CSE341: Programming Languages Lecture 24 Racket Modules, Abstraction with Dynamic Types; Racket Contracts Dan Grossman Fall 2011Another modules lecture • Recall lecture 12: SML modules. Key points: – Namespace management for larger programs (structures) – Hiding bindings inside the module (gcd, reduce) – Using an abstract type to enforce invariants Fall 2011 2 CSE341: Programming Languages signature RATIONAL = sig type rational exception BadFrac val make_frac : int * int -> rational val add : rational * rational -> rational val toString : rational -> string end structure Rational :> RATIONAL = …Racket is different • More flexible namespace management – Convenient ways to rename during export/import – (In other languages, could write wrapper modules) • Dynamic typing still has ways to create abstract types – Just need to be able to make a new type at run-time – This is what struct does; Scheme has nothing like it • By default, each file is a module – Not necessary but convenient • State-of-the-art contract system – Arbitrary dynamic checks of cross-module calls with blame assignment Fall 2011 3 CSE341: Programming LanguagesBut first… Worth emphasizing that modules are not necessary for creating abstract types: local scope and closures are enough Recall our rationals example (but note Racket has built-in rationals): Interface: – make-frac rejects 0 denominator – add adds two rationals – print-rat prints a rational in reduced form Can implement this by maintaining these invariants: – num and den fields kept in reduced form – den is always positive Fall 2011 4 CSE341: Programming LanguagesWrong approach [see lec24_non_modules.rkt] This uses local scope to hide gcd and reduce, but it exposes the rat constructor, so clients can make bad rationals – So to be "safe", add and print-rat can re-check invariants Fall 2011 5 CSE341: Programming Languages (struct (rat num den) (define rat-funs (letrec ([gcd (lambda (x y) …)] [reduce (lambda (x y) …)] [make-frac (lambda (x y) …)] [add (lambda (r1 r2) …)] [print-rat (lambda (r) …)]) (list make-frac add print-rat))) (define make-frac (car rat-funs)) (define add (cadr rat-funs)) (define print-rat (caddr rat-funs))Right approach [see lec24_non_modules.rkt] So we also need to hide the rat constructor! – Also hide mutators if you create them – Choose to hide accessors to keep representation opaque – This code doesn't "export" rat?, but doing so a good idea Fall 2011 6 CSE341: Programming Languages (define rat-funs (let () (struct (rat num den) (define (gcd x y) …) (define (reduce x y) …) (define (make-frac x y) …) (define (add r1 r2) …) (define (print-rat r) …) (list make-frac add print-rat))) (define make-frac (car rat-funs)) (define add (cadr rat-funs)) (define print-rat (caddr rat-funs))The key trick • By hiding the constructor and accessors, clients cannot make rationals or access their pieces directly • Clients can still pass non-rationals to add or print-rat, but any rational will satisfy the invariants • Technique requires fundamentally on semantics of struct – Make a new (dynamic) type of thing – If struct were sugar for cons cells, then clients could use cons to make bad rationals • So… to support abstract datatypes, dynamically typed languages need ways to make "new types of things" – Scheme traditionally had no such support • Again, making rat? public makes perfect sense Fall 2011 7 CSE341: Programming LanguagesRacket modules • The normal and convenient way puts bindings in a file and provides only the ones that should be public – Unlike SML, no separate notion of signature – module decides what to provide • Default is private – (But REPL for "Run" of a file is "inside" that file's module) – Which is why previous lectures used (provide (all-provided-out)) – Can provide some of struct's functions • See lec24_rationals.rkt – (provide make-frac add print-rat rat?) Fall 2011 8 CSE341: Programming LanguagesIt's the same trick • Modules take care of hiding bindings • struct takes care of making a new type • This doesn't work if rationals are implemented with an existing type like cons – Clients could use cons? to figure that out and then make bad rationals • Common misconception: Dynamically typed languages can't support abstract types – Some may not, but they could Fall 2011 9 CSE341: Programming LanguagesUsing modules [see lec24_client.rkt] • Clients get a module's bindings with the require form – Many variations, using a file-name string is the simplest – Can also get only the bindings you want, either by listing them with the only-in syntax or listing what you don't want with the except-in syntax • Convenient for avoiding name conflicts • See the manual for details – Can also rename bindings: rename-in and prefix-in • The provider can also rename when exporting • Overall: convenient namespace management is a nice thing Fall 2011 10 CSE341: Programming Languages (require "rationals.rkt")Contracts • A contract is a pre- and post-condition for a function – Software methodology of "design-by-contract" – If a function fails, blame either the caller or callee • Old idea; Racket's modules on the cutting edge • Can provide functions with a contract – Any predicate (boolean-returning function) on arguments and result – Any cross-module call will have its arguments and result checked at run-time (could be expensive) to assign blame • Intra-module calls (e.g., recursion) not checked • (You're not responsible for the details, just the high-level idea) Fall 2011 11 CSE341: Programming LanguagesExample lec24_rationals_contracts.rkt provides another implementation of a rationals library with contracts on each export It maintains different (weaker) invariants, putting more work on clients, with contracts checking that work: • Exports rat constructor, but contract requires integer arguments and positive denominator from client – Maintains these invariants • Exports rat-num, rat-den, and rat? • Does not keep rationals in reduced form – add doesn't care and doesn't reduce – print-rat does care (contract checks it); up to client to either call reduce-rat or "know" the rational is reduced Fall 2011 12 CSE341: Programming LanguagesExample provide (Note: needs DrRacket 5.2) • contract-out exports
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