Want to follow along? http://www.cs.york.ac.uk/~ndm/ads.pdf Ada Ada: Gen : Generi erics cs Neil Mitchell
Aga gain, aga in, again, ag in, again ain Dave writes a linked list package for characters Sue writes a linked list package for integers Ed writes a linked list package for booleans What if Dave had written a linked list package for <anything>? Sue and Ed could have gone to the pub! Generics Generics allows Dave to do this
Gen Generic? eric? Function Value Value Procedure Value Action Generic Function Type Function Generic Package Type Package
Swap_ Sw ap_Flo loat at procedure procedur e Swap_Float(X,Y: in out : in out Float) is is T : Float; begin begin T := X; X := Y; Y := T; end; end;
Sw Swap ( ap (gener generic ic proc proced edure) ure) generic generic type Item is private type is private; procedur procedure Swap(X,Y: in out in out Item); procedur procedure e Swap(X,Y: in out in out Item) is is T: Item; begin T := X; X := Y; Y:= T; end begin end Swap; -- an instantiation, which we use procedure procedur e Swap_Float is new is new Swap(Float);
Gen Generic p eric pack ackage sp age spec ecific ificat ation ion generic generic type type Element is private is private; package List is package is type type List is private is private; Nil : constant constant List; function Null_Query(L : List) return function return Boolean; function function Cons(Head : Element; Tail : List) return List; return ... -- other useful methods private private ... -- as before Not Note: end List; end Save as “ list.ads ”
Generic p Gen eric pack ackage b age bod ody package body List is package body is function Null_Query (L : List) return function return Boolean is is begin return L = Nil; begin return end end Null_Query; function Cons(Head : Element; Tail : List) function return return List is is begin begin return new return new Cell'(Content => Head ,Next => Tail); end Cons; end Not Note: end end List; Save as “ list.adb ”
Using Us ing a ge a gener neric pac ic packa kage ge with List; -- import with procedure procedure Test is is -- instantiate package package List_Integer is new is new List(Integer); -- use Ns : List_Integer.List := List_Integer.Nil; begin begin Ns := List_Integer.Cons(Head => 6, Tail => Ns); ...; Not Note: end Test; end Save as “ test.adb ”
Gen Generic typ eric type pa e paramet rameters ers generic generic type Element is <somet type is <something> hing>; package List is package is ted private = use as parameter type, limi mited private declare variables te + assign and test pri priva vate te = l = limited pri imited priva vate for equality (<>) = pri (<>) = priva vate te + treat as discrete type ( T’First , T’Range , etc)
Proced Pro cedures as ures as Pa Param rameters eters Requ Request: est: print a list procedure Put(L : List); Impossible! An item in list is generic We don’t know how to write it to the screen Solu Solution ion: the user tells us how with procedure Element_Put(E : in Element);
Implement Im lementing ing Put Put generic generic type type Element is private is private; with procedure Element_Put(E : in Element); with procedure package package List is is procedure procedure Put(L : in in List); ... -- as before end List; end procedure procedure Put(L : in in List) is is begin begin if not if not Null_Query(L) then then Element_Put(Head(L));
Using Us ing Put Put with Ada.Text_IO, List; with procedure procedure Test is is package List_Char is new package is new List (Element => Character ,Element_Put => Ada.Text_IO.Put); Hi : List_Char.List := ...; begin begin List_Char.Put(Hi); end end Test;
One p ne pack ackage, on ge, one e val value? ue? Each List package provides an unlimited number of values: ◦ Every operation takes a List parameter ◦ Which data structure to operate on ◦ Abstract Data Type (ADT) An alternative is to have one value in one package ◦ No more saying which data structure ◦ Can sometimes be simpler ◦ Abstract Object
Abst bstract ract Ob Object ject: Spe : Specific ificat ation ion gener neric ic type type Element is is p priv ivate te; with proc rocedu edure re Put(E : in in Element); packa ckage ge One_List is is -- Note: no exported type or constants! -- All the state is in the body -- A new facility (replaces constant Nil) procedure ure Reset; function on Null_Query return return Boolean; procedure ure Cons(Head : in in Element); ... -- Note: no private section! end d One_List;
Abst bstract ract Ob Object ject: Bod : Body with th List; packa ckage ge body body One_List is is pack pa ckag age List_Element is is n new ew List (Element => Element, Put => Put); The_List : List_Element.List; function on Null_Query return return Boolean is is begi egin ret n return urn List_Element.Null_Query(The_List); end end Null_Query; procedure ure Cons(Head : in Element) is is begin begin The_List := List_Element.Cons(Head, The_List); end end Cons; begin gin Reset; end d One_List;
Abst bstract ract Ob Object ject: Use : Use with th One_List; pr proce cedur ure Classify is is package e Marks is new is new One_List(Integer, Num_Out); package e Age is new is new One_List(Natural, Num_Out); begin Marks.Cons(99); -- cheated, not caught Marks.Cons(70); -- revised hard Marks.Cons(-5); -- cheated, caught, expelled Marks.Put; Marks.Reset; -- get rid of last years marks if Age.Null_Query then Age.Cons(21);
What t hat to o do do no now? w? Work through the exercises ◦ No more of us talking at the start of practicals ◦ Go from where you are ◦ Get as far as you can If you are struggling ◦ Stick your hand up, get some help now ◦ You will be expected to be able to program Ada for the open assessment ◦ You will not get Ada help during that time
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