3309

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.

1 files changed, 156 insertions, 47 deletions

diff --git a/055shape_shifting_container.cc b/055shape_shifting_container.cc
index a539e34f..131028a5 100644
--- a/055shape_shifting_container.cc
+++ b/055shape_shifting_container.cc
@@ -1,5 +1,14 @@
 //:: Container definitions can contain 'type ingredients'
 
+//: pre-requisite: extend our notion of containers to not necessarily be
+//: atomic types
+:(before "End is_mu_container(type) Special-cases")
+if (!type->atom)
+  return is_mu_container(root_type(type));
+:(before "End is_mu_exclusive_container(type) Special-cases")
+if (!type->atom)
+  return is_mu_exclusive_container(root_type(type));
+
 :(scenario size_of_shape_shifting_container)
 container foo:_t [
   x:_t
@@ -267,10 +276,10 @@ def main [
 
 :(before "End element_type Special-cases")
 replace_type_ingredients(element, type, info);
-:(before "Compute Container Size(element)")
-replace_type_ingredients(element, type, info);
-:(before "Compute Exclusive Container Size(element)")
-replace_type_ingredients(element, type, info);
+:(before "Compute Container Size(element, full_type)")
+replace_type_ingredients(element, full_type, container_info);
+:(before "Compute Exclusive Container Size(element, full_type)")
+replace_type_ingredients(element, full_type, exclusive_container_info);
 :(before "Compute Container Address Offset(element)")
 replace_type_ingredients(element, type, info);
 if (contains_type_ingredient(element)) return;  // error raised elsewhere
@@ -296,63 +305,42 @@ bool contains_type_ingredient(const reagent& x) {
 
 bool contains_type_ingredient(const type_tree* type) {
   if (!type) return false;
-  if (type->value >= START_TYPE_INGREDIENTS) return true;
-  assert(!is_type_ingredient_name(type->name));
+  if (type->atom) return type->value >= START_TYPE_INGREDIENTS;
   return contains_type_ingredient(type->left) || contains_type_ingredient(type->right);
 }
 
 // replace all type_ingredients in element_type with corresponding elements of callsite_type
-// todo: too complicated and likely incomplete; maybe avoid replacing in place?
 void replace_type_ingredients(type_tree* element_type, const type_tree* callsite_type, const type_info& container_info) {
   if (!callsite_type) return;  // error but it's already been raised above
   if (!element_type) return;
-
-  // A. recurse first to avoid nested replaces (which I can't reason about yet)
-  replace_type_ingredients(element_type->left, callsite_type, container_info);
-  replace_type_ingredients(element_type->right, callsite_type, container_info);
+  if (!element_type->atom) {
+    replace_type_ingredients(element_type->left, callsite_type, container_info);
+    replace_type_ingredients(element_type->right, callsite_type, container_info);
+    return;
+  }
   if (element_type->value < START_TYPE_INGREDIENTS) return;
-
   const int type_ingredient_index = element_type->value-START_TYPE_INGREDIENTS;
   if (!has_nth_type(callsite_type, type_ingredient_index)) {
     raise << "illegal type " << names_to_string(callsite_type) << " seems to be missing a type ingredient or three\n" << end();
     return;
   }
+  *element_type = *nth_type_ingredient(callsite_type, type_ingredient_index, container_info);
+}
 
-  // B. replace the current location
-  const type_tree* replacement = NULL;
-  bool zig_left = false;
-  {
-    const type_tree* curr = callsite_type;
-    for (int i = 0; i < type_ingredient_index; ++i)
-      curr = curr->right;
-    if (curr && curr->left) {
-      replacement = curr->left;
-      zig_left = true;
-    }
-    else {
-      // We want foo:_t to be used like foo:number, which expands to {foo: number}
-      // rather than {foo: (number)}
-      // We'd also like to use it with multiple types: foo:address:number.
-      replacement = curr;
-    }
-  }
-  if (element_type->right && replacement->right && zig_left) {  // ZERO confidence that this condition is accurate
-    element_type->name = "";
-    element_type->value = 0;
-    element_type->left = new type_tree(*replacement);
-  }
-  else {
-    string old_name = element_type->name;
-    element_type->name = replacement->name;
-    element_type->value = replacement->value;
-    assert(!element_type->left);  // since value is set
-    element_type->left = replacement->left ? new type_tree(*replacement->left) : NULL;
-    if (zig_left || final_type_ingredient(type_ingredient_index, container_info)) {
-      type_tree* old_right = element_type->right;
-      element_type->right = replacement->right ? new type_tree(*replacement->right) : NULL;
-      append(element_type->right, old_right);
-    }
+const type_tree* nth_type_ingredient(const type_tree* callsite_type, int type_ingredient_index, const type_info& container_info) {
+  bool final = final_type_ingredient(type_ingredient_index, container_info);
+  const type_tree* curr = callsite_type;
+  for (int i = 0; i < type_ingredient_index; ++i) {
+    assert(curr);
+    assert(!curr->atom);
+//?     cerr << "type ingredient " << i << " is " << to_string(curr->left) << '\n';
+    curr = curr->right;
   }
+  assert(curr);
+  if (curr->atom) return curr;
+  if (!final) return curr->left;
+  if (!curr->right) return curr->left;
+  return curr;
 }
 
 bool final_type_ingredient(int type_ingredient_index, const type_info& container_info) {
@@ -482,7 +470,128 @@ def main [
 ]
 +error: illegal type "foo" seems to be missing a type ingredient or three
 
-//: 'merge' on shape-shifting containers
+//:: fix up previous layers
+
+//: We have two transforms in previous layers -- for computing sizes and
+//: offsets containing addresses for containers and exclusive containers --
+//: that we need to teach about type ingredients.
+
+:(before "End compute_container_sizes Non-atom Cases")
+const type_tree* root = root_type(type);
+type_info& info = get(Type, root->value);
+if (info.kind == CONTAINER) {
+  compute_container_sizes(info, type, pending_metadata);
+  return;
+}
+if (info.kind == EXCLUSIVE_CONTAINER) {
+  compute_exclusive_container_sizes(info, type, pending_metadata);
+  return;
+}
+
+:(before "End Unit Tests")
+void test_container_sizes_shape_shifting_container() {
+  run("container foo:_t [\n"
+      "  x:number\n"
+      "  y:_t\n"
+      "]\n");
+  reagent r("x:foo:point");
+  compute_container_sizes(r);
+  CHECK_EQ(r.metadata.size, 3);
+}
+
+void test_container_sizes_shape_shifting_exclusive_container() {
+  run("exclusive-container foo:_t [\n"
+      "  x:number\n"
+      "  y:_t\n"
+      "]\n");
+  reagent r("x:foo:point");
+  compute_container_sizes(r);
+  CHECK_EQ(r.metadata.size, 3);
+  reagent r2("x:foo:number");
+  compute_container_sizes(r2);
+  CHECK_EQ(r2.metadata.size, 2);
+}
+
+void test_container_sizes_compound_type_ingredient() {
+  run("container foo:_t [\n"
+      "  x:number\n"
+      "  y:_t\n"
+      "]\n");
+  reagent r("x:foo:address:point");
+  compute_container_sizes(r);
+  CHECK_EQ(r.metadata.size, 2);
+  // scan also pre-computes metadata for type ingredient
+  reagent point("x:point");
+  CHECK(contains_key(Container_metadata, point.type));
+  CHECK_EQ(get(Container_metadata, point.type).size, 2);
+}
+
+void test_container_sizes_recursive_shape_shifting_container() {
+  run("container foo:_t [\n"
+      "  x:number\n"
+      "  y:address:foo:_t\n"
+      "]\n");
+  reagent r2("x:foo:number");
+  compute_container_sizes(r2);
+  CHECK_EQ(r2.metadata.size, 2);
+}
+
+:(before "End compute_container_address_offsets Non-atom Cases")
+const type_tree* root = root_type(type);
+type_info& info = get(Type, root->value);
+if (info.kind == CONTAINER) {
+  compute_container_address_offsets(info, type);
+  return;
+}
+if (info.kind == EXCLUSIVE_CONTAINER) {
+  compute_exclusive_container_address_offsets(info, type);
+  return;
+}
+
+:(before "End Unit Tests")
+void test_container_address_offsets_in_shape_shifting_container() {
+  run("container foo:_t [\n"
+      "  x:number\n"
+      "  y:_t\n"
+      "]\n");
+  reagent r("x:foo:address:number");
+  compute_container_sizes(r);
+  compute_container_address_offsets(r);
+  CHECK_EQ(SIZE(r.metadata.address), 1);
+  CHECK(contains_key(r.metadata.address, set<tag_condition_info>()));
+  set<address_element_info>& offset_info = get(r.metadata.address, set<tag_condition_info>());
+  CHECK_EQ(SIZE(offset_info), 1);
+  CHECK_EQ(offset_info.begin()->offset, 1);  //
+  CHECK(offset_info.begin()->payload_type->atom);
+  CHECK_EQ(offset_info.begin()->payload_type->name, "number");
+}
+
+void test_container_address_offsets_in_nested_shape_shifting_container() {
+  run("container foo:_t [\n"
+      "  x:number\n"
+      "  y:_t\n"
+      "]\n"
+      "container bar:_t [\n"
+      "  x:_t\n"
+      "  y:foo:_t\n"
+      "]\n");
+  reagent r("x:bar:address:number");
+  CLEAR_TRACE;
+  compute_container_sizes(r);
+  compute_container_address_offsets(r);
+  CHECK_EQ(SIZE(r.metadata.address), 1);
+  CHECK(contains_key(r.metadata.address, set<tag_condition_info>()));
+  set<address_element_info>& offset_info = get(r.metadata.address, set<tag_condition_info>());
+  CHECK_EQ(SIZE(offset_info), 2);
+  CHECK_EQ(offset_info.begin()->offset, 0);  //
+  CHECK(offset_info.begin()->payload_type->atom);
+  CHECK_EQ(offset_info.begin()->payload_type->name, "number");
+  CHECK_EQ((++offset_info.begin())->offset, 2);  //
+  CHECK((++offset_info.begin())->payload_type->atom);
+  CHECK_EQ((++offset_info.begin())->payload_type->name, "number");
+}
+
+//:: 'merge' on shape-shifting containers
 
 :(scenario merge_check_shape_shifting_container_containing_exclusive_container)
 container foo:_elem [