| Commit message (Collapse) | Author | Age | Files | Lines |
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The old approach with '&' and '@' modifiers turned out to be a bad idea
because it introduces notions of precedence. Worse, it turns out you
want different precedence rules at different times as the old test
alluded:
x:@number:3 # we want this to mean (address number 3)
x:address:@number # we want this to mean (address array number)
Instead we'll give up and focus on a single extensible mechanism that
allows us to say this instead:
x:@:number:3
x:address:@:number
In addition it allows us to shorten other types as well:
x:&:@:num
type board = &:@:&:@:char # for tic-tac-toe
Hmm, that last example reminds me that we don't handle abbreviations
inside type abbreviation definitions so far..
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Clean up another case (after commit 3309) of premature support for
shape-shifting recipes, where early layers had code without
corresponding tests.
One addendum to commit 3309: the proximal cause for triggering the
rewrite of type_trees was that I realized to_string() and variants were
lying to me while debugging; they couldn't distinguish between `(a . b)`
and `((a) . b)`
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Allow type-trees to be ordered in some consistent fashion. This could be
quite inefficient since we often end up comparing the four sub-trees of
the two arguments in 4 different ways. So far it isn't much of a time
sink.
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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
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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
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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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Correction for syntax highlighting.
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Fix CI.
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Thanks Stephen Malina for helping run into this hole in support for
compound types.
When I created that assert (commit 2381, Nov 2015) I was thinking only
of type ingredients, and didn't realize that compound types could have
internal nodes with zero values.
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Always show instruction before any transforms in error messages.
This is likely going to make some errors unclear because they *need* to
show the original instruction. But if we don't have tests for those
situations did they ever really work?
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Warn if 'put' or 'put-index' has a mismatch in the type of the product,
not just the name. It won't do any harm, but could be misleading to a
later reader. In both instructions, the product is just for
documentation.
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Raise an error if a 'put' or 'put-index' doesn't match ingredient and
product. That wouldn't do what you would expect.
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Never mind, always quote direct quotes from code in error messages.
Dilated reagents are the uncommon case.
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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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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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Update refcounts of address elements when copying containers.
Still lots to do; see todo list at end of 036refcount.cc.
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Since I switched to a Mac laptop (commit 2725) I've been lax in running
test_all_layers because I have to ssh into a server and whatnot. I
should just get CI setup somewhere..
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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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This commit covers instructions 'put', 'put-index' and 'maybe-convert'.
Next up are the harder ones: 'copy' and 'merge'. In these cases there's
a non-scalar being copied, and we need to figure out which locations
within it need to update their refcount.
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It's a bit of a trade-off because we need to store copies of
container metadata in each reagent (to support shape-shifting
containers), and metadata is not lightweight and will get heavier. But
it'll become more unambiguously useful when we switch to a compiler.
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This continues a line of thought sparked in commit 2831. I spent a while
trying to avoid calling size_of() at transform-time, but there's no
getting around the fact that translating names to addresses requires
knowing how much space they need.
This raised the question of what happens if the size of a container
changes after a recipe using it is already transformed. I could go down
the road of trying to detect such situations and redoing work, but that
massively goes against the grain of my original design, which assumed
that recipes don't get repeatedly transformed. Even though we call
transform_all() in every test, in a non-testing run we should be loading
all code and calling transform_all() just once to 'freeze-dry'
everything.
But even if we don't want to support multiple transforms it's worth
checking that they don't occur. This commit does so in just one
situation. There are likely others.
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Be more consistent that 'return' is the name of the instruction, and
'reply' just a synonym. Maybe I should take it out. It wouldn't affect
the recipe/ingredient terminology while I teach..
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Finally after much massaging, the 'address' and 'new' layers are
adjacent.
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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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As outlined at the end of 2797. This worked out surprisingly well. Now
the snapshotting code touches fewer layers, and it's much better
behaved, with less need for special-case logic, particularly inside
run_interactive(). 30% slower, but should hopefully not cause any more
bugs.
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Simplify 2790 by simply not computing any type->value inside
parse_type_tree. It now only generates names, and it turns out the
consumers handle the absence of values anyway. Now parse_type_tree no
longer pollutes the Type_ordinal table with type ingredients.
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The issue alluded to in the previous 2789 is now fixed. I'm not happy
with my solution, though. I pollute Type_ordinal with type ingredients
in parse_type_tree and simply ignore such entries later on. I'd much
rather avoid the pollution in the first place, but I'm not sure how to
do that..
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This should eradicate the issue of 2771.
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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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