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Knowledge Base›MIT CADR›The MIT Lisp Machine Inspector

The MIT Lisp Machine Inspector

The Inspector is an interactive browser and live editor for Lisp objects. It does not flatten an object into inert text: each displayed field retains enough identity for the user to follow references, inspect function definitions, and, where the implementation marks the field settable, replace data in the running system. Its three-pane history makes navigation visible while the separate history pane preserves older objects.

That power makes the Inspector both a documentation tool and a potentially destructive debugger. Modify changes the actual object, array, binding, or memory location being inspected; it does not edit a private copy.

This page treats the public MIT CADR System 46 source and the maintained LM-3 System 303 source as separate evidence sets. Runtime observations apply only to the System 303-0 load band described below.

Evidence sets

MIT CADR System 46

The public System 46 tree is pinned at Git revision 8e978d7.

File Role Bytes SHA-256
src/lmwin/inspct.80 Inspector frame, object printers, navigation, and modification 37,465 137fe25adf7df785548e098da2053ce7bac709e021a5abc7039e9aac218bb600
src/lmwind/operat.27 contemporary operations-manual source 85,337 a5ab658210dc09891b0886b58af705368e33a41f013073c8b9a637d99ab0f02d

LM-3 System 303

The maintained LM-3 System repository is pinned separately at Fossil check-in 4df393c, tag system-303.

File Role Bytes SHA-256
l/sys/window/inspct.lisp current Inspector frame, object dispatch, commands, and mutation paths 60,805 b38e92064d6827528064bca27fe0e36d1ab1d61e54431fe69e3efcf9bf5f083a
l/sys/window/basstr.lisp System-key registration and generic selection behavior 81,846 8ba3a16e726ed043e6585c7a68b7096bb2dcc5d6f05476afd89f84a48dff2645

The LM-3 repository is a maintained restoration. Features found only in its System 303 source are not attributed to the older System 46 snapshot without independent evidence.

Purpose and entry points

The System 46 manual describes the Inspector as a program for displaying and modifying Lisp data structures. The source supports three principal entry paths:

  • System I selects or creates an Inspector;
  • Inspect in the System Menu selects or creates one;
  • (INSPECT object) enters an Inspector from Lisp.

The process model differs by source line. In System 46, System I is registered as a specialized call to (TV:INSPECT), and INSPECT allocates a resource frame and runs its top level in the caller's process. In System 303, System I and the System Menu select or create an INSPECT-FRAME with its own process, while the function call (INSPECT object) deliberately allocates a processless resource frame and runs its command loop in the caller's process. The latter arrangement lets evaluated forms see the caller's dynamic special bindings; a standalone System 303 application does not inherit an arbitrary Listener's dynamic environment in that way.

Other programs can embed Inspector panes rather than launching the complete application. The source explicitly anticipates use inside debugging and error handling interfaces.

Frame and navigation model

The complete frame has five functional regions:

  1. a Lisp interaction pane;
  2. a history pane;
  3. a one-line command menu;
  4. three stacked object-display panes by default;
  5. the who line supplied by the window system.

The arrangement changed. System 46 gives the interaction pane a three-line row and places History and the menu together in the next horizontal constraint. The observed System 303 frame places interaction at upper left and History at upper right, with the menu as a full-width row immediately below them.

The three newest navigation objects occupy the object panes, with the newest at the bottom. Following a reference normally pushes the existing displays upward. A middle-click variant preserves the source display in the second-from-bottom pane and puts the followed object at the bottom, allowing a direct before/after comparison.

The history pane retains previously inspected objects beyond those three visible slots. Selecting a history item brings it back into the display stack; the middle-button history operation removes the item and invalidates its cached display.

Display records and caching in System 303

System 303 represents each cached display with seven pieces of state:

  • the object;
  • its printing function;
  • an argument for that function;
  • the generated lines or selectable items;
  • the current top item;
  • the pane label;
  • an optional incremental item generator.

This cache is observable behavior. If another computation mutates an object, the existing Inspector text can remain stale until the user chooses DeCache, presses Clear-Screen, or otherwise invalidates that display. Modify invalidates the affected object after changing it. The cache is a display optimization, not a snapshot or isolation mechanism.

The command menu

Both inspected source lines define the same six visible menu items, but not in the same order. System 46 orders them Exit, Return, Modify, DeCache, Clear, Set \\. System 303 and the observed frame order them Exit, Return, Clear, DeCache, Modify, Set \\.

Item Effect
Exit leave the command loop; System 46 returns NIL, while System 303 returns the current * object
Return prompt for a value selected by keyboard or mouse, then leave returning that value
Modify restrict pointer selection to settable displayed fields and replace the selected value
DeCache invalidate cached representations so objects are regenerated
Clear clear history, cache, and object panes
Set \\ prompt for a value selected by keyboard or mouse and bind the Inspector-local \\ variable to it

Modify is a modal pointer operation. In System 303, only fields whose display records carry a setter remain sensitive. A right click aborts the selection. After a field is chosen, the replacement can be typed and evaluated or obtained by selecting a displayed object with the mouse.

The menu is not the only way to modify. System 303 also binds Hyper-left on a settable field to the direct modification path.

Keyboard command inventory

System 46

Key or input Effect in inspct.80
Control-Z exit the Inspector
Break enter a Lisp break loop
Quote read, evaluate, and print a form without inspecting its result
Rubout ignored by the outer command loop
ordinary form read and evaluate it, then inspect the result

The ordinary line editor still handles corrections while it is reading a form. “Rubout ignored” describes the outer Inspector command dispatch, not a claim that typed Lisp can never be corrected.

System 303

Key or input Effect in inspct.lisp
Control-Z or Abort signal an abort from the current inspection
Control-V scroll the bottom Inspector pane down by roughly one screen
Meta-V scroll the bottom Inspector pane up by roughly one screen
Break enter a Lisp break loop
Clear-Screen clear the display cache and fully redisplay
End return the current * object from the processless Inspector loop
Help display the Inspector's in-program help and wait for acknowledgment
Delete clear history, cache, and all three object panes
Control-\\ prompt for a value and set the Inspector-local \\ variable to it
Rubout ignored by the outer command loop
Quote read, evaluate, and print a form without inspecting its result
ordinary form read and evaluate it, then inspect the result

System 46 explicitly maintains * as the last inspected object. System 303 adds ** and ***, so the three variables mirror the visible three-object stack. End and Exit return * in System 303; Return instead asks for an explicit value. These exits are useful when INSPECT is being used as an expression-level chooser rather than merely as a standalone application.

Pointer interaction inventory

Selectable displayed values support these core gestures:

Gesture Result
left click inspect the value, pushing the display history
middle click inspect the value while preserving the source pane for comparison
right click follow the value as a function definition where meaningful
Hyper-left click System 303: modify that field directly if it has a setter

The function-following gesture is type-aware. System 303 can follow a symbol's function, an instance's function, a closure or entity, or another valid function spec. It does not simply treat every printed token as a function name.

Within the history pane, left click reinspects an entry; middle click removes that entry and decaches it. The frame also inherits the generic right-button System Menu gesture from the window system.

What the Inspector understands

The object dispatcher chooses a specialized presentation from the object's runtime type. This is not CLOS pretty-printing layered over a generic list: the Inspector knows how to expose implementation structures and live locations.

System 46 object classes

The pinned System 46 dispatcher has specialized paths for:

  • stack frames;
  • named structures;
  • instances and their instance variables;
  • arrays and array leaders;
  • lists;
  • symbols;
  • select-method objects;
  • closures and entities;
  • FEFs, the machine's compiled-function representation.

Its array browser exposes leaders and elements of one-dimensional arrays. Multi-dimensional arrays fall back to printing the object rather than enumerating all indexed elements. Its FEF view includes disassembly.

Additions and extensions in System 303

System 303 preserves those categories and adds or broadens:

  • arbitrary-rank arrays, enumerated with row-major indices converted back to subscripts;
  • locatives, whose referenced contents are displayed directly;
  • a catch-all presentation for otherwise unknown objects;
  • richer compiled and interpreted lexical-closure displays;
  • incremental item generation for large structures.

Named structures are tested before more general categories, preserving their type-specific descriptions even when their representation overlaps another Lisp type.

Fields that can be modified

The Inspector does not make every printed item editable. The setter inventory is part of its data model and differs between the source lines.

System 46

The inspected code provides setters for:

  • a symbol's value, function, and property list;
  • list cars and corresponding list locations;
  • instance variables;
  • closure variables;
  • named-structure slots;
  • one-dimensional array elements and array leaders;
  • select-method mappings.

FEFs are displayed and disassembled, but the inspected System 46 code does not establish a general settable FEF-constant interface. Multi-dimensional array elements likewise lack the later generic indexed path.

System 303

The maintained source provides setters for:

  • named-structure slots;
  • instance variables;
  • ordinary closure slots and lexical-closure slots;
  • unshadowed interpreted-closure variables;
  • select-method tails and keyword functions;
  • symbol values, functions, and property lists;
  • locative contents;
  • list cars and list locations;
  • array leaders and elements of arbitrary-rank arrays;
  • FEF constants.

Special or shadowed interpreted variables are intentionally not marked settable by that display path. A prospective FEF function setter is present only as commented-out code; this page does not count it as a feature.

These operations can violate application and runtime invariants. The Inspector has no transactional undo layer, and the source does not turn Modify into a copy-on-write operation.

Findings not apparent from the short manual description

  • (INSPECT object) in System 303 runs a processless frame in the caller's process specifically so evaluated forms can see dynamic special bindings; the older System 46 INSPECT call also runs its resource frame in the caller.
  • The visible object text is cached and can become stale even though mutation acts on live objects.
  • System 303's arbitrary-rank array view computes real multidimensional subscripts rather than displaying only a flat row-major number.
  • Mouse sensitivity is capability-based: Modify narrows selection to display items carrying real setters.
  • The right-button value gesture follows function identity and is distinct from ordinary structural navigation.
  • System 303 exposes locatives and implementation-level closure state that are not ordinary application records.
  • FEF constants are mutable in the maintained System 303 implementation, but a different FEF function setter is merely commented out. Source presence alone is not treated as an implemented command.

Runtime observation: System 303-0

A fresh Xvfb computer-use session selected the Inspector with System I, then evaluated and inspected '(museum listener inspector peek). The initial frame visibly had the interaction, history, six-item menu, and three Inspector panes described by the source. The top two object panes read Empty; after the form was entered, the history pane showed the expression and the bottom pane showed the resulting object under the label a list.

Item Recorded value
Session core-env-20260718, generation 1; 2026-07-18 03:55:07–04:06:40 EDT; boot ID 3ee4cfb0-9636-4c5c-8661-a1b04ab7aa07; non-resumed
Load band System 303-0; banner Experimental System 303.0, ZWEI 129.0, microcode 323
Disk base/private-start SHA-256 bb16e46ad81decfe1efe691d36b6aa4ce3fd4ffb82474365de3520989d397cb5; base unchanged after stop
Public revisions L d1250f90044f09b6c92014a9aef65f9574e1bcbf8a7163004e53cc6dbed0f2d6; System 4df393c68d7f083ce42d5c377039d26043cc18a9031ace28258dc97f4137eb91; usim 330d8248ec2e12af071e287920e681600f75df9ffd854aada5f8a64c9adad64d; usite 8f717978b458b40adf1e238aaf177f5bc54ef46881268e03b787ba57b0d30a0e; Chaos db2953fde68d726a605d1d1699bab6c926ef252bd4991f692bae6ee5a634764e
Private copy copied 2026-07-18 03:55:03 EDT at those revisions; System tree 21f5215de973aa6ccbddb817f2d64edd95ee1014c3028a9b0711ea7c741b807e, Chaos 34ab197641aae909e9a224edc307020fddec263e732207a74573d51dac0daa87, usite adbb720339db225e6635977a869cf3f3d50b507e614b37a976f4a6548d212a81; copy/start hashes matched and changed flags were false
Emulator start and execution SHA-256 707a77d23e28ea1c45ae0eb0145dc181fa7ba649b9defc30044d4f847ac2c5be
Machine artifacts promh.mcr 2c667f99f014a7130a55b255d31df02588d9396beace78abfe9325269e4ff3e6; promh.sym e9e3dd6a541511dd9541ae96b99dae19cb185d8b79fa09959f21fa52224f233d; ucadr.sym 9071decf16fa8f11d7970c4662db0d6e95600fe43ec86ac41c77b37dbd7caa2a
Toolchain manifest 3adae999bbe420182f22adc2499fcc82449a46eaf580a362de9c0e718fa6b37d; Guix channel 230aa373f315f247852ee07dff34146e9b480aec; Python 3.11.14, Xorg 21.1.21, ImageMagick 6.9.13-5, xdotool 3.20211022.1
Host window LOCAL-CADR [running], XID 2097202, x=0, y=0, 768×963

The relevant ordered input was System I, followed by the expression '(museum listener inspector peek). No modification gesture or other destructive Inspector operation was attempted.

Two raw 768×963 captures remain in the ignored session:

Raw capture State PNG SHA-256 Decoded-pixel SHA-256
0008-inspector-initial.png initial Inspector frame c524f90d50659ca59fb59b8788e3e1f30e980d2265ded8a5657526aa604b9e08 19c519454d3a1ca16ead54034043bc97e81ab8432a05bc2d691f5f6b4c0b8cb7
0009-inspector-list.png inspected list a8c104631118a10749f4b0a6b6059dfe902c190e4c20075d8f94fcb683c137b3 178b6ccd02a999bebaa6e5171d33f6980d6196f210078286dcb6483ec7398044

The image-by-image review selected only the synthetic-list state for the curated CADR screenshot catalog. The blank initial frame remains ignored because it adds little evidence beyond the populated layout below.

The System 303 Inspector with its interaction pane at upper left, history at upper right, and a researcher-created list in the bottom object pane.

Runtime observation: the LM-3 System 303 Inspector displaying '(museum listener inspector peek), captured 2026-07-18. Underlying software and display material remain the property of their respective rightsholders; reproduced here for criticism, scholarship, and historical documentation under 17 U.S.C. section 107. No affiliation or endorsement is implied.

The 6,781-byte run record has SHA-256 2d877843f58a8a261bafe68afca846919963dec5584a39bbb5275fcbe6615e22. Shutdown was clean: forced_stop and state_may_be_incomplete are false, the emulator and Xvfb both exited zero, and the base disk was unchanged. The computer-use harness article explains the evidence schema and repeat procedure.

Limits and open questions

  • No live mutation was performed. The setter inventory is established from source, while the visible modal selection and resulting memory change remain to be exercised on a deliberately disposable test object.
  • The runtime pass did not open the Inspector's Help screen or exercise every scrolling and history gesture. Those bindings are source-established, not claimed as individually runtime-confirmed.
  • No runnable System 46 band was used. Its behavior here is source- and manual-grounded only.
  • The exact first release that introduced locatives, arbitrary-rank array browsing, **/***, or Hyper-left modification has not been established.
  • The populated-list capture received a separate per-image publication review; the unused initial-frame capture remains ignored and unreviewed for publication.

Sources

Source links and pinned revisions last verified 2026-07-18.

  • MIT CADR System 46, INSPCT.80, Inspector implementation.
  • MIT CADR System 46, OPERAT.27, window-system operations manual source.
  • LM-3 System 303, window/inspct.lisp, maintained Inspector implementation.
  • LM-3 System 303, window/basstr.lisp, System-key and selection infrastructure.
  • Local System 303-0 Xvfb observation, session core-env-20260718, generation 1, 2026-07-18; raw evidence identified above.
Try “search window system”, “open genera”, or “help”.