An Emacs Helm extension that retrieves citations from the ADS abstract service and inserts a \cite{...} command in the buffer.

helm-ads-extension.el

;;
;; An Emacs Helm extension that retrieves citations from the ADS abstract service 
;; and inserts a \cite{...} command in the buffer. The citation key is the bibcode
;; returned by ADS.
;;
;; To use, call (helm-ads)
;;

(require 'url)
(require 'json)

(setq ads-token "YOUR ADS ABSTRACT SERVICE TOKEN")

(defun join-strings (list-of-strings separator)
  (mapconcat #'identity list-of-strings separator))

(defun background-color ()
  (face-attribute 'default :background))

(defun ads-entry-string (doc)
  `(
    ,(format "%s\t%s\t%s\n'%s'"
             (gethash "year" doc)
             (join-strings (gethash "author" doc) ", ")
             ;; Include the pattern in the result so Helm displays
             ;; all our results. Use background color so it's
             ;; invisible, at least when it's not selected.
             (propertize (format "(pattern: %s)" helm-pattern) 'font-lock-face
                         `(:foreground ,(background-color)))
             (join-strings (gethash "title" doc) " "))
    . ,(gethash "bibcode" doc)
    )
  )

(defun ads-search-query (query-str)
  (let ((a-url (format "https://api.adsabs.harvard.edu/v1/search/query?q=%s&fl=bibcode,year,author,title" query-str))
        (url-request-method "GET")
        (url-request-extra-headers
         `(("Authorization" . ,(format "Bearer %s" ads-token))))
        (json-object-type 'hash-table)
        )

    (let ((json-response 
           (with-current-buffer
               (url-retrieve-synchronously a-url)
             (goto-char url-http-end-of-headers)
             (json-read)))
          )
      (let ((docs (gethash "docs" (gethash "response" json-response))))
        (mapcar 'ads-entry-string docs)
      ))))

(defun helm-ads-search ()
  (let ((result (ads-search-query helm-pattern)))
    (if (eq (length result) 0) `((,(format "No results found for pattern: %s" helm-pattern) . "")) result)
  ))

(setq helm-source-ads-citation-search
  '((name . "ADS Abstract Service")
    (volatile)
    (delayed)
    (multiline)
    (requires-pattern . 2)
    (candidates . helm-ads-search)
    (action . (lambda (bibcode) (insert (format "\\cite{%s}" bibcode))))
  ))

(defun helm-ads ()
  "Bring up an ADS search interface in helm. Use this to add citations."
  (interactive)
  (let ((helm-input-idle-delay 0.5))
    (helm :sources '(helm-source-ads-citation-search)
          :buffer "*helm-ads*"))
  )

;; Example keybinding
(add-hook 'LaTeX-mode-hook
	  (lambda () (local-set-key (kbd "C-c C-c") 'helm-ads)))