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Yet another attempt at trying to clean up commit 3216. I think this solution
might finally let me stop agonizing over the problem. State variables for
distinguishing call-sites are a reasonable mechanism, orthogonal to waypoints
and the hook functions to hold them.
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https://www.reddit.com/r/badcode/comments/5xo3qv/its_better_to_have_it_all_on_one_line_right/delodzn
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Eliminate a long-standing under-abstraction.
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Be more disciplined about tagging 2 different concepts in the codebase:
a) Use the phrase "later layers" to highlight places where a layer
doesn't have the simplest possible self-contained implementation.
b) Use the word "hook" to point out functions that exist purely to
provide waypoints for extension by future layers.
Since both these only make sense in the pre-tangled representation of
the codebase, using '//:' and '#:' comments to get them stripped out of
tangled output.
(Though '#:' comments still make it to tangled output at the moment.
Let's see if we use it enough to be worth supporting. Scenarios are
pretty unreadable in tangled output anyway.)
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This was a large commit, and most of it is a follow-up to commit 3309,
undoing what is probably the final ill-considered optimization I added
to s-expressions in Mu: I was always representing (a b c) as (a b . c),
etc. That is now gone.
Why did I need to take it out? The key problem was the error silently
ignored in layer 30. That was causing size_of("(type)") to silently
return garbage rather than loudly complain (assuming 'type' was a simple
type).
But to take it out I had to modify types_strictly_match (layer 21) to
actually strictly match and not just do a prefix match.
In the process of removing the prefix match, I had to make extracting
recipe types from recipe headers more robust. So far it only matched the
first element of each ingredient's type; these matched:
(recipe address:number -> address:number)
(recipe address -> address)
I didn't notice because the dotted notation optimization was actually
representing this as:
(recipe address:number -> address number)
---
One final little thing in this commit: I added an alias for 'assert'
called 'assert_for_now', to indicate that I'm not sure something's
really an invariant, that it might be triggered by (invalid) user
programs, and so require more thought on error handling down the road.
But this may well be an ill-posed distinction. It may be overwhelmingly
uneconomic to continually distinguish between model invariants and error
states for input. I'm starting to grow sympathetic to Google Analytics's
recent approach of just banning assertions altogether. We'll see..
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Follow-up to commit 3321: move get_base_type() more thoroughly to layer
55. The notion of a base_type doesn't really make sense before we
introduce type ingredients and shape-shifting containers, and it
simplifies early layers a *lot* even including the cost of that *ugly*
preamble in layer 55 to retrofit all the places.
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Don't crash on bad types.
I need to be more careful in distinguishing between the two causes of
constraint violations: bad input and internal bugs. Maybe I should
create a second assert() to indicate "this shouldn't really be an
assert, but I'm too lazy to think about it right now."
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size_of(type_tree*) is a mess; clean it up with an eye to the final
tangled version.
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I was under the impression that I only needed static array lengths for
container members, but these are *payload* types for allocations. So we
need to compute the size of a dynamic array.
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We weren't checking within (static) array elements for addresses.
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Extract a helper to compute the element type of an array. As a side
effect, the hack for disambiguating array:address:number and
array:number:3 is now in just one place.
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Standardize on calling literate waypoints "Special-cases" rather than
"Cases". Invariably there's a default path already present.
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Programming languages need some higher-level language construct that's
neither an interface nor a class nor an object but a *collection of
mutually recursive functions with a well-defined set of entry points and
common ingredients. Perhaps the solution here is the Haskell "save your
boilerplate" paper. For now I'm going to include the purpose in
auxiliary variable names that aren't really necessary for the core
processing of a function.
Thanks Caleb Couch for reporting this issue.
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Thanks Caleb Couch for running into this with $print.
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Tag all transforms as idempotent or not.
I'd fallen off this wagon.
I might even be getting it wrong. Something a type system should
automatically verify.
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One more place we were missing expanding type abbreviations: inside
container definitions.
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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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Highlight a couple of places where it turns out that we're flying by the
seat of our pants with heuristics, and we don't really understand how to
precompute metadata for a program's types.
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Bugfix in filesystem creation. I'm sure there are other fake-filesystem
bugs.
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When updating refcounts for a typed segment of memory being copied over
with another, we were only ever using the new copy's data to determine
any tags for exclusive containers. Looks like the right way to do
refcounts is to increment and decrement separately.
Hopefully this is a complete fix for the intermittent but
non-deterministic errors we've been encountering while running the edit/
app.
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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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I'm seeing *extremely* rare crashes due to some problem with negative
refcounts in the edit/ app. It's not using any concurrency at all, so
that's not the issue. Setting a tripwire to try and catch it. I'm going
to run:
mu --trace edit
..all the time for a while. And periodically restart when the trace
makes the program too sluggish to continue.
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Reorganize data structure for lambda cells. Create our first real unit
test for the compiler in the process.
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This wouldn't have failed silently; I have that fig leaf. If someone had
tried to copy an exclusive container containing an exclusive container
containing an address Mu would have crashed on them.
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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 reorganization in preparation for implementing recursive abandon().
Refcounts are getting incredibly hairy. I need to juggle containers
containing other containers, and containers *pointing* to other
containers. For a while I considered getting rid of address_element_info
entirely and just going by types for every single
update_refcount. But that's definitely more work, and it's unclear that
things will be cleaner/shorter/simpler. I haven't measured the speedup,
but it seems worth optimizing every pointer copy to make sure we aren't
manipulating types at runtime.
The key insight now is a) to continue to compute information about
nested containers at load time, because that's the common case when
updating refcounts, but b) to compute information about *pointed* values
at run-time, because that's the uncommon case.
As a result, we're going to cheat in the interpreter and use type
information at runtime just for abandon(), just because the
corresponding task when we get to a compiler will be radically
different. It will still be tractable, though.
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This is hopefully quite thorough. I handle nested containers, as well as
exclusive containers that might contain addresses only when the tag has
a specific value.
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To support exclusive containers a recursive answer will be easier to
reuse than the current iterative one. First step: figure out the precise
boundary and interface of the recursive function.
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