Advanced Functional Programming

thetheorybehindmanythingscommonlywrittenbyfunctionalprogrammers. Oneofthemainachievementsoftheprojecthasbeentomakepeopleaware ofthefactthatmanyalgorithmscanbedescribedinadata-independentway. ThePolyPsystemintroducedinthesenotesisoneofthetranslationstothe Haskell-worldofthistheoreticalunderpinning. Thethirdlecture,onGenericProgramTransformation,canalsobeseenas anapplicationofthetheoryintroducedinlecturetwo. Manye?ciency-improving programtransformationscanbeperformedinamechanicalway,andthesewould nothavebeenpossiblewithoutinsightintothecorrectnessofsuchtransfor- tionsgainedinthelectureonGenericProgramming. Thefourthlecture,onDesigningandImplementingCombinatorLanguages, introducesaneasytowriteformalismforwritingdownthecatamorphismsint- ducedinearlierchapters. Itisshownhowquitecomplicatedcatamorphisms,that at?rstsightseemratherforbiddingbymakingextensiveuseofhigher-orderdo- VI Preface mains,canactuallybedevelopedinastep-wisefashion,usinganattributegr- marview;itisfurthermoreshownhowtorelatethiswayofprogrammingwith conceptsfromtheobject-orientedworldthusmakingclearwhatthestrengths andweaknessesofeachworldare. The?fthlecture,titledUsingMetaML:AStagedProgrammingLanguage, introducestheconceptofpartialevaluation. Itservesasanotherinstanceof thequestfor“themostgenericofwritingprogramsatthelowestcost”. The stagingtechniquesshowhowcoststhatwereintroducedbyaddingextralevels ofabstraction,maybemovedfromrun-timetocompile-time. Ithasbeencommonknowledgetousersofmodernfunctionallanguagesthat thetypesystemcanbeagreathelpinshorteningprogramsandreducingerrors. Intheextremeonemightseeatypeasapredicatecapturingtheproperties ofanyexpressionwiththattype. InthesixthlectureonCayenne–Spiceup yourProgrammingwithDependentTypesitisshowninwhatdirectionfunctional languagesaremostlikelytodevelop,andwhatmaybeexpectedofthenewtype systemstobeintroduced. Thelastlecture,titledHaskellasanAutomationController,showsthat writingfunctionalprogramsdoesnothavetoimplythatoneisboundtoremain isolatedfromtherestoftheworld. Beingabletocommunicatewithsoftware writtenbyothersinauniformway,isprobablyoneofthemostinteresting newdevelopmentsincurrentcomputerscience. Itappearsthattheconceptofa monadtogetherwiththeHaskelltypingrules,isquiteadequatetodescribethe interfacebetweenHaskellprogramsandtheouterworld. Finallywewanttothankeveryonewhocontributedtothisschoolandmade itsuchasuccessfulevent:sponsors,localsystemmanagers,localorganizers, students,andlastbutnotleastthelecturers. Weareconvincedthateveryone presentattheschoolenjoyedthiseventasmuchaswedid,andweallhopethat youwillfeelsomeofthespiritofthiseventwhenstudyingtheselecturenotes. March1999 DoaitseSwierstra PedroHenriques Jos´eOliveira VII Sponsorship Theschoolhasreceivedgeneroussponsorshipfrom: FCT-Fundac˜¸aoparaaCiˆenciaeTecnologia,Minist´eriodaCiˆenciae Tecnologia AdegaCooperativadePontedeLima AgˆenciaAbreu CGD-CaixaGeraldeDep´ositos CIUM-CentrodeInform´aticadaUniversidadedoMinho DI-DepartamentodeInform´aticadaUniversidadedoMinho GEPL-GrupodeEspeci?cac˜¸aoeProcessamentodeLinguagens LESI-Direc¸c˜aodeCursodeEngenhariadeSistemaseInform´atica Enabler Lactolima Latic´?niosdasMarinhas,Lda NovabasePorto-SistemasdeInforma¸c˜aoSA PrimaveraSoftware ProjectoCamila-GrupodeM´etodosFormais Sidereus-SistemasdeInforma¸c˜aoeConsultoriaInformat´icaLda SIBS-SociedadeInterbanc´ariadeServico¸s VieiradeCastro LocalCommittee: Jos´eAlmeida,Minho Lu´?sBarbosa,Minho Jos´eBarros,Minho M. Joao ˜ Frade,Minho PedroHenriques,Minho F. M´arioMartins,Minho F. LuisNeves,Minho CarlaOliveira,Minho JorgePinto,Lix JorgeRocha,Minho CesarRodrigues,Minho Joa˜oSaraiva,Minho M. Joa˜oVaranda,Minho IX TableofContents SortingMorphisms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 LexAugusteijn 1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 MorphismsonLists. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.