Motivation

To add extensibility to the Clojure reader in a composable way.

General Goals

• new syntax for "tagged" literals in the reader
• tags can be namespaced
• non-namespace-qualified tags are reserved for Clojure
• when the reader encounters a tagged literal, it invokes a function associated with that tag
• users may define new tags and tag-reader functions
• users may override functions for built-in tags

Non-goals

• Common Lisp-style reader macros
• Custom parsing of the character stream
  • Reader will parse tagged literals as data and pass that to the tag function

Open Questions

• What is the syntax for tags?
  • Keywords, like #:tag form
    • One character longer
    • Slightly easier to parse
  • Symbols, like #tag form
    • One character shorter
    • Slightly harder to parse
    • Ambiguous with record constructors?
      • Not if we assume that: 
        • 1) record names always contain a dot
          • Which they must, because deftypes occur in namespaces
        • 2) user-defined tags are always namespace-qualified
        • 3) built-in tags do not contain a dot
• What happens when the reader encounters a tag for which no function is defined?
  • Error (YES)
  • Return form unchanged?
  • Return form with extra metadata?
    • Only possible if form supports metadata, ignore otherwise?

• How to associate tags with reader functions?
  • file containing mapping from tags to Vars
    • "magic" file name gets loaded before other files
    • doesn't load any code directly
    • If 2 JARs on classpath have a magic file, who wins?
      • throw an exception
  • map stored in dynamic var
    • thread-local binding for each source file

      • created in Compiler.load and Compiler.compile

    • can be set! within a source file
    • can be rebound when invoking the reader at runtime

References / See Also

• CLJ-890
• Clojure Instant Literal & Library