export GOBIN=$HOME/gopl/bin
export GOPATH=$HOME/gopl
export PATH=$PATH:/$HOME/gopl/bin
go get gopl.io
go build gopl.io/ch1/helloworld
go run gopl.io/ch1/helloworld
go install gopl.io/ch1/helloworld
$GOPATH/bin/helloworld
go work init src/ch1
go work use src/ch2
go run module_name