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Evalator is a new kind of REPL for Emacs.

What makes it different from a normal REPL? With evalator you can:

  • Evaluate expressions using any language*
  • See the results of an expression as it is typed
  • Transform data in a stepwise fashion
  • Generate code
  • Interact with Emacs using a language other than Elisp

* Provided you've installed the appropriate extension package.

Language Support

By default, evalator uses Elisp as its evaluation language, but it supports other languages through extension packages. These packages define an evaluation context that can be passed to the evalator command in order to set the evaluation language for that particular session.

Currently available evaluation contexts

Don't see a context package for your favorite language? Write one! See evalator-elisp.el and evalator-clojure.el for examples. Open an issue in this repo with any questions and I'll be happy to help.

Here's a practical example to demonstrate what evalator's capable of:

A Practical Example

Suppose you have several Emacs packages and want to generate an elisp configuration file for each one in your current directory. Each file should be named setup-$PACKAGE.el with $PACKAGE equaling the package name. Each file should contain the following line.

(provide 'setup-$PACKAGE)

Watch the example below to see how easy this is with evalator.


Here's what happened:

  • Copy all package names
  • Call M-x evalator to start an evalator session
  • Paste the package names into a lisp list '()
  • Build a string for the shell command I want executed for each package
  • Call the Emacs command shell-command with each of the strings from before
  • Exit evalator using C-g
  • Verify the files are created with the right contents

See the Basic Usage section for a detailed walkthrough.


Evalator is available through the MELPA repository. You can install it by using Emacs's built-in package manager package.el.

If you haven't configured package.el, see here for a short guide on setting it up to use MELPA.

Once configured, you can run M-x package-install then enter evalator to install evalator.



Add the following to your init file to setup key bindings for the primary evalator commands.

;;Suggested keybindings
(global-set-key (kbd "C-c e e") 'evalator)
(global-set-key (kbd "C-c e x") 'evalator-explicit)
(global-set-key (kbd "C-c e r") 'evalator-resume)
(global-set-key (kbd "C-c e i") 'evalator-insert-equiv-expr)

Auto Detect Context###

If you've installed packages that provide other evalaution context's for evalator, you may want it to automatically detect what evalaution context to use. For example, if you're in a buffer using ruby-mode then you'd like your evalator session to use Ruby. Likewise, if you're in a buffer using clojure-mode then use Clojure.

(setq evalator-config-mode-context-alist nil)
(add-to-list 'evalator-config-mode-context-alist '(ruby-mode . evalator-ruby-context))
(add-to-list 'evalator-config-mode-context-alist '(clojure-mode . evalator-clojure-context))

evalator-config-mode-context-alist is an association list(alist) of pairs with the following form:


evalator uses this alist in determining what context to use for a session. If a context wasn't passed explicity in a call to evalator or evalator-explicit, then the current buffer's major-mode is searched for within the alist. If an entry is found, the context associated with that major-mode is used for the session. Otherwise an elisp evaluation context is used.

Basic Usage


Basic Usage

Help is available anytime in an evalator session by running C-h m. This will open a buffer containg help for evalator followed by help for helm.

Start evalator by typing M-x evalator. A helm buffer will be opened with a prompt to enter an expression. Enter a valid expression to generate the initial "candidates". A candidate is simply a result that is displayed. We call them candidates to stay consistent with helm, which evalator uses as a front-end. Once you've entered a valid expression press RET to confirm your expression.

Evalator provides immediate feedback by attempting to evaluate your expression as it is typed. As soon as you've entered a valid expression the candidates are updated to show what their new value will be if you confirm your expression. Your expression prompt also will change color from red to green to indicate its validity.

Obviously you can only confirm your expression when the expression is valid. If you try confirming an invalid expression the error message is displayed in the echo area.

You can further transform the candidates you've generated by typing more expressions. Subsequent expressions are executed on each candidate. That is evalator's behavior in normal mode. You may refer to the current candidate by inserting the special arg character into your expression by pressing C-c ;.

Modes and special args will be explained in more detail in the following sections.


You can start an evalator session with either M-x evalator or M-x evalator-explicit. These commands start an evalator session in either normal mode or explicit mode, respectively.

To explain the difference between these two modes, we first need to define what a "candidate" is. If evalator was started in normal mode (M-x evalator) and the result of an expression is an atomic value like an integer or string, then the result will be displayed as a single candidate. If the result is a collection like a list or vector, then each of the elements of that collection are displayed as a separate candidate.

If evalator was started in explicit mode (M-x evalator-explicit) then the result is always displayed as a single candidate. One benefit to using explicit mode is that you can generate the equivalent code of a session using M-x evalator-insert-equiv-expr and insert that into a buffer*.

So how do you choose what mode to use?

Use normal mode if you want to transform collection data and you don't care about code generation.

Use explicit mode otherwise.

* The implementation of evalator-insert-equiv-expr is pretty naive currently so it's output may be sub-optimal if you used numbered special args like Ⓔ0 in any of your expressions.

Special Args

When writing an expression in evalator, you use a special arg to refer to a particular candidate. You can easily insert this arg into the the expression prompt by pressing C-c ;, which is the key binding for evalator-action-insert-special-arg. This arg is the character by default. If the last element was a collection then adding an integer N after the special arg, causes the Nth element from that candidate to be substituted.

Here's what you would add to make the elisp context use @ as its special arg:

(setq evalator-elisp-special-arg "@")

Or you could change the default special arg globally with the following:

(setq evalator-context-special-arg-default "@") 

Public API

The commands below can be bound to a shortcut or executed using M-x

evalator - Starts an evalator session

evalator-explicit - Starts an evalator session in explicit mode

evalator-resume - Resumes last evalator session

evalator-insert-equiv-expr - Inserts the equivalent expression of the last evalator session into the current buffer. NOTE: The last session must have been run in explicit mode for this to work.

Key Actions

Below is a table of evalator specific key actions that can be used within the evalator session. For helm specific commands such as candidate navigation and marking, refer to helm's documentation or run C-h m to open a help buffer that contains evalator help followed by helm help.

Shortcut Command and Description
RET uevalator-action-confirm-make-or-transformu

Accepts initial candidates or transformation then updates history. If transforming candidates, the expression is evaluated on each candidate with the special arg referring to the candidate's value
C-c ; C-; evalator-action-insert-special-arg

Insert's the current evaluation context's special arg.
C-c C-c evalator-action-confirm-transform-collection

Accepts transformation and updates history. Expression is evaluated once awith the special arg referring to the selected candidates list
C-c C-e evalator-action-execute-in-elisp

Executes the expression in Elisp on each selected candidate. The history is not updated and the candidates are not transformed
C-j evalator-action-next

Goes to next history state
C-l evalator-action-previous

Goes to previous history state

You can exit the evalator session by quitting the minibuffer. This command is usually bound to C-g.


Emacs package for interactive evaluation and transformation of data.



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