macrostep: interactive macro-expander

macrostep is an Emacs minor mode for interactively stepping through the expansion of macros in Emacs Lisp source code. It lets you see exactly what happens at each step of the expansion process by pretty-printing the expanded forms inline in the source buffer, which is temporarily read-only while macro expansions are visible. You can expand and collapse macro forms one step at a time, and evaluate or instrument the expansions for debugging with Edebug as normal (but see “Bugs and known limitations”, below). Single-stepping through the expansion is particularly useful for debugging macros that expand into another macro form. These can be difficult to debug with Emacs’ built-in macroexpand, which continues expansion until the top-level form is no longer a macro call.

Both globally-visible macros as defined by defmacro and local macros bound by (cl-)macrolet or another macro-defining form can be expanded. Within macro expansions, calls to macros and compiler macros are fontified specially: macro forms using macrostep-macro-face, and functions with compiler macros using macrostep-compiler-macro-face. Uninterned symbols (gensyms) are fontified based on which step in the expansion created them, to distinguish them both from normal symbols and from other gensyms with the same print name.

As of version 0.9, it is also possible to extend macrostep to work with other languages with macro systems in addition to Emacs Lisp. An extension for Common Lisp (via SLIME) is in the works; contributions for other languages are welcome. See “Extending macrostep” below for details.

Key-bindings and usage

The standard keybindings in macrostep-mode are the following:

e, =, RET

expand the macro form following point one step

c, u, DEL

collapse the form following point

q, C-c C-c

collapse all expanded forms and exit macrostep-mode

n, TAB

jump to the next macro form in the expansion

p, M-TAB

jump to the previous macro form in the expansion

It’s not very useful to enable and disable macrostep-mode directly. Instead, bind macrostep-expand to a key in emacs-lisp-mode-map, for example C-c e:

(define-key emacs-lisp-mode-map (kbd "C-c e") 'macrostep-expand)

You can then enter macrostep-mode and expand a macro form completely by typing C-c e e e ... as many times as necessary.

Exit macrostep-mode by typing q or C-c C-c, or by successively typing c to collapse all surrounding expansions.

Customization options

Type M-x customize-group RET macrostep RET to customize options and faces.

To display macro expansions in a separate window, instead of inline in the source buffer, customize macrostep-expand-in-separate-buffer to t. The default is nil. Whichever default behavior is selected, the alternative behavior can be obtained temporarily by giving a prefix argument to macrostep-expand.

To have macrostep ignore compiler macros, customize macrostep-expand-compiler-macros to nil. The default is t.

Customize the faces macrostep-macro-face, macrostep-compiler-macro-face, and macrostep-gensym-1 through macrostep-gensym-5 to alter the appearance of macro expansions.

Locally-bound macros

As of version 0.9, macrostep can expand calls to a locally-bound macro, whether defined by a surrounding (cl-)macrolet form, or by another macro-defining macro. In other words, it is possible to expand the inner local-macro forms in both the following examples, whether local-macro is defined by an enclosing cl-macrolet –

(cl-macrolet ((local-macro (&rest args)
                `(expansion of ,args)))
  (local-macro (do-something)))

– or by a macro which expands into cl-macrolet, provided that its definition of macro is evaluated prior to calling macrostep-expand:

(defmacro with-local-macro (&rest body)
  `(cl-macrolet ((local-macro (&rest args)
                   `(expansion of ,args)))
     ,@body))

(with-local-macro
    (local-macro (do something (else)))

See the with-js macro in Emacs’s js.el for a real example of the latter kind of macro.

Expansion of locally-bound macros is implemented by instrumenting Emacs Lisp’s macro-expander to capture the environment at point. A similar trick is used to detect macro- and compiler-macro calls within expanded text so that they can be fontified accurately.

Expanding sub-forms

By moving point around in the macro expansion using macrostep-next-macro and macrostep-prev-macro (bound to the n and p keys), it is possible to expand other macro calls within the expansion before expanding the outermost form. This can sometimes be useful, although it does not correspond to the real order of macro expansion in Emacs Lisp, which proceeds by fully expanding the outer form to a non-macro form before expanding sub-forms.

The main reason to expand sub-forms out of order is to help with debugging macros which programmatically expand their arguments in order to rewrite them. Expanding the arguments of such a macro lets you visualise what the macro definition would compute via macroexpand-all.

Extending macrostep for other languages

Since version 0.9, it is possible to extend macrostep to work with other languages besides Emacs Lisp. In typical Emacs fashion, this is implemented by setting buffer-local variables to different function values. Six buffer-local variables define the language-specific part of the implementation:

• macrostep-sexp-bounds-function
• macrostep-sexp-at-point-function
• macrostep-environment-at-point-function
• macrostep-expand-1-function
• macrostep-print-function
• macrostep-macro-form-p-function

Typically, an implementation for another language would set these variables in a major-mode hook. See the docstrings of each variable for details on how each one is called and what it should return. At a minimum, another language implementation needs to provide macrostep-sexp-at-point-function, macrostep-expand-1-function, and macrostep-print-function. Lisp-like languages may be able to reuse the default macrostep-sexp-bounds-function if they provide another implementation of macrostep-macro-form-p-function. Languages which do not implement locally-defined macros can set macrostep-environment-at-point-function to ignore.

Note that the core macrostep machinery only interprets the return value of macrostep-sexp-bounds-function, so implementations for other languages can use any internal representations of code and environments which is convenient. Although the terminology is Lisp-specific, there is no reason that implementations could not be provided for non-Lisp languages with macro systems, provided there is some way of identifying macro calls and calling the compiler / preprocessor to obtain their expansions.

Bugs and known limitations

You can evaluate and edebug macro-expanded forms and step through the macro-expanded version, but the form that eval-defun and friends read from the buffer won’t have the uninterned symbols of the real macro expansion. This will probably work OK with CL-style gensyms, but may cause problems with make-symbol symbols if they have the same print name as another symbol in the expansion. It’s possible that using print-circle and print-gensym could get around this.

Please send other bug reports and feature requests to the author.

Acknowledgements

Thanks to:

• John Wiegley for fixing a bug with the face definitions under Emacs 24 & for plugging macrostep in his EmacsConf presentation!
• George Kettleborough for bug reports, and patches to highlight the expanded region and properly handle backquotes.
• Nic Ferrier for suggesting support for local definitions within macrolet forms
• Luís Oliveira for suggesting and implementing SLIME support

macrostep was originally inspired by J. V. Toups’s ‘Deep Emacs Lisp’ articles (part 1, part 2, screencast).

Changelog

• v0.9, 2015-10-01:
  • separate into Elisp-specific and generic components
  • highlight and expand compiler macros
  • improve local macro expansion and macro form identification by instrumenting macroexpand(-all)
• v0.8, 2014-05-29: fix a bug with printing the first element of lists
• v0.7, 2014-05-11: expand locally-defined macros within (cl-)macrolet forms
• v0.6, 2013-05-04: better handling of quote and backquote
• v0.5, 2013-04-16: highlight region, maintain cleaner buffer state
• v0.4, 2013-04-07: only enter macrostep-mode on successful macro-expansion
• v0.3, 2012-10-30: print dotted lists correctly. autoload definitions.