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d86597d499
- Change Clojure code to eliminate errors and to match commented output > Change edn/read-string to clojure.edn/read-string (no _require_ in text) > Change map->menu-item to map->MenuItem to match defrecord and output - Modify the text to make it easier to understand the given example
116 lines
3.2 KiB
Clojure
116 lines
3.2 KiB
Clojure
---
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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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- ["Jonathan D Johnston", "https://github.com/jdjohnston"]
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---
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Extensible Data Notation (EDN) is a format for serializing data.
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EDN is a subset of the syntax used by Clojure. Reading data defined by EDN is
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safer than that defined by the full Clojure syntax, especially from untrusted
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sources. EDN is restricted to data, no code. It is similar in intent to JSON.
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Though it is more commonly used in Clojure, there are implementations of EDN
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for many other languages.
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The main benefit of EDN over JSON and YAML is that it is extensible. 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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; Anything 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 preceded 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.
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; You can namespace symbols by using /. Whatever precedes / is
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; the namespace of the symbol.
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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 associate 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 whitespace.
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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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; defrecord defined, among other things, map->MenuItem which will take a map
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; of field names (as keywords) to values and generate a user.MenuItem record
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; To transform EDN to Clojure values, I will need to use the built-in EDN
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; reader, clojure.edn/read-string
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(clojure.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, pass to clojure.edn/read-string an option map
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; with a :readers map that maps tag symbols to data-reader functions, like so
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(clojure.edn/read-string
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{:readers {'MyYelpClone/MenuItem map->MenuItem}}
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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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