成功删除stringutil.a,但会自动返回

I'm following "How to Write Go Code" and tries to delete stringutil.a under $GOPATH/pkg/darwin_amd64/github.com/user. The delete is successful but the file comes back on its own. I'm confused. What is happening?

zps-MacBook-Air:haibin haibin$ rm stringutil.a
zps-MacBook-Air:haibin haibin$ ls -lah
total 0
drwxr-xr-x   2 haibin  staff    68B Feb 15 00:57 .
drwxr-xr-x  17 haibin  staff   578B Feb 15 00:39 ..
zps-MacBook-Air:haibin haibin$ ls -lah
total 8
drwxr-xr-x   3 haibin  staff   102B Feb 15 00:57 .
drwxr-xr-x  17 haibin  staff   578B Feb 15 00:39 ..
-rw-r--r--   1 haibin  wheel   2.4K Feb 15 00:57 stringutil.a

You need to delete the stringutil.go source file that is under the src tree. The *.a file is a binary file which results from the compilation (and possibly linking) of source files.