ifs considered Harmful

Transcription

1 ifs cosidered Harmful Or; how to elimiate 90% of your bugs ad 99% of your techical debt i oe easy step. Jules May JulesMay.co.uk

2 Codebase survey Bugs: all ~2.5 millio lies 5 years developmet 6-25 developers Bugs: primary Bugs: secodary

3 goto cosidered harmful EWD 215: A case agaist the goto statemet (1968) Edsger Wybe Dijkstra ( ) Goto statemet cosidered harmful, Commu. ACM 11 (1968), 3:

4 goto cosidered harmful - why? What a program looks like: Static map Spatial - locatios What a program meas: Dyamic process Temporal - istats Executio coordiate

5 Executio coordiate 10: a=10; 11: b=20; 12: c=a+b; 13: prit (c); 14: d=sqrt (c); 15: ed;

6 Executio coordiate 10: a=10; 11: b=20; 12: c=a+b; 13: prit (c); 14: d=sqrt (c); 15: ed; Rt:9178: Rt:9179: Rt:9180: Rt:4124: Rt:4126: Rt:4125: Rt:4128: Rt:4127: 10: 11: 12: 13: 14: 15:

7 Executio coordiate 10: a=10; 11: for b=(1..3) { 12: c=a+b; 13: prit (c); 14: d=sqrt (c); 15: 13: 14: 12: 11(1): 11(2): 11(3): 15: 10:

8 Executio coordiate - fail? 10: a=9;b=-10 11: if b=11 goto 18; 12: b=b+1; 13: if b>12 goto 10; 14: c=a+b; 15: if c<0 goto 11; 16: prit c; 17: d=sqrt (c); 18: goto 12; 19: ed; 10: 11: 12: 13: 14: 15: 16: 17: 18: 19:

9 Executio coordiate - take-away Coordiate is tuple of (locatio or repetitio) q e.g. [11(2), 13, Rt:4126] Shows us (for structured code): q Locatio i code q Momet i executio time q Etire history leadig up to that momet. Not all gotos violate structure : q retur, cotiue, break q loops q assembler

10 Executio coordiate - the problem 10: for b=(-1..1) { 11: if (b<0) 12: prit (GORED); 13: else 14: prit (GOBLUE); 15: prit (b); 16: if (b<0) 17: prit ( DR\ ); 18: else 19: prit ( CR\ ); 20:

11 Executio coordiate - the problem 10: for b=(-1..1) { 11: if (b<0) 12: prit (GORED); 13: else 14: prit (GOBLUE); 15: prit (b); 16: if (b<0) > 17: 18: prit ( DR\ ); else 19: prit ( CR\ ); 20: 10: for b=(-1..1) { 11: if (b<0) { 12: prit (GORED); 13: prit (b); 14: prit ( DR\ ); 13: else { 14: prit (GOBLUE); 15: prit (b); 16: prit ( CR\ ); 19: 20:

12 Executio coordiate - summary goto Destroys temporal idetity Destroys history Creates flow spaghetti if..if Coflates temporal idetity Destroys history Creates flow wormholes Imply goto from code structure Not all gotos are evil Imply decisio from code structure Not all coditios are evil

13 ifs cosidered harmful Need a ew otio of if-structure : q Coditios are cocetrated q Decisios are implied Need ew mechaisms for if q Do t overstay your welcome q Specialist classes & dowcastig Need to express those i covetioal laguages

14 ifs cosidered harmful Assert

15 ifs cosidered harmful- assert float sqrt (a) { assert (isnumeric (a)) or error ( Numeric type required ); assert (a>=0) or error throw () DomaiError ( ); r = // do some computatio o a retur r; Priciple: ot to cotrol the flow, but to abort it. Species: assert ( ) or error (); -- Geerate compile-time error (if possible) assert ( ) or retur (); -- retur default value or ull object assert ( ) or throw ; -- refuse to process; cotract violatio assert ( ) or die (); -- do shutdow or recovery process

16 ifs cosidered harmful- assert I practice: It s easy: q if (!coditio) throw ; assert () or error eeds compiler support.

17 ifs cosidered harmful Do t overstay your welcome

18 ifs cosidered harmful - Values umber fact (umber ) { if (==1) retur 1; else retur *fact(-1); fact (1)=1; fact (umber )=*fib(-1); fact (strig s)= Ca t fact +s; strig fact (strig s) { retur Ca t fact + s;

19 ifs cosidered harmful - Values for b=(-1..1) { if (b<0) prit (GORED); else prit (GOBLUE); prit (b); if (b<0) prit ( DR\ ); else prit ( CR\ ); for b=(-1..1) { prit fmt (b); strig fmt (umber b<0){ retur GORED + b + DR\ ; strig fmt (umber b){ retur GOBLUE + b + CR\ ; strig fmt (umber b=0){ retur GOBLACK + il\ ;

20 ifs cosidered harmful - Values I practice: Do coditios before aythig else, q e.g. fact example Use ordiary polymorphism over specialist classes q e.g. fmt (plusmoey (b)) { ; Use Haskell!

21 ifs cosidered harmful Specialists

22 ifs cosidered harmful - specialists class littleclass { class bigclass { littleclass c = ull; bigclass () { // ctor. might set c; bigmethod () { if (isnull (c)) { // do somethig else { // do somethig similar ivolvig c;

23 ifs cosidered harmful - specialists littleclass c = 4; -- meas: c = littleclass (4); littleclass c = ull; -- meas c = ull; -- should mea: c = littleclass (ull); Null poiter No iterface at all Needs if!isnull() o each use All uses must be checked at rutime Null object Exposes the class iterface Ca be used aively All uses ca be checked at compile time

24 ifs cosidered harmful - specialists class littleclass { class bigclass { littleclass c = littleclass ull; -- ow (ull); meas: c = littleclass (ull); bigclass () { // ctor. might set c; bigmethod () { if (isnull (c)) { // do somethig else { // do somethig ivolvig c

25 ifs cosidered harmful Specialists ad dowcasts

26 ifs cosidered harmful - dowcasts Every method has a implied coditio: if (this.isistaceof (thisclass)) { class littleclass { private littleclass () { // declaratios // littleclass methods () -> littlenull (); littleclass (it k) -> littleit (k); littleclass (object obj) -> littlethig (obj); class littlenull:littleclass { littlenull () { //ctor // implemetatios class littleit:littleclass { littleit (it k) { //ctor // implemetatios class littlethig:littleclass { littlethig (object obj) { // implemetatios

27 ifs cosidered harmful - dowcasts for b=(-1..1) { if (b<0) prit (GORED); else prit (GOBLUE); prit (b); if (b<0) prit ( DR\ ); else for b=(-1..1) { prit (moey (b)); class moey:umber { //strig umber:tostrig () iherited private moey (umber ()) { private prit class ( CR\ ); plusmoey:moey { tostrig () { retur GOBLUE + :tostrig() + CR\ ); private class miusmoey:moey { tostrig () { retur GORED + :tostrig() + DR\ ); moey (umber ) -> >=0? plusmoey () : miusmoey ();

28 ifs cosidered harmful - dowcasts I practice: Use abstract base classes with static factory methods, q e.g. x=myclass:ew ( ); Expose the subclasses directly q e.g. moey b = plusmoey (4); Precompile q e.g. abstract dowcastable class myclass { ; ew myclass( ) -> myclass:factory ( )

29 ifs cosidered harmful - Summary goto is harmful. It causes spaghetti code, which is ureliable. Actually, it was always if that was harmful. It causes wormhole code, which is ureliable.

30 ifs cosidered harmful - Summary The atidote to goto is flow-structure Structured laguages replace goto with: q for q while q throw The atidote to if is decisio-structure A decisio-structured laguage would replace if with: q assert () or q value-polymorphism q dowcasts

31 ifs cosidered harmful - Summary If you re careful, you ca do structured programmig with gotos. If you re careful, you ca do decisiostructured programmig with ifs q while you re waitig for laguage techology to catch up.

32 Take-aways (Misuse of) if causes more bugs tha all other causes combied. This should t be ews it s goto all over agai; We eed decisio-structured laguage costructs; I the meatime, good programmig hygiee helps.

33 ifs cosidered harmful Questios? Bugs: primary Bugs: secodary JulesMay.co.uk