixed arrays• ARRAY I OF T <: ARRAY J OF Tif NUMBER(I) = NUMBER(j)4Subtypingof object types• NULL <: T OBJECT ... END <: TOBJECT ... END <: REFANYExamples• T1 = OBJECT i: INTEGER; END;T2 = OBJECT i: INTEGER; END; ST1 = T1 OBJECT j: INTEGER; END;ST2 = T2 OBJECT j: INTEGER; END;• T1 <: T2?• ST1 <: T1?• ST2 <: T2?• ST1 <: T2?• What happens with branding?5(Simplified) Assignment rules• Type T is assignable to Type U if– T <: U– T and U are ordinal types with at least one member in common– U <: T and T is an array type or reference type• Why the exception in the third case?• Note the implicit safe casts!Run-time checks• S <: T;VAR s: S; t: T;t := s;• What checks does this need?• What representation does this need?6Opaque types• The information hiding mechanism based on subtyping• TYPE T <: UU = OBJECT i: INTEGER; END;T, an opaque type, is some subtype of U• REVEAL T = U OBJECT j: INTEGER; END;T is “revealed”: must be consistent with its opaque declarationRevelations• Revelations can be incrementalTYPE T <: UU = OBJECT i: INTEGER; END;V = U OBJECT ch: CHAR; END;• REVEAL T <: V;REVEAL T = V OBJECT j: INTEGER; END;• Can reveal different views to different clients (trusted, etc.).7An example of using opaque types• INTERFACE Counter;TYPE T <: Public;Public = OBJECT METHODS next(): INTEGER; END;END Counter• INTERFACE CounterFriends IMPORT Counter;REVEAL Counter.T <: U;TYPE U = Counter.Public OBJECT last_value: INTEGER; END;END CounterFriends• MODULE Counter EXPORTS Counter, CounterFriends;REVEAL T = U OBJECT otherstate: INTEGER; END;END Counter.Continuing with example• MODULE TrustedClient; IMPORT Counter, CounterFriends;BEGINEND TrustedClient• MODULE OtherClient; IMPORT Counter;BEGIN• END OtherClient8Unsafe parts of Modula-3• Unsafe operations are restricted to modules especially marked as unsafe– Explicit deallocation: Untraced references may be deallocated only unsafe modules– Unchecked type casts: Called LOOPHOLE!– ...What about other languages?• Java– Similar to Modula-3, BUT rather than specifying subtyping rules, it specifies conversions– Different information hiding model– Interfaces! (more soon)• C++/C– Unsafe: everything can be converted to everything else using casts– Otherwise not too dissimilar to Java9Next topic: Object orientation•