Reducing Costs with Duck Typing. Structural

Transcription

1 Reducing Costs with Duck Typing Structurl 1

2 Duck Typing In computer progrmming with object-oriented progrmming lnguges, duck typing is lyer of progrmming lnguge nd design rules on top of typing. Typing is concerned with ssigning type to ny object. Duck typing is concerned with estblishing the suitbility of n object for some purpose. With norml typing, suitbility is ssumed to be determined by n object s type only. clss clss vcuous! With nominl typing, suitbility is ssumed to depend on the clims mde when the object ws creted In contrst in duck typing, n object's suitbility is determined by the presence of certin methods nd properties (with pproprite mening), rther thn the ctul type of the object. The nme of the concept refers to the duck test, ttributed to Jmes Whitcomb Riley, which my be phrsed s follows: When I see bird tht wlks like duck nd swims like duck nd qucks like duck, I cll tht bird duck.[1] Some confusion present here unnecessry! 2

3 Metz: Duck types re public interfces tht re not tied to ny specific clss A Ruby object is like prtygoer t msquerde bll tht chnges msks to suit the theme. It cn expose different fce to every viewer; it cn implement mny different interfces. Grce n object s type is in the eye of the beholder. Users of n object need not, nd should not, be concerned bout its clss. It s not wht n object is tht mtters, it s wht it does. 3

4 Structurl vs. Nominl Structurl typing (Duck Typing) type T describes set of properties (so, it s like predicte). An object o hs type T if it stisfies tht predicte. Grce hs Structurl typing. Nominl Typing (Clss Typing + ) type T is identified with both set of properties nd nme. An object o hs type T if it ws mde by clss tht sys tht its objects hve type T. The lnguge is designed in such wy tht this lso mens tht it must hve the properties of T. Even though p might hve ll the right properties, it s not of type T unless the clss tht mde it sys so. 4

5 Sttic typing: the type of every expression cn be determined before the progrm strts to execute. The progrmmer is usully (but not lwys) required to nnotte declrtions with types. Dynmic typing: no ttempt is mde to determine the type of n expression until the progrm is executing. Grdul Typing: type nnottions re optionl; when given, they enble the types of some expressions to be evluted before the progrms strts to execute. Others will be checked t runtime. 5

6 Explore consequences of not using Duck Types some object Trip Mechnic prepre(mechnic) prepre_bicycles(bicycles) some object Trip Mechnic Figure 5.1 Trip prepres itself by sking mechnic to prepre the bicycles. Seems OK, so long s we hve only mechnics 6

7 But when there re other preprers some object Trip Mechnic Coordintor Driver prepre(preprers) loop [preprers] type-cse, lt [Mechnic] prepre_bicycles(bicycles) instnce-of, brnding with strings, re ll eqully evil [TripCoordintor] buy_food(customers) [Driver] gs_up(vehicle) fill_wter_tnk(vehicle) some object Trip Mechnic Coordintor Driver Figure 5.2 Trip knows too mny concrete clsses nd methods. 7

8 The Root of the Problem If your design imgintion is constrined by clss nd you 4ind yourself unexpectedly deling with objects tht don t understnd the messge you re sending, your tendency is to go hunt for messges tht these new objects do understnd. Insted: crete new messges tht ll the objects cn resonbly understnd 8

9 Wht do the trgets hve in common? They ll help trip mke preprtions Wht kind of thing is Preprer? At this point it hs no concrete existence; it s n bstrction, n greement bout the public interfce on n ide. It s figment of design. some object loop some object prepre(preprers) [preprers] Trip Trip prepre_trip(self) request_dditionl info dditionl info response Preprer Preprer Figure 5.4 Trip collbortes with the preprer duck. 9

10 Documenting Duck Types When you crete duck types you must both document nd test their public interfces. document the existence of the type test tht certin objects hve tht type 10

11 Polymorphism Functionl progrmmers: function is (prmetriclly) polymorphic if it cn be pplied to rguments of more thn one type, nd trets them ll uniformly Metz: messge is polymorphic if there re mny objects tht hve corresponding method, nd they do relted things Blck: method is polymorphic if it cn be pplied to rguments of more thn one type. It does this by sending messges tht re polymorphic in Metz's sense. 11

12 Sttic typing does not prevent Duck typing Clss typing prevents duck typing Clss typing sys: I don't cre how cpble you re, or how good you re t your job if you don t come from the right clss, I won t even let you try Even in Jv, you cn do Duck typing if you use interfces s types, not clsses. 12

13 Wht s Wrong with Sttic Typing? Progrms tht re well-typed Progrms tht work After Simon Peyton Jones Zone of Abysml Pin 13

14 Wht s Wrong with Sttic Typing? Progrms tht re well-typed Progrms tht work Accomplished by inventing ever-more powerful type systems tht re ever-hrder to understnd After Simon Peyton Jones Smller Zone of Abysml Pin 14