Merge pull request #1694 from jsyeo/feature-edn

[edn/en] Add a short tutorial for EDN

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+---
+language: edn
+filename: learnedn.edn
+contributors:
+  - ["Jason Yeo", "https://github.com/jsyeo"]
+---
+
+Extensible Data Notation or EDN for short is a format for serializing data.
+
+The notation is used internally by Clojure to represent programs and it also
+used as a data transfer format like JSON. Though it is more commonly used in
+Clojure land, there are implementations of EDN for many other languages.
+
+The main benefit of EDN over JSON and YAML is that it is extensible, which we
+will see how it is extended later on.
+
+```Clojure
+; Comments start with a semicolon.
+; Anythng after the semicolon is ignored.
+
+;;;;;;;;;;;;;;;;;;;
+;;; Basic Types ;;;
+;;;;;;;;;;;;;;;;;;;
+
+nil         ; also known in other languages as null
+
+; Booleans
+true
+false
+
+; Strings are enclosed in double quotes
+"hungarian breakfast"
+"farmer's cheesy omelette"
+
+; Characters are preceeded by backslashes
+\g \r \a \c \e
+
+; Keywords start with a colon. They behave like enums. Kind of
+; like symbols in Ruby.
+:eggs
+:cheese
+:olives
+
+; Symbols are used to represent identifiers. They start with #.
+; You can namespace symbols by using /. Whatever preceeds / is
+; the namespace of the name.
+#spoon
+#kitchen/spoon ; not the same as #spoon
+#kitchen/fork
+#github/fork   ; you can't eat with this
+
+; Integers and floats
+42
+3.14159
+
+; Lists are sequences of values
+(:bun :beef-patty 9 "yum!")
+
+; Vectors allow random access
+[:gelato 1 2 -2]
+
+; Maps are associative data structures that associates the key with its value
+{:eggs        2
+ :lemon-juice 3.5
+ :butter      1}
+
+; You're not restricted to using keywords as keys
+{[1 2 3 4] "tell the people what she wore",
+ [5 6 7 8] "the more you see the more you hate"}
+
+; You may use commas for readability. They are treated as whitespaces.
+
+; Sets are collections that contain unique elements.
+#{:a :b 88 "huat"}
+
+;;;;;;;;;;;;;;;;;;;;;;;
+;;; Tagged Elements ;;;
+;;;;;;;;;;;;;;;;;;;;;;;
+
+; EDN can be extended by tagging elements with # symbols.
+
+#MyYelpClone/MenuItem {:name "eggs-benedict" :rating 10}
+
+; Let me explain this with a clojure example. Suppose I want to transform that
+; piece of edn into a MenuItem record.
+
+(defrecord MenuItem [name rating])
+
+; To transform edn to clojure values, I will need to use the built in EDN
+; reader, edn/read-string
+
+(edn/read-string "{:eggs 2 :butter 1 :flour 5}")
+; -> {:eggs 2 :butter 1 :flour 5}
+
+; To transform tagged elements, define the reader function and pass a map
+; that maps tags to reader functions to edn/read-string like so
+
+(edn/read-string {:readers {'MyYelpClone/MenuItem map->menu-item}}
+                 "#MyYelpClone/MenuItem {:name \"eggs-benedict\" :rating 10}")
+; -> #user.MenuItem{:name "eggs-benedict", :rating 10}
+
+```
+
+# References
+
+- [EDN spec](https://github.com/edn-format/edn)
+- [Implementations](https://github.com/edn-format/edn/wiki/Implementations)
+- [Tagged Elements](http://www.compoundtheory.com/clojure-edn-walkthrough/)