How can load a .el configure file with a specified mode in emacs - emacs

Question

Usually, I put the confugire .el files in src directory for all kinds of languages. Such as Go, the go-conf.el file:
(add-hook 'before-save-hook 'gofmt-before-save)
(add-hook 'go-mode-hook (lambda ()
(local-set-key (kbd "M-.") 'godef-jump)))
(add-hook 'go-mode-hook (lambda ()
(local-set-key (kbd "M-,") 'godef-jump-back)))
(add-to-list 'load-path "/usr/local/go/src/github.com/dougm/goflymake")
(add-hook 'after-init-hook #'global-flycheck-mode)
(require 'flycheck)
(require 'go-autocomplete)
(require 'auto-complete-config)
(ac-config-default)
)
(provide 'go-conf)
Then, in init.el, I write this line
(require 'go-conf)
Although go-conf can be loaded successfully, emacs launches slowly. It is because that emacs loads go-conf whatever files are opened. I can not tolerate it.
It is better that only when Go file is opened, go-conf is loaded.
I modify the init.el as :
(add-hook 'go-mode-hook '(lambda ()
(require 'go-conf)
(go-conf)
))
But it does not work!!
who can help me?

Answer 1

Your code seems to assume that the whole Emacs only has a single buffer and mode, whereas that is not the case. E.g. (add-hook 'before-save-hook 'gofmt-before-save) affects all buffers, whether they're using go-mode or not. Same for (add-hook 'after-init-hook #'global-flycheck-mode). Emacs is designed such that you can start it once and then edit hundreds of different files at the same time in that one Emacs session. So you should probably rewrite your code along the lines of:
(defun my-go-lang-config ()
(add-hook 'before-save-hook #'gofmt-before-save nil 'local)
(local-set-key (kbd "M-.") 'godef-jump)
(local-set-key (kbd "M-,") 'godef-jump-back)
(add-to-list 'load-path "/usr/local/go/src/github.com/dougm/goflymake")
(require 'go-autocomplete))
(add-hook 'go-mode-hook #'my-go-lang-config)
(require 'auto-complete-config)
(ac-config-default)
(global-flycheck-mode 1)
where the last three lines are part of your "generic" config (not specific to support for the Go language), meaning that you want to use flycheck and auto-complete whenever it's available rather than only in go-mode.

Answer 2

Your code to add to the hook doesn't work because the hook is run only after the mode is turned on, and the mode is not defined until the library is loaded. It makes no sense to load the same library in the mode hook.
If Emacs becomes slow after loading some library, it is probably due to that library. Is is slow after loading the library even if you do not turn the mode on?
You can try byte-compiling the library code. That can sometimes make a big difference in performance. You can use M-x byte-compile to compile a given file.
If compiling does not help, and if you do not seen anything suspect in buffer *Messages* (e.g., warnings that seem like they might be pertinent), then consider contacting the library maintainer, reporting the problem and asking for a remedy.

Answer 3

If go-mode itself is already available (most likely loaded on demand via an addition to auto-mode-alist, which is probably taken care of automatically if it was installed as an ELPA package), and you're just looking to load your custom library at the same time, then you can use eval-after-load:
(eval-after-load 'go-mode
'(require 'go-conf))
Make sure that the parent directory for your go-conf.el library is in the load-path, of course, otherwise require won't find it.