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Reader Literals in Clojure

Austin Clojure Meetup

Hosted by Capital Factory

Monday, February 4th, 2013

Presenter: Steve Suehs, Principal Engineer, Datical

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The Reader

  • Parses stream -> Data

which also gives us

  • Parses stream -> Code

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Reader Literals

  • allows extension of reader without allowing change of the language
  • contrasts with other LISP's reader extensions, which could change semantics

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Reader Literals

#foo/bar form
#foo/bar [1 2 3]
  • magic sequence that the reader will map to a function and invoke
    • namespaced with slash
    • root space reserved for Clojure
  • reader will pass next form to that function

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Reader Literals (cont'd)

#foo/bar [1 2 3]
#nascan/file "archives/2012/myblog.zip"
#nascan/file { :path "archives/2012/myblog.zip"  
                :size 12345678
                :scantime  223048}
#nas.File {...} ;; note no slash! 
           ;;while a reader literal, 
           ;; used by Clojure for objects and classes.

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Why?

S expressions are so powerful, why not use functions?

The difference is subtle but important. With the reader literals we can represent a data object and not rely on executable code, which means we can use it to operate with other systems or map to other implementations in a different context.

The reader literals also mean we can change the function that the reader will call with the next form.

The date-time instance is a good example of both.

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Reader Literals has roots in EDN

or maybe the other way around.

  • EDN = Extensible Data Notation
  • Used by Clojure and Datomic
  • Intended to be used by other languages
  • Looks like it is extracted from Clojure

If you took the programming out of Clojure and just left the data parts, it would look like EDN.

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Default Data Readers

user=> default-data-readers
{inst #'clojure.instant/read-instant-date, 
 uuid #'clojure.uuid/default-uuid-reader}

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Let us Code

Fire up a repl and play.

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nastimer example

I was having a problem with a disk usage program and my home NAS so I played around with literals.

  • If the data_readers.clj exists, it must have at least an empty map, or reader will fail, and repl may not start.

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Possible Pitfalls

Clojure searches for data_readers.clj at the "root of the classpath".

I suspect this will be perilous, but time will tell.

Counter examples:

  • hibernate configuration pain
  • memcache configuration pain
  • singletons not

Worth digging into the reader internals and proposing a solution.

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This Presentation on Github

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Research Links

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