82 lines
3.2 KiB
Org Mode
82 lines
3.2 KiB
Org Mode
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* [[https://newbedev.com/how-to-hook-emacs-up-to-a-json-service][How to hook emacs up to a json service? | Newbedev]] :website:
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[2021-12-18 Sat 14:37]
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** Article
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*** How to hook emacs up to a json service?
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Emacs has pretty good built-in support for this kind of thing, at least at the
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basic level of making requests and parsing responses. Use =url-retrieve= or
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=url-retrieve-synchronously= to fetch data from a remote service. As the names
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suggest, one is an asynchronous call which takes a callback, the other a
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blocking call which returns a buffer containing the response. Load them by
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including =(require 'url)= in your Elisp file.
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The =url-= functions are documented in a separate Info manual from the rest of
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Elisp, named "URL", but some useful features go unmentioned there. For GET
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requests where arguments are passed in the URL, the =url-build-query-string=
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function is useful for constructing a string of query parameters from an a-list
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of keys and values. For POST, PUT, DELETE and other requests, you may need to
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let-bind the variables =url-request-data=, =url-request-method= and
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=url-request-extra-headers=. They have informative docstrings.
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A final confusing thing about using these calls for HTTP URLs is that they
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leave the HTTP response headers in the same buffer as the response body, which
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may not be what you expect. A simple way to deal with this is to use the
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(undocumented) =url-http-end-of-headers= variable to skip over the headers before
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processing the body, though I suspect there might be better ways.
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Use the =json-read= function to parse JSON responses, and bind the variables
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=json-array-type=, =json-object-type=, and =json-key-type= to control how JSON types
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are converted to Lisp types. This function is obtained by including =(require
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'json)=. XML responses can be parsed using =xml-parse-region= or
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=libxml-xml-parse-region=. The latter requires Emacs to be compiled with libxml
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support, the former is implemented in Elisp.
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Putting this together, the skeleton of a request to a JSON service looks
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something like this:
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#+begin_example
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(url-retrieve
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"http://example.com/api/some/request"
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(lambda (events)
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(goto-char url-http-end-of-headers)
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(let ((json-object-type 'plist)
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(json-key-type 'symbol)
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(json-array-type 'vector))
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(let ((result (json-read)))
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;; Do something with RESULT here
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))))
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#+end_example
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In the callback, you can work with =result= like any other Lisp value. The
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=plist-get=, =pcase= and =cl-destructuring-bind= functions are often useful for
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extracting pieces of the result. Use =(require 'pcase)= to get the =pcase= macro,
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=(require 'cl-lib)= to get =cl-destructuring-bind=.
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\\
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This is what I use to lookup currency exchange rates from a json webservice at
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rate-exchange.appspot.com:
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#+begin_example
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(defun my-json-get (url)
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(interactive)
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(progn
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(require 'json)
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(with-current-buffer (url-retrieve-synchronously url)
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(goto-char (point-min))
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(re-search-forward "^$")
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(json-read))))
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(defun my-currency-exchange-rate (from to)
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(let ((xurl (format "http://rate-exchange.appspot.com/currency?from=%s&to=%s" from to)))
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(assoc-default 'rate (my-json-get xurl))))
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#+end_example
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Example usage:
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#+begin_example
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(my-currency-exchange-rate "USD" "SEK")
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#+end_example
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