CSE 341:Programming LanguagesAutumn 2005Lecture 12 — Modules and Abstract TypesCSE 341 Autumn 2005, Lecture 12 1ModulesLarge programs benefit from more structure than a list of bindings.Breaking into parts allows separate 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.CSE 341 Autumn 2005, Lecture 12 2Structure 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.CSE 341 Autumn 2005, Lecture 12 3Signature basics(For those interested in learning more, we’re doing only opaquesignatures on structure definitions.)A signature signature SQUID = 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 :> SQUID = struct bindings end:• Ensures Name is a legal implementation of SQUID.• Ensures code outside of Name assumes nothing more than whatSQUID provides.Hence signatures are what really enable separate reasoning.CSE 341 Autumn 2005, Lecture 12 4Signature matchingIs Name a legal implementation of SQUID.• Clearly it must define everything in SQUID.• It can define more (unavailable outside of Name).• SQUID can restrict the type of polymorphic functions.• SQUID 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.CSE 341 Autumn 2005, Lecture 12 5RememberKey tools for modularity/information hiding in ML: structures andsignatures (and functors, which we’re skipping).A signature that “hides more” makes it easier to:• Replace the structure implementation without breaking clients.• Reason about how clients use the structure.Note: See the extended example code for this lecture for more details...CSE 341 Autumn 2005, Lecture 12
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