.. image:: http://cscott.net/Projects/TurtleScript/images/hello-world.png :align: right

Project Goals

The TurtleScript experiment attempts to provide a simple logo-like programming environment which is based on a "real" programming language. It draws heavy inspiration from Etoys_, Scratch_, BYOB_, Elements_, TileScript_, Turtle Art, and Open Blocks. As a browser-based programming environment, it builds on ideas from the Lively Kernel_ and Lively Qt. The TileScript paper describes our own motivations well, although we've chosen a slightly different path.

The ultimate goal is to construct a Sugar_-like activity environment with a pervasive and scalable View Source_ capability. Any visible item can be interrogated and almost all source code viewed, modified, and saved for later use in an interactive fashion. The choice of JavaScript is intended to leverage modern browser technology, which already provides an advanced runtime environment and sandbox. It is also a commercially-important programming language, which is (sadly) important to many vocationally-minded educators.

I've also been recently (re)inspired by the Smalltalk community, and in particular by Ian Piumarta's work on the small self-describing systems cola_ and maru_. The SELF bytecode format described in Chambers et al's OOPSLA 89_ paper was a further incentive to investigate minimal executable representations. The hope is that a few powerful optimizations can be employed against a tiny set of fundamental operations to create a compact and high-performance environment which is truly "turtles all the way down_".

.. _Etoys: http://wiki.laptop.org/go/Etoys .. _Scratch: http://scratch.mit.edu/ .. _BYOB: http://byob.berkeley.edu/ .. _Elements: http://www.chirp.scratchr.org/blog/?p=24 .. _TileScript: http://tinlizzie.org/jstile/ .. _TileScript paper: http://tinlizzie.org/jstile/#TileScript .. _Turtle Art: http://wiki.laptop.org/go/Turtle_Art .. _Open Blocks: http://education.mit.edu/drupal/openblocks .. _Lively Kernel: http://www.lively-kernel.org/index.html .. _Lively Qt: http://lively.cs.tut.fi/qt/ .. _Sugar: http://wiki.laptop.org/go/Sugar .. _View Source: http://wiki.laptop.org/go/View_Source .. _cola: http://piumarta.com/software/cola/ .. _maru: http://piumarta.com/software/maru/ .. _OOPSLA 89: http://selflanguage.org/documentation/published/implementation.html .. _turtles all the way down: http://en.wikipedia.org/wiki/Turtles_all_the_way_down

Warnings and Caveats

This README is a description of a work-in-progress. It is likely to get increasingly outdated over time. I'll attempt to periodically overhaul it to match reality, but it may always contain historical fragments, false starts, and loose ends.

State of the world: 2013-06-03

I recently wrote a "native" interpreter for TurtleScript in Mozilla's Rust_ programming language. The source code for that project can be found at http://github.com/cscott/rusty-turtle.

In the process I improved the REPL_ for TurtleScript, built out the standard library a bit, fixed bugs, and added node_ compatibility and a test suite.

More recently I've begun to think about a TurtleScript-to-asm.js-to-LLVM compiler, which would let me investigate object layout and performance optimizations without resorting to Rust, C, or another low-level language. (Unfortunately asm.js is not a proper subset of TurtleScript at present, since TurtleScript doesn't have the switch statements or bitwise operators. This gap can be mended.) Accordingly, asm-llvm.js_ is the beginnings of an asm.js-to-LLVM compiler, written in TurtleScript.

.. _Rust: http://www.rust-lang.org .. _REPL: http://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop .. _node: http://nodejs.org .. _asm.js: http://asmjs.org .. _LLVM: http://llvm.org/ .. _asm-llvm.js: https://github.com/cscott/TurtleScript/blob/master/asm-llvm.js

State of the world: 2011-05-19

The source code for the project is hosted at http://github.com/cscott/TurtleScript.

It begins with a parser for the "Simplified JavaScript" of Douglas Crockford_ in parse.js. I've extended the language very slightly: it now supports block comments and ' in identifiers, and represents blocks in the parse tree in a more uniform fashion. I've also hoisted all variable declarations to the top of a block, to more accurately reflect their scope. Some further improvements are discussed in Interesting Parser Tasks, but the base language is not expected to change much more.

.. figure:: http://cscott.net/Projects/TurtleScript/images/compile.png :alt: Bytecode output :align: right :target: tdop.html_

Simple bytecode compiler/interpreter, 2011

There are a few backends which process the parsed text. The first to be implemented (jcompile.js, July 2010) simply emitted JavaScript from the parse tree which can be eval'ed by the browser's standard JavaScript environment. At the moment, this isn't very interesting |---| but it allows us to modify the parsed language in various ways and still emit ECMA-standard JavaScript which can take advantage of browsers' highly-tuned JavaScript implementations. Some possible extensions are described in the Interesting Compiler Tasks section.

In May 2011 I wrote a simple bytecode compiler/interpreter for the language, inspired by Piumarta's maru_ system. You can test the parser and the bytecode compiler and interpreter using tdop.html, which is a highly modified version of Douglas Crockford's original parser demonstration. The bytecode instruction set is simple, but not simple enough; Simplifying the Environment_ discusses improvements.

.. figure:: http://cscott.net/Projects/TurtleScript/images/tiles1.png :alt: 2010 graphical tiles :align: right :target: tile2.html_

Original CSS experiments, 2010

Back in 2010 I implemented some simple tile-based renderers for the parse tree. These used jQuery_ to render the tree as CSS-styled HTML. A CSS-styling demo is at tile2.html_ and tiles.html_ displays "editable" interactive source using jQuery UI_. I was dissatisfied with these results. The markup process was very slow, and rendering into HTML/CSS added a lot of additional difficulty and complexity. The promise of clean semantic HTML for the program source was not fulfilled: the actual generated HTML needed to be exceedingly crufty in order to get the rendering close to what I wanted. The interaction model also failed to satisfy: jQuery UI had a lot of problems with horizontal layouts, and the real time re-layout during drag operations made the display stutter unacceptably.

.. figure:: http://cscott.net/Projects/TurtleScript/images/tiles2.png :alt: 2011 graphical tiles :align: right :target: ctiles.html_

Tile Rendering using , 2011

I revisited the rendering code in 2011. As discussed in the Rendering Ideas_ section, I wanted to explore tile-based representations that were nonetheless faithful to the "traditional" text-based layout of source. This time I decided to skip the complexity of the HTML/CSS layers and render directly to a canvas. The result can be seen at ctiles.html. This renderer is written in Simplified JavaScript with a very small canvas API, and can display itself. Further discussion can be found in the Interaction Ideas_ and Renderer Tasks_ sections.

It is my intention to drive towards an initial application similar to Turtle Art, with an on-screen turtle controlled by the script tiles. This will be a public demonstration of the ideas behind the TurtleScript project. The on-screen palette will include a tab for each imported namespace, which displays tiles for each name (method or variable) defined in that namespace. So the activity would initially offer a run() method to be defined, starting with var turtle = imports.turtle (borrowing the gjs module system, which is built on the single global imports definition). This is sufficient to put the standard forward(...), turn(...), pen up(), pen down(), and color(...) definitions on the first page of the palette. By dragging these into the run() function, we can create a simple turtle program. There should be at least three other buttons visible, to run/stop the turtle or clear the screen.

But you should also be able to open the turtle namespace to look at how the commands are defined (in Simplified JavaScript), and thus to add your own turtle commands! These will show up in the turtle palette like any other commands.

Ideally you should also be able to drill down all the way to the parser, compiler, and renderer, to effect even more fundamental changes to the language driving the turtle. In fact, you can keep digging all the way into the object system and runtime. Turtles all the way down!

See Helping out_ to contribute.

.. _Douglas Crockford: http://www.crockford.com/javascript/ .. _parse.js: https://github.com/cscott/TurtleScript/blob/master/parse.js .. _jcompile.js: https://github.com/cscott/TurtleScript/blob/master/jcompile.js .. _maru: http://piumarta.com/software/maru/ .. _tdop.html: http://cscott.net/Projects/TurtleScript/tdop.html .. _parser demonstration: http://javascript.crockford.com/tdop/index.html .. _jQuery: http://jquery.com/ .. _jQuery UI: http://jqueryui.com/ .. _tile2.html: http://cscott.net/Projects/TurtleScript/tile2.html .. _tiles.html: http://cscott.net/Projects/TurtleScript/tiles.html .. _ctiles.html: http://cscott.net/Projects/TurtleScript/ctiles.html .. _canvas API: https://github.com/cscott/TurtleScript/blob/master/ccanvas.js .. _gjs module system: http://cananian.livejournal.com/58744.html

Interesting Parser Tasks

There are a few different things one could do to extend the "Simplified JavaScript" language. The four that I think are interesting are:

1. Transform variable names.

   It would be better to present "readable" variable names, including embedded spaces to separate words. Typical underscore or StudlyCaps conventions can be reversibly transformed to their corresponding readable names in the parse tree, and converted back in the compiler pass. For example, "make_parse" in the source becomes "make parse" in the parse tree, and vice-versa.

   Further, we'd like to avoid the requirement to reserve keywords from use as variable names. Since we'd like to localize the keyword names in the future, we especially don't want to have to reserve all possible keyword names in all possible languages. Better would be to use a standard convention to transform the names when parsing/compiling, such that $if in the source gets rendered as the plain name if in the parse tree, and vice-versa.

   We will also want to reserve some names for use by the compiler. A reasonable solution is to transform names to protect (say) "leading quot; for the exclusive use of the compiler.

   To protect ourselves from JavaScript implementations which don't support full Unicode in identifiers, we might also want to transform names to escape/unescape these characters.

   Once the variable names are independent of the keywords, both variable names and keywords ought to be fully translatable. A good demo would be to translate a good chunk of the system code (and comments!) and allow real-time switching between display languages.

2. Introduce a yada yada yada_ operator.

   When programming interactively, we will often have some "holes" in the program which haven't yet been filled in. For instance, we might have dragged a "while" tile in from a palette, but haven't yet filled in the test expression or the contents of the loop block.

   From Perl 6 we borrow the ... operator, pronounced "yada yada yada". This is used to represent a "hole" in the program which hasn't yet been filled in. By adding this to the formal syntax we simplify serializing/compiling/viewing programs with holes.

   The yada yada yada operator can be compiled to Object.yada_yada_yada() or some other placeholder or global method. By default it will probably throw an exception or enter the debugger.

3. Add an imports global.

   This is a trivial change to the top-level scope of the parser, but it is the hook on which the module mechanism will hang. The existing code should be rewritten to use the imports global, which we'll hand-populate with our modules until we've got a "real" loader running.

4. Preserve comments and new lines.

   Comments are an important part of the documentation of a program, and shouldn't get discarded during the parse. Similarly, newlines are an important part of the formatting of the program text, which is useful even when doing graphical rendering (see Rendering Ideas_, below). Newlines can be attached to parser tokens. In the simplest case, each token would have a boolean flag to indicate whether it was followed by a newline. I haven't yet figured out whether a boolean is sufficient, or whether we actually need to count how many newlines occur. I assume we should count them all initially, and chose the ignore the quantity at a later stage if that turns out to be best.

In contrast, I don't believe these are pressing (or even desirable):

1. Add throw, try, catch, and finally keywords. Add delete and in operators.

   Exceptions add a lot to the expressivity of the language. I expect that their function can be implemented in the library, however, without requiring additional syntax in the base language. The extensions.js_ file demonstrates how these might be implemented as library methods. The implementations of these methods will need to be primitive (and thus will not be introspectable), but we can retain our simplified syntactic vocabulary.

2. Add more/better looping constructs.

   Simplified JavaScript only has a while loop. For beginning programmers, a for i = 1 to 5 { ... } or repeat(5) { ... } sort of loop might be easier to understand. A standard library function (taking a function as a block) or a macro or "build your own tile" feature might be a better way to add this feature. (In particular, I've found myself using the standard Arrays.forEach_ method extensively when writing Simplified JavaScript.)

.. _extensions.js: https://github.com/cscott/TurtleScript/blob/master/extensions.js .. _yada yada yada: http://search.cpan.org/~tmtm/Yada-Yada-Yada-1.00/Yada.pm .. _Arrays.forEach: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/forEach

Interesting Compiler Tasks

Extending the compiler in ways which change the semantics of the language must be done with care: we don't want to end up defining our own "JavaScript-like" language, or negatively impact portability (or editability) of existing JavaScript code. Certain tweaks may be warranted, however, if they simplify the implementation of (and reflection into) the rest of the system. Here are some interesting compiler extensions:

1. Providing "real" block scope for variables in JavaScript, either by transforming var to let in Mozilla-based browsers, or by creating new anonymous functions at block level to implement the necessary scoping.

   This just simplifies the programming model to better match most users' expectations. Very little existing code depends on the lack of block scope, although naive code written for our Simplified JavaScript environment might then fail to run in a native JavaScript environment.

2. Support yield.

   Generators/yield_ are a powerful language extension, especially when implementing asynchronous computation. They are implemented in the Mozilla JavaScript engines, but not in Webkit or V8. It would be helpful to be able to use yield, even when running in these other browsers.

   The importance of this feature depends on the details of the event model we adopt. Adding yield introduces an incompatibility with ECMAScript 5 browsers, but not with Mozilla JavaScript engines.

3. Allow serialization of (running) program state.

   JavaScript currently provides "real" information hiding, in the form of a function's closure object. Variables defined in function scope can be accessed within the function, but not from outside the scope. This prevents proper serialization of a created function, since the scope can not be saved or reconstructed. Transforming::

   function () { var v = ... }

   to something like::

   function($scope) { $scope.v = ... }

   allows us to manually manage the scope chain, including serializing and deserializing a function's closure [1]_. The $scope parameter can be stored as a scope property of the Function object.

4. Bind this properly in inner functions.

   This is a proposal by Crockford_. Function expressions should bind this from their scope at definition time; only method invocation should change the this binding. With an explicit scope parameter, as described above, this can be implemented by defining $scope.this at function creation time, compiling the this literal as (this || $scope.this), as implement (non-this-binding) function invocation as f.call(null, ...).

   As with the previous tweak, most existing JavaScript code avoids use of this in inner functions, or manually overrides the default this via a bind utility function. Existing code is thus expected to work in our environment, but naive Simplified JavaScript code will fail to run in a native JavaScript environment.

5. Extend properties of Function objects.

   Every function object should have a scope property, as proposed above, as well as name and arguments parameters, as in the proposal by Crockford_. A parsed property might link to the Simplified JavaScript parse tree of the function's source. It would also be nice to add a means to access the function object itself from within the function body. This would allow a function to access to its own name, arguments, scope, and parsed properties and any other properties explicitly added to the Function. For example, a user framework might add an owner property to each method defined in a prototype, pointing at the prototype object itself, in order to allow the function to access to the prototype chain involved in the function's dispatch.

   Most existing code would be unaffected by the presence of additional properties of Function objects, and most naive user code will not need to access these properties.

6. A hidden property mechanism for objects.

   For serialization we'll probably want to add a hidden $id field to every serializable object; we may wish to add other hidden properties to support the scope transformation and other needs. For $id, it probably makes the most sense to do this by overriding Object.create() and ensuring that the new $id property is not enumerable_.

   As an alternative, one might consider adding a "meta object" above each "real" object in the object's prototype chain. Properties can be added to the "meta object" without being enumerable, assuming that the developer is using the hasOwnProperty prophylactic_.

   If a "meta object" mechanism is required, the goal would be to avoid any changes to the semantics of the language. This would purely be an implementation aid for efficient hidden properties.

.. [1] Note that there's a bug in ECMA-262 3rd edition which allows standard JavaScript to access the hidden scope object via::

 function f() { this.scope = this; }
 try {
   throw f;
 } catch (e) {
   e();
 }
 ... = scope;

See ECMA-262 5th edition, Annex D, 12.4 and 13 for details. Transformation of the parse tree is a much better way to make the scope object accessible! We will have to transform variable names slightly in order to avoid the bugs corrected by ECMA-262 5th edition: in particular, making properties of Object visible as identifiers in scope.

.. _proposal by Crockford: http://www.crockford.com/javascript/recommend.html .. _Generators/yield: https://developer.mozilla.org/en/JavaScript/Guide/Iterators_and_Generators .. _not enumerable: https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Global_Objects/Object/defineProperty .. _prophylactic: http://javascript.crockford.com/code.html

Simplifying the Environment

The existing bytecode compiler/interpreter is simple, but it could be even simpler. With fewer basic forms, we can get better mileage out of a small set of powerful optimizations: inlining, constant propagation, and memoization. Here's a task list:

1. Transform all the binary and unary operators into method calls. They will become simple invoke operations in bytecode. The tricky part is just ensuring that method lookup/dispatch works properly on primitives, and that the various type coercions are done correctly.

2. Remove jumps from the bytecode. Use dispatch to the boolean results of comparisons instead. See the ifElse and while operators in extensions.js_. An example::

   var i = 0; (function() { i += 1; }).while(this, function() { return i < 5; });

   function pluralize(str, n) { return str + ((n==1).ifElse(this, function() { return ""; }, function() { return "s"; })); }

3. Remove the five get_slot/set_slot variants and replace with get_getter and get_setter messages sent to the object's map. The mapof operator is the only new bytecode operator needed. The result from get_getter/get_setter is a function, so these will be immediately followed by an invocation to actually perform the get/set.

   The implementation of get_getter for a map representing an array will indirect through the field::

   ArrayMap.get_getter = function(field) { return field.array_getter(this); }

   Then we can make a special "numeric string" subclass of string, used for strings which can be parsed as uint32_t numbers (ie, valid array indices) and represented internally as a tagged integer. (If length > 10 or any of the first 10 characters is not a digit, then it's not a numeric string. Negative integers are not numeric strings.) This lets us implement array indexing efficiently as a method of NumericString::

   NumericString.array_getter = function(map) { // this function creation and its subsequent invocation should // be inlined. val idx = this.asUint32(); return native_func(obj) { return memory.get(obj + OFFSET + idx * 8); } } // all other fields use normal object lookup. String.array_getter = function(map) { // this should also be inlinable. return ObjectMap.get_getter.call(map, this); }

   We've now reduced all runtime type tests to the same basic dispatch mechanism, which we can optimize using specialization and inlining.

4. Rewrite bytecode interpreter to operate on object representations stored in a Typed Array. This can include a proper object model and garbage collector. Use NaN boxing, possibly based more-or-less directly on SpiderMonkey's jsval.h but with the addition of a NumericString type as described above.

5. Write a simple bytecode interpreter in C which can operate on system images created by the JavaScript implementation above. Bind it to a canvas, run it in NaCl_ as a demo? At this point you'd have a system which was turtles all the way down to bytecode.

6. Construct a REPL loop for interactive use of the system. Maybe integrate this with the tile demo, so that you can see a tile representation of the current frame, including bound method bodies, and you can type commands at a proper to update the frame/compute results. This may involve writing some code which can convert from a native object representation to an equivalent parse tree, which would look something like: { foo: 'bar', bat: function() { ... } }. We'd need a way to link a binterp function ID with the corresponding widget tree.

7. Efficient compiler which does an interpretation of the bytecode during the first execution, propagating constants and memoizable function results.

.. _extensions.js: https://github.com/cscott/TurtleScript/blob/master/extensions.js .. _Typed Array: http://www.khronos.org/registry/typedarray/specs/latest/ .. _object model: http://piumarta.com/software/cola/objmodel2.pdf .. _NaN boxing: http://blog.mozilla.com/rob-sayre/2010/08/02/mozillas-new-javascript-value-representation/ .. _jsval.h: http://hg.mozilla.org/tracemonkey/annotate/9c869e64ee26/js/src/jsval.h .. _NaCl: http://en.wikipedia.org/wiki/Google_Native_Client

Rendering Ideas

I originally had two conflicting ideas for rendering the Simplified JavaScript parse tree:

1. Move towards a traditional text representation.

   Text-based languages are easy to read and understand for a reason: many years of experience have been used to improve and refine them. We want to move away from the keyboard and towards a more intuitive touch-based editing mechanism, but why throw the baby out with the bathwater?

   In this concept, we still use some subtle puzzle-piece styling cues, but try to fit these "in between the lines". The basic layout should be almost identical to what you'd see in your text editor, with very good syntax coloring.

   Liberal use of the "yada yada yada" operator would be used to indicate drop points, along with dynamic highlight effects as you drag over places where an existing construct (block, argument list, variable declaration, etc) can be extended.

2. Puzzle pieces.

   Scratch_, Turtle Art, and Open Blocks are successful with kids. Try to learn from these representations and copy the details which make them successful. One key might be switching to more "open" layouts of block groupings, using a "C" shape open at one side instead of a box enclosing all the parts. Similarly, the space for the test expression in a if or while, or the argument list in a call, could be left open at the right hand side to allow the expression/list to grow outside the tile without forcing the tile itself to expand horizontally.

Current code leaves heavily towards the first option, although we use puzzle piece styling as much as possible. The original code used a "stacking" 3d look which made deeply-nested expressions look too "tall"; the current look using a single 3d level, with pieces fitting into indents so that the combination of pieces is still flat.

Additional thoughts:

1. Repeated binary expressions (... + ... + ... or ... && ... && ...) need to be flattened, instead of exposing the parse tree details. Explicit piece boundaries should only be shown where precedence levels vary, where they serve to visually indicate "parentheses" in the traditional text representation.

2. It may be possible to aggressively use a "click to expand" representation, so that the rendering of a long function or namespace is not overwhelmingly complex. Initially we might only see a list of top level symbols, with expander boxes. Clicking on the expander would show the definition of that symbol. (This could visually relate to the way the object browser represents non-primitive field values: in both cases an "expander" would be used to show/edit a complex value.)

3. I believe we want to explicitly represent "line breaks", rather than allow constructs to extend indefinitely to the right. My original thought was to just add a "new line" flag to the binop node and to the function call nodes (both the "binary" and "ternary" forms). Setting the newline flag on the binop would arrange the "right" and "left" operands vertically. Setting the newline flag on the function invocation would arrange the arguments vertically. Similar flags would allow you to toggle vertical/horizontal orientations for the arguments of function definitions, and for the array and object constructor forms.

   My current thinking is that all tokens should have a "newlinesAfter" count, and as many places as possible should support adding newlines to the rendering, using a uniform gesture.

   An alternative is to make layout "smarter" so that the correct orientation is selected automatically. It's probably possible to reach a happy medium in which automatic line breaks happen in reasonable places but the user is still able to customize the display for additional clarity/expressiveness.

4. I'd prefer that syntactic extension to the base language occur through the definition of new graphical block types, which can desugar to the basic AST structures; thus, the block widget is a type of macro. We still need a means to represent the macro textually, so that there is a lossless conversion between text and graphical forms, but correspondence might be accomplished by simple convention, like being imported from a path rooted at macros::

   var ForBlockMacro = imports.macros.ForBlockMacro; var foo = function() { var i; ForBlockMacro(function() { i=0; }, function() { return i < 5; }, function() { i+=1; }, function() { /* body */ }); }

   A user without a definition for ForBlockMacro would see a graphical representation corresponding to the text above. But if the ForBlockMacro function includes an asWidget() method, it could define its own graphical representation which could suppress the function() and return cruft to yield a graphical representation identical to the traditional syntactic form::

   for ( i=0 ; i < 5 ; i+=1 ) { /* body */ }

   But this resemblance is purely visual; the underlying source language and syntax remains unchanged. More radical visual changes could also be accomplished, but display of macros can also be toggled off to yield more traditional (if verbose) syntax.

Renderer Tasks

The following is a potential implementation order for additional rendering tasks:

1. Split crender.js_ to separate out the Widget definitions from the code which transforms a parse tree into widgets. Perhaps make the AST node definitions their own separate module as well, instead of conflating them with token objects in parse.js_?

2. Move parenthesization of expressions based on precedence from the transform code into the widget rendering. Parentheses should automatically appear around a binop if its operator precedence is lower than its context.

3. Add the ability to losslessly render Widgets back into Simplified JavaScript source and/or a parse tree.

4. Add basic 'pick' functionality. (Possibly split Widget representation into Composite/Composable at the same time, as is done in Lessphic_.)

5. Allow dragging widgets (but not actual editing yet).

6. Allow editing trees via drag and drop (but not yet editing/creating names).

7. Click to edit literals, including name literals. (Modal dialog is fine at first.)

8. Name literal browser/palettes, for each access to all the names that are in scope. Perhaps combine this with an object browser which can display active objects and let you drag/drop slot names.

.. _crender.js: https://github.com/cscott/TurtleScript/blob/master/crender.js .. _parse.js: https://github.com/cscott/TurtleScript/blob/master/parse.js .. _Lessphic: http://piumarta.com/software/cola/canvas.pdf

Interaction Ideas

I hope that TurtleScript will be used to explore interaction models for programming on touchscreen devices. Here are some of my current ideas:

1. Managing flicker (avoiding resize).

   Dragging pieces into a dynamically-resizing rendering causes excessive flicker as the various drop targets expand/contract. The flicker may cause the drop target itself to move, which may make it impossible to drop the piece in a desired location.

   To solve this problem, the drop targets should be identified without resizing the rendering; any expansion should occur only after the drop. For example, border colors might highlight to indicate that a drop may occur between two existing tiles. When you drag a block out, it should be replaced by a "yada yada yada" element of the exact same size so that the parent widget does not immediately change. Only after the drop should the yada yada yada shrink.

   Alternatively, one could explore an "explicit resize" model, where the user uses an explicit pinch/spread gesture to expand or contract an element (block body, say). This gives more control of layout to the user, at the cost of forcing them to perform additional actions to "tidy up" the display. Perhaps "double tap to shrink fit" is the main gesture -- after you drag out a large block body, the placeholder yada yada yada stays the same large size until you double tap it. The benefit is entirely avoiding automatic resize (and thus flicker) during editing.

   Some additional study of existing block-based systems is warranted.

2. Clone by default.

   It's more common to copy (and then modify) a part than to reorder the parts of a program. The default behavior when dragging a piece which is currently part of some structure (not free floating on the workspace) should be to drag a clone. A separate double-tap or swipe gesture should be used to delete the original, if a move was actually desired.

3. Tap to break apart.

   It's visually confusing to show all the possible drop targets or subcomponents for every expression and statement. Introducing a uniform "tap to break apart" gesture would allow hiding these details unless/until they are necessary. Each tap would reveal the boundaries in one additional level of structure (the individual statements in a function, for instance). Additional taps on a subcomponent would allow drilling down to additional levels of detail (exposing the parts of an assignment statement, for instance).

4. Pervasive "undo".

   Each change to a program should be easily reversible. Similarly, editing the state of a live object should also be reversible: it should be possible to go "back in time" before the execution of a function or assignment of a field. (Clearing the turtle's drawing canvas might even use this mechanism.)

   In practice this is probably implemented by serializing logarithmically-spaced program states and recording mutations and executions. We can then revert to the state at a previous time by deserializing an appropriate older state and then replaying all interactive mutations/function executions which occurred between that state and the desired point in time. This is the approach used by recent work, such as Jockey_, Flashback_, and libckpt_, and results in time travel time complexity proportional to the distance traveled.

.. _Jockey: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.88.2071 .. _Flashback: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.130.6878 .. _libckpt: http://www.cs.utk.edu/~plank/plank/papers/USENIX-95W.html

Environment

This section contains more tentative thoughts about the overall application environment.

1. Building on the shoulders of HTML/CSS/DOM/JavaScript (or not)

   One original goal was to attempt to leverage the existing HTML elements and DOM rather than invent our own GUI framework. We'd use DOM event model (with some sugar). Applications should serialize to an HTML/CSS tree with JavaScript bindings; probably other bits like "the current contents of a canvas" could be serialized as well. Perhaps CSS and the DOM can be unified with JavaScript/JSON using something like CSS JSON_ and JsonML_ to mitigate the number of different syntaxes involved.

   At the moment, I feel that the complexity this adds to the environment isn't warranted. We should be able to harness/embed HTML/CSS, but we shouldn't use it as a building block. Perhaps some "Simplified HTML" subset can be employed. As a limit case, perhaps only elements? (That's what we're doing now.)

2. Work on serialization format.

   First step towards a serializable environment is to write a simple module loader. Assuming we've written a module (JavaScript plus its visible DOM tree and event bindings) to disk, what does it look like? How do we re-load it? For speed we want to leverage the existing native HTML, JavaScript and JSON parsers in the browser. Four possible solutions (perhaps there are others):

   a. The module is an HTML file loaded via