proposal: cmd/go: conditional/optional dependency for go mod

[dfang]

What version of Go are you using (go version)?

$ go version
go version go1.12.1 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/mj/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/mj/go"
GOPROXY="https://goproxy.io"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.12.1/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.12.1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/xb/dsd0_2b92x7bsl92xlzj6fbm0000gn/T/go-build051189324=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

main.go

package main

import (
    "github.com/jinzhu/gorm"
  _ "github.com/jinzhu/gorm/dialects/sqlite"
)

func main() {
  db, err := gorm.Open("sqlite3", "test.db")
  if err != nil {
    panic("failed to connect database")
  }
  defer db.Close()
}

go mod init gorm-demo
go mod tidy
go mod graph

when you run go mod graph, you can see so many dependencies,

Output

github.com/jinzhu/gorm@v1.9.8 github.com/denisenkom/go-mssqldb@v0.0.0-20190423183735-731ef375ac02
github.com/jinzhu/gorm@v1.9.8 github.com/erikstmartin/go-testdb@v0.0.0-20160219214506-8d10e4a1bae5
github.com/jinzhu/gorm@v1.9.8 github.com/go-sql-driver/mysql@v1.4.1
github.com/jinzhu/gorm@v1.9.8 github.com/jinzhu/inflection@v0.0.0-20180308033659-04140366298a
github.com/jinzhu/gorm@v1.9.8 github.com/jinzhu/now@v1.0.0
github.com/jinzhu/gorm@v1.9.8 github.com/lib/pq@v1.1.0
github.com/jinzhu/gorm@v1.9.8 github.com/mattn/go-sqlite3@v1.10.0
just for mssql
github.com/denisenkom/go-mssqldb@v0.0.0-20190423183735-731ef375ac02 cloud.google.com/go@v0.37.4

github.com/denisenkom/go-mssqldb@v0.0.0-20190423183735-731ef375ac02 golang.org/x/crypto@v0.0.0-20190325154230-a5d413f7728c

github.com/denisenkom/go-mssqldb@v0.0.0-20190423183735-731ef375ac02 gopkg.in/check.v1@v1.0.0-20180628173108-788fd7840127

github.com/denisenkom/go-mssqldb@v0.0.0-20190423183735-731ef375ac02 gopkg.in/yaml.v2@v2.2.2
just for cloud.google.com/go
cloud.google.com/go@v0.37.4 github.com/golang/mock@v1.2.0

cloud.google.com/go@v0.37.4 honnef.co/go/tools@v0.0.0-20190106161140-3f1c8253044a

cloud.google.com/go@v0.37.4 google.golang.org/grpc@v1.19.0

cloud.google.com/go@v0.37.4 google.golang.org/genproto@v0.0.0-20190404172233-64821d5d2107

cloud.google.com/go@v0.37.4 google.golang.org/api@v0.3.1

cloud.google.com/go@v0.37.4 golang.org/x/tools@v0.0.0-20190312170243-e65039ee4138

cloud.google.com/go@v0.37.4 golang.org/x/time@v0.0.0-20181108054448-85acf8d2951c

cloud.google.com/go@v0.37.4 golang.org/x/text@v0.3.1-0.20180807135948-17ff2d5776d2

cloud.google.com/go@v0.37.4 golang.org/x/sync@v0.0.0-20190227155943-e225da77a7e6

cloud.google.com/go@v0.37.4 golang.org/x/oauth2@v0.0.0-20190226205417-e64efc72b421

cloud.google.com/go@v0.37.4 golang.org/x/lint@v0.0.0-20190301231843-5614ed5bae6f

cloud.google.com/go@v0.37.4 golang.org/x/exp@v0.0.0-20190121172915-509febef88a4

cloud.google.com/go@v0.37.4 go.opencensus.io@v0.20.1

cloud.google.com/go@v0.37.4 github.com/jstemmer/go-junit-report@v0.0.0-20190106144839-af01ea7f8024

cloud.google.com/go@v0.37.4 github.com/googleapis/gax-go/v2@v2.0.4

cloud.google.com/go@v0.37.4 github.com/google/pprof@v0.0.0-20181206194817-3ea8567a2e57

cloud.google.com/go@v0.37.4 github.com/google/martian@v2.1.0+incompatible

cloud.google.com/go@v0.37.4 github.com/google/go-cmp@v0.2.0

cloud.google.com/go@v0.37.4 github.com/google/btree@v0.0.0-20180813153112-4030bb1f1f0c

cloud.google.com/go@v0.37.4 github.com/golang/protobuf@v1.2.0
and many more ....

What did you expect to see?

what if i only use postgres db, please don't install mssql, sqlite dependencies for me.

i'm not sure this feature calls, maybe conditional dependencies ?

this pattern is common in some library that as a provider .....

What did you see instead?

a huge dependencies tree

Extra information (FYI)

i was a ruby on rails developer, as my experience, package management in node.js is sucking, big node_modules, installed failed, unmet dependencies ...

however in ruby world, package management (bundler) is nearly perfect, maybe go mod designer can learn something from that

https://bundler.io/man/gemfile.5.html#INSTALL_IF
https://bundler.io/v1.10/whats_new.html#install-if
https://bundler.io/guides/groups.html