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One more place we were missing expanding type abbreviations: inside
container definitions.
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It might be too much, particularly if students start peeking inside .mu
files early. But worth a shot for not just to iron out the kinks in the
abbreviation system.
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Rip out everything to fix one failing unit test (commit 3290; type
abbreviations).
This commit does several things at once that I couldn't come up with a
clean way to unpack:
A. It moves to a new representation for type trees without changing
the actual definition of the `type_tree` struct.
B. It adds unit tests for our type metadata precomputation, so that
errors there show up early and in a simpler setting rather than dying
when we try to load Mu code.
C. It fixes a bug, guarding against infinite loops when precomputing
metadata for recursive shape-shifting containers. To do this it uses a
dumb way of comparing type_trees, comparing their string
representations instead. That is likely incredibly inefficient.
Perhaps due to C, this commit has made Mu incredibly slow. Running all
tests for the core and the edit/ app now takes 6.5 minutes rather than
3.5 minutes.
== more notes and details
I've been struggling for the past week now to back out of a bad design
decision, a premature optimization from the early days: storing atoms
directly in the 'value' slot of a cons cell rather than creating a
special 'atom' cons cell and storing it on the 'left' slot. In other
words, if a cons cell looks like this:
o
/ | \
left val right
..then the type_tree (a b c) used to look like this (before this
commit):
o
| \
a o
| \
b o
| \
c null
..rather than like this 'classic' approach to s-expressions which never
mixes val and right (which is what we now have):
o
/ \
o o
| / \
a o o
| / \
b o null
|
c
The old approach made several operations more complicated, most recently
the act of replacing a (possibly atom/leaf) sub-tree with another. That
was the final straw that got me to realize the contortions I was going
through to save a few type_tree nodes (cons cells).
Switching to the new approach was hard partly because I've been using
the old approach for so long and type_tree manipulations had pervaded
everything. Another issue I ran into was the realization that my layers
were not cleanly separated. Key parts of early layers (precomputing type
metadata) existed purely for far later ones (shape-shifting types).
Layers I got repeatedly stuck at:
1. the transform for precomputing type sizes (layer 30)
2. type-checks on merge instructions (layer 31)
3. the transform for precomputing address offsets in types (layer 36)
4. replace operations in supporting shape-shifting recipes (layer 55)
After much thrashing I finally noticed that it wasn't the entirety of
these layers that was giving me trouble, but just the type metadata
precomputation, which had bugs that weren't manifesting until 30 layers
later. Or, worse, when loading .mu files before any tests had had a
chance to run. A common failure mode was running into types at run time
that I hadn't precomputed metadata for at transform time.
Digging into these bugs got me to realize that what I had before wasn't
really very good, but a half-assed heuristic approach that did a whole
lot of extra work precomputing metadata for utterly meaningless types
like `((address number) 3)` which just happened to be part of a larger
type like `(array (address number) 3)`.
So, I redid it all. I switched the representation of types (because the
old representation made unit tests difficult to retrofit) and added unit
tests to the metadata precomputation. I also made layer 30 only do the
minimal metadata precomputation it needs for the concepts introduced
until then. In the process, I also made the precomputation more correct
than before, and added hooks in the right place so that I could augment
the logic when I introduced shape-shifting containers.
== lessons learned
There's several levels of hygiene when it comes to layers:
1. Every layer introduces precisely what it needs and in the simplest
way possible. If I was building an app until just that layer, nothing
would seem over-engineered.
2. Some layers are fore-shadowing features in future layers. Sometimes
this is ok. For example, layer 10 foreshadows containers and arrays and
so on without actually supporting them. That is a net win because it
lets me lay out the core of Mu's data structures out in one place. But
if the fore-shadowing gets too complex things get nasty. Not least
because it can be hard to write unit tests for features before you
provide the plumbing to visualize and manipulate them.
3. A layer is introducing features that are tested only in later layers.
4. A layer is introducing features with tests that are invalidated in
later layers. (This I knew from early on to be an obviously horrendous
idea.)
Summary: avoid Level 2 (foreshadowing layers) as much as possible.
Tolerate it indefinitely for small things where the code stays simple
over time, but become strict again when things start to get more
complex.
Level 3 is mostly a net lose, but sometimes it can be expedient (a real
case of the usually grossly over-applied term "technical debt"), and
it's better than the conventional baseline of no layers and no
scenarios. Just clean it up as soon as possible.
Definitely avoid layer 4 at any time.
== minor lessons
Avoid unit tests for trivial things, write scenarios in context as much as
possible. But within those margins unit tests are fine. Just introduce them
before any scenarios (commit 3297).
Reorganizing layers can be easy. Just merge layers for starters! Punt on
resplitting them in some new way until you've gotten them to work. This is the
wisdom of Refactoring: small steps.
What made it hard was not wanting to merge *everything* between layer 30
and 55. The eventual insight was realizing I just need to move those two
full-strength transforms and nothing else.
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array length = number of elements
array size = in locations
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When you pass an ingredient to a recipe using 'start-running' it mostly
behaves identically to performing a regular function call. However, if
the calling function completed before the new routine had a chance to
run, the ingredients passed in ran the risk of being reclaimed.
In response, let's always increment refcounts at the time of a function
call rather than when the ingredients are read inside the callee.
Now the summary of commit 3197 is modified to this:
Update refcounts of products after every instruction, EXCEPT:
a) when instruction is a non-primitive and the callee starts with
'local-scope' (because it's already not decremented in 'return')
OR:
b) when instruction is primitive 'next-ingredient' or
'next-ingredient-without-typechecking'
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Never mind, just close your nose and replace that function parameter
with a global variable.
This may not always be the solution for the problem of layers being
unable to add parameters and arguments, but here it works well and it's
unclear what problems the global might cause.
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Replace an integer with a boolean across two layers of function calls.
It has long been one of the ugliest consequences of my approach with
layers that functions might need to be introduced with unnecessary
arguments simply because we have no clean way to add parameters to a
function definition after the fact -- or to add the default argument
corresponding to that parameter in calls. This problem is exacerbated by
the redundant argument having to be passed in through multiple layers of
functions. In this instance:
In layer 20 we define write_memory with an argument called
'saving_instruction_products' which isn't used yet.
In layer 36 we reveal that we use this argument in a call to
should_update_refcounts_in_write_memory() -- where it is again not used
yet.
Layer 43 finally clarifies what we're shooting for:
a) In general when we need to update some memory, we always want to
update refcounts.
b) The only exception is when we're reclaiming locals in a function
that set up its stack frame using 'local-scope' (signalling that it
wants immediate reclamation). At that point we avoid decrementing
refcounts of 'escaping' addresses that are being returned, and we also
avoid incrementing refcounts of products in the caller instruction.
The latter case is basically why we need this boolean and its dance
across 3 layers.
In summary, write_memory() needs to update refcounts except if:
we're writing products for an instruction,
the instruction is not a primitive, and
the (callee) recipe for the instruction starts with 'local-scope'.
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Standardize quotes around reagents in error messages.
I'm still sure there's issues. For example, the messages when
type-checking 'copy'. I'm not putting quotes around them because in
layer 60 I end up creating dilated reagents, and then it's a bit much to
have quotes and (two kinds of) brackets. But I'm sure I'm doing that
somewhere..
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Fix CI failure.
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More thorough redo of commit 2767 (Mar 12), which was undone in commit
2810 (Mar 24). It's been a long slog. Next step: write a bunch of mu
code in the edit/ app in search of bugs.
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More consistent labeling of waypoints. Use types only when you need to
distinguish between function overloadings. Otherwise just use variable
names unless it's truly not apparent what they are (like that the result
is a recipe in "End Rewrite Instruction").
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Now that we no longer have non-shared addresses, we can just always
track refcounts for all addresses.
Phew!
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I realize that there's still a serious problem with refcounts.
Everything's fine as long as I copy those shared addresses manually
elsewhere, but there's a couple of places where I just do a memcopy
right now without any extra smarts: in 'copy' and 'merge' instructions.
I need to replace support for arbitrary types in these instructions, and
replace it with transforms to generate the right code. Mu basically
needs copy constructors and destructors, so that containers can
decrement the refcounts of any elements (or elements of elements, or
elements of elements of elements..) that are shared addresses.
But my confidence in this whole approach is shaken. Maybe I should stop
this project. It's turning into a language+OS design project where I was
hoping that being a toy would shelter me from these concerns. I just
want to explore turning manual tests into reproducible automatic ones.
Maybe I should just build libraries for each interface to hardware
(network, disk, screen, keyboard, ...) in C++11 or something. Use no
high-level libraries for sockets, files, etc. Instead rely on just the
kernel syscalls, memory allocator, RAII, STL. Build things from scratch
atop those building blocks.
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Show more thorough information about instructions in the trace, but keep
the original form in error messages.
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This should eradicate the issue of 2771.
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This uncovered a second bug (besides 2766) -- I was manually doing the
work of 'new-fake-console' inside 'assume-console' but forgetting to
increment a refcount.
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I'm dropping all mention of 'recipe' terminology from the Readme. That
way I hope to avoid further bike-shedding discussions while I very
slowly decide on the right terminology with my students.
I could be smarter in my error messages and use 'recipe' when code uses
it and 'function' otherwise. But what about other words like ingredient?
It would all add complexity that I'm not yet sure is worthwhile. But I
do want separate experiences for veteran programmers reading about Mu on
github and for people learning programming using Mu.
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I'm going to stop wasting precious first-line characters on 'bugfix:'.
It's going to be all bugfixes for a while I think.
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I really have only one warning left: when somebody redefines a function.
I think I'm going to just turn that into an error as well and drop the
notion of warnings altogether. Anytime we find something wrong we stop
running the program. This is a place where hygiene is justified.
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Delete all the [] that has crept in since 2377 in November.
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Stack of plans for cleaning up replace_type_ingredients() and a couple
of other things, from main problem to subproblems:
include type names in the type_tree rather than in the separate properties vector
make type_tree and string_tree real cons cells, with separate leaf nodes
redo the vocabulary for dumping various objects:
do we really need to_string and debug_string?
can we have a version with *all* information?
can we have to_string not call debug_string?
This commit nibbles at the edges of the final task, switching from
member method syntax to global function like almost everything else. I'm
mostly using methods just for STL in this project.
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Somehow this never transferred over from the Arc version until now.
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We don't yet actually maintain the refcount. That's next.
Hardest part of this was debugging the assume-console scenarios in layer
85. That took some detailed manual diffing of traces (because the output
of diff was no good).
New tracing added in this commit add 8% to .traces LoC. Commented out
trace() calls (used during debugging) make that 45%.
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This is the one major refinement on the C programming model I'm planning
to introduce in mu. Instead of Rust's menagerie of pointer types and
static checking, I want to introduce just one new type, and use it to
perform ref-counting at runtime.
So far all we're doing is updating new's interface. The actual
ref-counting implementation is next.
One implication: I might sometimes need duplicate implementations for a
recipe with allocated vs vanilla addresses of the same type. So far it
seems I can get away with just always passing in allocated addresses;
the situations when you want to pass an unallocated address to a recipe
should be few and far between.
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Reorganize layers in preparation for a better way to manage heap
allocations without ever risking use-after-free errors.
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