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