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Hack Emacs - The REPL.org

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Overview of Programming with Emacs

  • Advantages
    • Extremely broad language support
    • Excellent generic text editing capabilities
    • Works in a console and in a windowing system
    • Easily programmable
    • Extremely good version control system support (see magit)
  • Disadvantages
    • Not designed to semantically analyze the code on-the-fly
    • Weak when it comes to things like e.g. “Intellisense”

Addressing the disadvantages

  • Time honored approach: the REPL
  • Allows for development of large programs in small, testable pieces

The REPL

  • Stands for the “Read-Eval-Print Loop”
  • Very intuitive
  • Broad language support
    • Emacs Lisp (see “M-x ielm”)
    • Python
    • Ruby
    • Scala
    • Common Lisp (see SLIME)
    • Clojure (has SLIME support)
    • Javascript (see Rhino)
    • Perl (see Sepia)
    • R
    • Mathematica
    • Lua
    • …

Using the REPL

There are two very simple ways to use a REPL:

  1. Write code at the REPL, and when you get it working, paste it into your buffer in Emacs so you can save it
  2. Write code in your buffer and paste it in the REPL to test it. When it breaks, go back to the buffer and edit it and try again.

These are good ways to use a REPL, but they wear on you, so various languages and/or modes have added convenience functions that make using the REPL faster and easier. We’ll look at two today:

  1. Emacs Lisp
    1. “Embedded” REPL
    2. IELM
  2. Scheme

Final note: not all REPLs are created equal

The vary in their implementation

  • Perl and Scala don’t have real interpreters
  • SLIME-based implementations have servers running in the native language (lisp) that communicate with Emacs over a socket
  • Simple REPLs can use a comint-mode derivative with same basic mode-level functions to dispatch code to the buffer running the interpreter subprocess
    • This is how most first-cuts at a REPLs are implemented for “new” languages