_Macro Debugger_
================

Keywords: _syntax_, _syntax-browser_, _hygiene_,
          _referential transparency_, _lexical context_,
          _macro_, _expansion_, _macro-debugger_, _macro-stepper_

OUTLINE
=======

* Introduction
* APIs
  - expand.ss
  - syntax-browser.ss
* Using the macro stepper
* Using the syntax browser

Introduction
============

The macro-debugger collection contains two tools: a stepper for macro
expansion and a standalone syntax browser. The macro stepper shows the
programmer the expansion of a program as a sequence of rewriting
steps, using the syntax browser to display the individual terms. The
syntax browser uses colors and a properties panel to show the term's
syntax properties, such as lexical binding information and source
location.

_expand.ss_
===========

> (require (lib "expand.ss" "macro-debugger"))

This module provides a single procedure:

> (expand/step syntax-or-sexpr)

Expands the syntax (or S-expression) and opens a macro stepper frame
for stepping through the expansion.

_syntax-browser.ss_
===================

> (require (lib "syntax-browser.ss" "macro-debugger"))

This module provides the following procedures:

> (browse-syntax syntax) -> void

Creates a frame with the given syntax object shown. More information
on using the GUI is available below.

> (browse-syntaxes list-of-syntax) -> void

Like browse-syntax, but shows multiple syntax objects in the same
frame. The coloring partitions are shared between the two,
showing the relationships between subterms in different syntax
objects.

> (syntax-snip syntax) -> (is-a?/c snip%)

Like 'browse-syntax', but creates a snip that can be displayed in an
editor. In particular, evaluating
  (syntax-snip <stx>)
in DrScheme's interactions window will insert a syntax browser into
the interactions window.

Using the Macro Stepper
=======================

Navigation
----------

The stepper presents expansion as a linear sequence of rewriting
process, and it gives the user controls to step forward or backwards
as well as to jump to the beginning or end of the expansion process.

If the macro stepper is showing multiple expansions, then it also
provides "Previous term" and "Next term" buttons to go up and down in
the list of expansions. Horizontal lines delimit the current expansion
from the others.

Macro hiding
------------

Macro hiding lets one see how expansion would look if certain macros
were actually primitive syntactic forms. The macro stepper skips over
the expansion of the macros you designate as opaque, but it still
shows the expansion of their subterms.

The bottom panel of the macro stepper controls the macro hiding
policy. The user changes the policy by selecting an identifier in the
syntax browser pane and then clicking one of "Hide module", "Hide
macro", or "Show macro". The new rule appears in the policy display,
and the user may later remove it using the "Delete" button.

The stepper also offers coarser-grained options that can hide
collections of modules at once. These options have lower precedence
than the rules above.

Macro hiding, even with no macros marked opaque, also hides certain
other kinds of steps: internal defines are not rewritten to letrecs,
begin forms are not spliced into module or block bodies, etc.

Using the Syntax Browser
========================

Selection (bold)
----------------

The user can click on any part of a subterm to *select* it. To select
a parenthesized subterm, click on either of the parentheses. The
selected syntax is bolded. Since one syntax object may occur
inside of multiple other syntax objects, clicking on one occurrence
will cause all occurrences to be bolded.

The syntax browser displays information about the selected syntax
object in the properties panel on the right, when that panel is
shown. The selected syntax also determines the highlighting done by
the secondary partitioning (see below).

Primary partition (foreground color)
------------------------------------

The *primary partitioning* always assigns two syntax subterms the same
color if they have the same marks. In the absence of unhygienic
macros, this means that subterms with the same foreground color were
either present in the original pre-expansion syntax or generated by
the same macro transformation step.

Syntax colored in black always corresponds to unmarked syntax. Such
syntax may be original, or it may be produced by the expansion of a
nonhygienic macro.

Secondary partitioning (highlight)
----------------------------------

The user may select a *secondary partitioning* from a drop-down box
(or in the macro stepper, through the Syntax menu). This partitioning
applies only to identifiers. When the user selects an identifier, all
terms in the same equivalence class as the selected term are
highlighted in yellow.

The available secondary partitionings are:

* bound-identifier=?
* module-identifier=?
* module-or-top-identifier=?

  See the documentation for these predicates in the MzScheme manual.

* symbolic-identifer=?

  Two identifiers are symbolic-identifier=? if discarding all lexical
  context information yields the same symbol.

* same marks

  Two identifiers have the same marks if (barring nonhygienic macros)
  they were produced by the same macro transformation step.

* same source module

  The bindings of the two identifiers come from definitions in the
  same module.

* same nominal module

  The bindings of the two identifiers were imported into the current
  context by requiring the same module.

Properties
----------

When the properties pane is shown, it displays properties of the
selected syntax object. The properties pane has three tabbed pages:

  - Binding

      If the selection is an identifier, shows the binding information
      associated with the syntax object.

      *Note: See the warning in the section below.

      For more information, look up 'identifier-binding',
      'identifier-transformer-binding', and
      'identifier-template-binding' in the Help Desk.

  - Source

      Displays source location information about the syntax object.

  - Properties

      Displays properties (see 'syntax-property') of the selection
      when it has properties it knows the keys for.

Warnings about interpreting syntax
----------------------------------

The binding information of a *syntax object* may not be the same as
the binding structure of the *program* it represents. The binding
structure of a *program* is only determined after macro expansion is
complete.

  Example: (browse-syntax #'(lambda (foo) foo))
  The syntax browser will report that the inner 'foo' is unbound, even
  though in the *program* that this syntax represents, the inner 'foo'
  is bound to the outer 'foo'.

Notes and Limitations
---------------------

The syntax browser does not have a way of extending the set of
available secondary partitions.

The syntax browser does not have a way of extending the set of known
properties.

The syntax browser does not preserve the distinction between
parentheses and square brackets.
