Automatic package install is not available, package exists

Question

I want to automatically install packages that I don't have locally installed when I initialize Emacs. I use the following code to install some packages.

(package-initialize)
(mapc #'(lambda (pkg)
           (if (not (package-installed-p pkg))
                     (package-install pkg)))
            (list
             'json-mode
             ; ...

However, when loading Emacs, Emacs says the package is not available for installation. However, it exists on the packages buffer.

error: Package `json-mode' is not available for installation

Does anyone know what I am doing wrong?

Answer 1

You most likely need a package-refresh-contents call to download the list of available packages first:

(defvar my-useful-packages
  '(ace-jump-mode ace-jump-buffer ... ))

(defun my-check-and-maybe-install (pkg)
  "Check and potentially install `PKG'."
  (when (not (package-installed-p pkg))
    (when (not (require pkg nil t))
      (package-install pkg))))

(defun my-packages-reset()
  "Reset package manifest to the defined set."
  (interactive)
  (package-refresh-contents)
  (mapc 'my-check-and-maybe-install my-useful-packages))