How to Write Go Code - go.dev

Go programs are organized into packages. A package is a collection of source files in the same directory that are compiled together. Functions, types, variables ... WhyGo GetStarted Docs Packages Play Blog HowtoWriteGoCode Introduction ThisdocumentdemonstratesthedevelopmentofasimpleGopackageinsidea moduleandintroducesthegotool,thestandardwayto fetch,build,andinstallGomodules,packages,andcommands. Note:ThisdocumentassumesthatyouareusingGo1.13orlaterandthe GO111MODULEenvironmentvariableisnotset.Ifyouarelookingfor theolder,pre-modulesversionofthisdocument,itisarchived here. Codeorganization Goprogramsareorganizedintopackages.Apackageisacollection ofsourcefilesinthesamedirectorythatarecompiledtogether.Functions, types,variables,andconstantsdefinedinonesourcefilearevisibletoall othersourcefileswithinthesamepackage. Arepositorycontainsoneormoremodules.Amoduleisacollection ofrelatedGopackagesthatarereleasedtogether.AGorepositorytypically containsonlyonemodule,locatedattherootoftherepository.Afilenamed go.modtheredeclaresthemodulepath:theimportpath prefixforallpackageswithinthemodule.Themodulecontainsthepackagesin thedirectorycontainingitsgo.modfileaswellassubdirectories ofthatdirectory,uptothenextsubdirectorycontaininganother go.modfile(ifany). Notethatyoudon'tneedtopublishyourcodetoaremoterepositorybeforeyou canbuildit.Amodulecanbedefinedlocallywithoutbelongingtoarepository. However,it'sagoodhabittoorganizeyourcodeasifyouwillpublishit someday. Eachmodule'spathnotonlyservesasanimportpathprefixforitspackages, butalsoindicateswherethegocommandshouldlooktodownloadit. Forexample,inordertodownloadthemodulegolang.org/x/tools, thegocommandwouldconsulttherepositoryindicatedby https://golang.org/x/tools(describedmorehere). Animportpathisastringusedtoimportapackage.Apackage's importpathisitsmodulepathjoinedwithitssubdirectorywithinthemodule. Forexample,themodulegithub.com/google/go-cmpcontainsapackage inthedirectorycmp/.Thatpackage'simportpathis github.com/google/go-cmp/cmp.Packagesinthestandardlibrarydo nothaveamodulepathprefix. Yourfirstprogram Tocompileandrunasimpleprogram,firstchooseamodulepath(we'lluse example/user/hello)andcreateago.modfilethat declaresit: $mkdirhello#Alternatively,cloneitifitalreadyexistsinversioncontrol. $cdhello $gomodinitexample/user/hello go:creatingnewgo.mod:moduleexample/user/hello $catgo.mod moduleexample/user/hello go1.16 $ ThefirststatementinaGosourcefilemustbe packagename.Executablecommandsmustalwaysuse packagemain. Next,createafilenamedhello.goinsidethatdirectorycontaining thefollowingGocode: packagemain import"fmt" funcmain(){ fmt.Println("Hello,world.") } Nowyoucanbuildandinstallthatprogramwiththegotool: $goinstallexample/user/hello $ Thiscommandbuildsthehellocommand,producinganexecutable binary.Ittheninstallsthatbinaryas$HOME/go/bin/hello(or, underWindows,%USERPROFILE%\go\bin\hello.exe). TheinstalldirectoryiscontrolledbytheGOPATH andGOBINenvironment variables.IfGOBINisset,binariesareinstalledtothat directory.IfGOPATHisset,binariesareinstalledto thebinsubdirectoryofthefirstdirectoryin theGOPATHlist.Otherwise,binariesareinstalledto thebinsubdirectoryofthedefaultGOPATH ($HOME/goor%USERPROFILE%\go). Youcanusethegoenvcommandtoportablysetthedefaultvalue foranenvironmentvariableforfuturegocommands: $goenv-wGOBIN=/somewhere/else/bin $ Tounsetavariablepreviouslysetbygoenv-w,usegoenv-u: $goenv-uGOBIN $ Commandslikegoinstallapplywithinthecontextofthemodule containingthecurrentworkingdirectory.Iftheworkingdirectoryisnotwithin theexample/user/hellomodule,goinstallmayfail. Forconvenience,gocommandsacceptpathsrelative totheworkingdirectory,anddefaulttothepackageinthe currentworkingdirectoryifnootherpathisgiven. Soinourworkingdirectory,thefollowingcommandsareallequivalent: $goinstallexample/user/hello $goinstall. $goinstall Next,let'sruntheprogramtoensureitworks.Foraddedconvenience,we'll addtheinstalldirectorytoourPATHtomakerunningbinaries easy: #Windowsusersshouldconsulthttps://github.com/golang/go/wiki/SettingGOPATH #forsetting%PATH%. $exportPATH=$PATH:$(dirname$(golist-f'{{.Target}}'.)) $hello Hello,world. $ Ifyou'reusingasourcecontrolsystem,nowwouldbeagoodtimetoinitialize arepository,addthefiles,andcommityourfirstchange.Again,thisstepis optional:youdonotneedtousesourcecontroltowriteGocode. $gitinit InitializedemptyGitrepositoryin/home/user/hello/.git/ $gitaddgo.modhello.go $gitcommit-m"initialcommit" [master(root-commit)0b4507d]initialcommit 1filechanged,7insertion(+) createmode100644go.modhello.go $ ThegocommandlocatestherepositorycontainingagivenmodulepathbyrequestingacorrespondingHTTPSURLandreadingmetadataembeddedintheHTMLresponse(see gohelpimportpath). Manyhostingservicesalreadyprovidethatmetadataforrepositoriescontaining Gocode,sotheeasiestwaytomakeyourmoduleavailableforotherstouseis usuallytomakeitsmodulepathmatchtheURLfortherepository. Importingpackagesfromyourmodule Let'swriteamorestringspackageanduseitfromthehelloprogram. First,createadirectoryforthepackagenamed $HOME/hello/morestrings,andthenafilenamed reverse.gointhatdirectorywiththefollowingcontents: //PackagemorestringsimplementsadditionalfunctionstomanipulateUTF-8 //encodedstrings,beyondwhatisprovidedinthestandard"strings"package. packagemorestrings //ReverseRunesreturnsitsargumentstringreversedrune-wiselefttoright. funcReverseRunes(sstring)string{ r:=[]rune(s) fori,j:=0,len(r)-1;i