'&$%CSE 341:Programming LanguagesWinter 2006Lecture 12— Modules and Abstract TypesCSE341 Winter 2006, Lecture 12 1'&$%ModulesLarge programs benefit from more structure than a list of bindings.Breaking into parts allow s s eparate reasoning:• Application-level: in terms of module (in ML, structure) invariants• Type-checking level: in terms of module types• Implementation level: in terms of module code-generationBy providing a restricted interface (in ML, a signature), there are moreequivalent implementations in terms of the interface.Key restrictions:• Make bindings inaccessible• Make types abstract (know type exists, but not its definition)SML has a much fancier module system, but we’ll stick with the basics.Abstract types are a “top-5” feature of modern languages.CSE341 Winter 2006, Lecture 12 2'&$%Structure basicsSyntax: structure Name = struct bindings endIf x is a variable, exception, type, constructor, etc. defined in Name,the rest of the program refers to it via Name.x(You can also do open Name, which is often bad style, but convenientwhen testing.)So far, this is just namespace management, which is important forlarge programs, but not very interesting.CSE341 Winter 2006, Lecture 12 3'&$%Signature basics(For those interested in learning more, we’re doing only opaquesignatures on structure definitions.)A signature signature BLAH = sig ... end is like a type for astructure.• Describes what types a structure provides.• Describes what values a structure provides (and their types).Writing structure Name :> BLAH = struct bindings end:• Ensures Name is a legal implementation of BLAH.• Ensures code outside of Name assumes nothing more than whatBLAH provides.Hence signatures are what really enable separate reasoning.CSE341 Winter 2006, Lecture 12 4'&$%Signature matchingIs Name a legal implementation of BLAH.• Clearly it must define everything in BLAH.• It can define more (unavailable outside of Name).• BLAH can restrict the type of polymorphic functions.• BLAH can make types abstract.In particular, making a datatype abstract hides the constructors, soclients have no (direct) way to create or access-parts-of values of thetype.That’s often a good thing.CSE341 Winter 2006, Lecture 12 5'&$%RememberA signature that “hides more” makes it easier to:• Replace the structure implementation without breaking clients.• Reason about how clients use the structure.Note: The real “content” of this lecture is in the extended example.CSE341 Winter 2006, Lecture 12
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