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<pre id='vimCodeElement'>
<span class="Comment">//: Exclusive containers contain exactly one of a fixed number of 'variants'</span>
<span class="Comment">//: of different types.</span>
<span class="Comment">//:</span>
<span class="Comment">//: They also implicitly contain a tag describing precisely which variant is</span>
<span class="Comment">//: currently stored in them.</span>

<span class="Delimiter">:(before &quot;End Mu Types Initialization&quot;)</span>
<span class="Comment">//: We'll use this container as a running example, with two number elements.</span>
<span class="Delimiter">{</span>
type_ordinal tmp = put<span class="Delimiter">(</span>Type_ordinal<span class="Delimiter">,</span> <span class="Constant">&quot;number-or-point&quot;</span><span class="Delimiter">,</span> Next_type_ordinal++<span class="Delimiter">);</span>
get_or_insert<span class="Delimiter">(</span>Type<span class="Delimiter">,</span> tmp<span class="Delimiter">).</span>size = <span class="Constant">2</span><span class="Delimiter">;</span>
get<span class="Delimiter">(</span>Type<span class="Delimiter">,</span> tmp<span class="Delimiter">).</span>kind = EXCLUSIVE_CONTAINER<span class="Delimiter">;</span>
get<span class="Delimiter">(</span>Type<span class="Delimiter">,</span> tmp<span class="Delimiter">).</span>name = <span class="Constant">&quot;number-or-point&quot;</span><span class="Delimiter">;</span>
get<span class="Delimiter">(</span>Type<span class="Delimiter">,</span> tmp<span class="Delimiter">).</span>elements<span class="Delimiter">.</span>push_back<span class="Delimiter">(</span>reagent<span class="Delimiter">(</span><span class="Constant">&quot;i:number&quot;</span><span class="Delimiter">));</span>
get<span class="Delimiter">(</span>Type<span class="Delimiter">,</span> tmp<span class="Delimiter">).</span>elements<span class="Delimiter">.</span>push_back<span class="Delimiter">(</span>reagent<span class="Delimiter">(</span><span class="Constant">&quot;p:point&quot;</span><span class="Delimiter">));</span>
<span class="Delimiter">}</span>

<span class="Comment">//: Tests in this layer often explicitly setup memory before reading it as an</span>
<span class="Comment">//: array. Don't do this in general. I'm tagging exceptions with /raw to</span>
<span class="Comment">//: avoid errors.</span>
<span class="Delimiter">:(scenario copy_exclusive_container)</span>
<span class="Comment"># Copying exclusive containers copies all their contents and an extra location for the tag.</span>
recipe main [
  <span class="Constant">1</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">1</span>  <span class="Comment"># 'point' variant</span>
  <span class="Constant">2</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">34</span>
  <span class="Constant">3</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">35</span>
  <span class="Constant">4</span>:number-or-point<span class="Special"> &lt;- </span>copy <span class="Constant">1</span>:number-or-point/unsafe
]
<span class="traceContains">+mem: storing 1 in location 4</span>
<span class="traceContains">+mem: storing 34 in location 5</span>
<span class="traceContains">+mem: storing 35 in location 6</span>

<span class="Delimiter">:(before &quot;End size_of(type) Cases&quot;)</span>
if <span class="Delimiter">(</span>t<span class="Delimiter">.</span>kind == EXCLUSIVE_CONTAINER<span class="Delimiter">)</span> <span class="Delimiter">{</span>
  <span class="Comment">// size of an exclusive container is the size of its largest variant</span>
  <span class="Comment">// (So like containers, it can't contain arrays.)</span>
  long long int result = <span class="Constant">0</span><span class="Delimiter">;</span>
  for <span class="Delimiter">(</span>long long int i = <span class="Constant">0</span><span class="Delimiter">;</span> i &lt; t<span class="Delimiter">.</span>size<span class="Delimiter">;</span> ++i<span class="Delimiter">)</span> <span class="Delimiter">{</span>
    reagent tmp<span class="Delimiter">;</span>
    tmp<span class="Delimiter">.</span>type = new type_tree<span class="Delimiter">(</span>*type<span class="Delimiter">);</span>
    long long int size = size_of<span class="Delimiter">(</span>variant_type<span class="Delimiter">(</span>tmp<span class="Delimiter">,</span> i<span class="Delimiter">));</span>
    if <span class="Delimiter">(</span>size &gt; result<span class="Delimiter">)</span> result = size<span class="Delimiter">;</span>
  <span class="Delimiter">}</span>
  <span class="Comment">// ...+1 for its tag.</span>
  <span class="Identifier">return</span> result+<span class="Constant">1</span><span class="Delimiter">;</span>
<span class="Delimiter">}</span>

<span class="SalientComment">//:: To access variants of an exclusive container, use 'maybe-convert'.</span>
<span class="Comment">//: It always returns an address (so that you can modify it) or null (to</span>
<span class="Comment">//: signal that the conversion failed (because the container contains a</span>
<span class="Comment">//: different variant).</span>

<span class="Comment">//: 'maybe-convert' requires a literal in ingredient 1. We'll use a synonym</span>
<span class="Comment">//: called 'variant'.</span>
<span class="Delimiter">:(before &quot;End Mu Types Initialization&quot;)</span>
put<span class="Delimiter">(</span>Type_ordinal<span class="Delimiter">,</span> <span class="Constant">&quot;variant&quot;</span><span class="Delimiter">,</span> <span class="Constant">0</span><span class="Delimiter">);</span>

<span class="Delimiter">:(scenario maybe_convert)</span>
recipe main [
  <span class="Constant">12</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">1</span>
  <span class="Constant">13</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">35</span>
  <span class="Constant">14</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">36</span>
  <span class="Constant">20</span>:address:point<span class="Special"> &lt;- </span>maybe-convert <span class="Constant">12</span>:number-or-point/unsafe<span class="Delimiter">,</span> <span class="Constant">1</span>:variant
]
<span class="traceContains">+mem: storing 13 in location 20</span>

<span class="Delimiter">:(scenario maybe_convert_fail)</span>
recipe main [
  <span class="Constant">12</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">1</span>
  <span class="Constant">13</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">35</span>
  <span class="Constant">14</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">36</span>
  <span class="Constant">20</span>:address:number<span class="Special"> &lt;- </span>maybe-convert <span class="Constant">12</span>:number-or-point/unsafe<span class="Delimiter">,</span> <span class="Constant">0</span>:variant
]
<span class="traceContains">+mem: storing 0 in location 20</span>

<span class="Delimiter">:(before &quot;End Primitive Recipe Declarations&quot;)</span>
MAYBE_CONVERT<span class="Delimiter">,</span>
<span class="Delimiter">:(before &quot;End Primitive Recipe Numbers&quot;)</span>
put<span class="Delimiter">(</span>Recipe_ordinal<span class="Delimiter">,</span> <span class="Constant">&quot;maybe-convert&quot;</span><span class="Delimiter">,</span> MAYBE_CONVERT<span class="Delimiter">);</span>
<span class="Delimiter">:(before &quot;End Primitive Recipe Checks&quot;)</span>
case MAYBE_CONVERT: <span class="Delimiter">{</span>
  const recipe&amp; caller = get<span class="Delimiter">(</span>Recipe<span class="Delimiter">,</span> r<span class="Delimiter">);</span>
  if <span class="Delimiter">(</span>SIZE<span class="Delimiter">(</span>inst<span class="Delimiter">.</span>ingredients<span class="Delimiter">)</span> != <span class="Constant">2</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
    raise_error &lt;&lt; maybe<span class="Delimiter">(</span>caller<span class="Delimiter">.</span>name<span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;'maybe-convert' expects exactly 2 ingredients in '&quot;</span> &lt;&lt; to_string<span class="Delimiter">(</span>inst<span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;'</span><span class="cSpecial">\n</span><span class="Constant">&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
    <span class="Identifier">break</span><span class="Delimiter">;</span>
  <span class="Delimiter">}</span>
  reagent base = inst<span class="Delimiter">.</span>ingredients<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Constant">0</span><span class="Delimiter">);</span>
  canonize_type<span class="Delimiter">(</span>base<span class="Delimiter">);</span>
  if <span class="Delimiter">(</span>!base<span class="Delimiter">.</span>type || !base<span class="Delimiter">.</span>type<span class="Delimiter">-&gt;</span>value || get<span class="Delimiter">(</span>Type<span class="Delimiter">,</span> base<span class="Delimiter">.</span>type<span class="Delimiter">-&gt;</span>value<span class="Delimiter">).</span>kind != EXCLUSIVE_CONTAINER<span class="Delimiter">)</span> <span class="Delimiter">{</span>
    raise_error &lt;&lt; maybe<span class="Delimiter">(</span>caller<span class="Delimiter">.</span>name<span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;first ingredient of 'maybe-convert' should be an exclusive-container, but got &quot;</span> &lt;&lt; base<span class="Delimiter">.</span>original_string &lt;&lt; <span class="cSpecial">'\n'</span> &lt;&lt; end<span class="Delimiter">();</span>
    <span class="Identifier">break</span><span class="Delimiter">;</span>
  <span class="Delimiter">}</span>
  if <span class="Delimiter">(</span>!is_literal<span class="Delimiter">(</span>inst<span class="Delimiter">.</span>ingredients<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Constant">1</span><span class="Delimiter">)))</span> <span class="Delimiter">{</span>
    raise_error &lt;&lt; maybe<span class="Delimiter">(</span>caller<span class="Delimiter">.</span>name<span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;second ingredient of 'maybe-convert' should have type 'variant', but got &quot;</span> &lt;&lt; inst<span class="Delimiter">.</span>ingredients<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Constant">1</span><span class="Delimiter">).</span>original_string &lt;&lt; <span class="cSpecial">'\n'</span> &lt;&lt; end<span class="Delimiter">();</span>
    <span class="Identifier">break</span><span class="Delimiter">;</span>
  <span class="Delimiter">}</span>
  if <span class="Delimiter">(</span>inst<span class="Delimiter">.</span>products<span class="Delimiter">.</span>empty<span class="Delimiter">())</span> <span class="Identifier">break</span><span class="Delimiter">;</span>
  reagent product = inst<span class="Delimiter">.</span>products<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Constant">0</span><span class="Delimiter">);</span>
  if <span class="Delimiter">(</span>!canonize_type<span class="Delimiter">(</span>product<span class="Delimiter">))</span> <span class="Identifier">break</span><span class="Delimiter">;</span>
  reagent&amp; offset = inst<span class="Delimiter">.</span>ingredients<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Constant">1</span><span class="Delimiter">);</span>
  populate_value<span class="Delimiter">(</span>offset<span class="Delimiter">);</span>
  if <span class="Delimiter">(</span>offset<span class="Delimiter">.</span>value &gt;= SIZE<span class="Delimiter">(</span>get<span class="Delimiter">(</span>Type<span class="Delimiter">,</span> base<span class="Delimiter">.</span>type<span class="Delimiter">-&gt;</span>value<span class="Delimiter">).</span>elements<span class="Delimiter">))</span> <span class="Delimiter">{</span>
    raise_error &lt;&lt; maybe<span class="Delimiter">(</span>caller<span class="Delimiter">.</span>name<span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;invalid tag &quot;</span> &lt;&lt; offset<span class="Delimiter">.</span>value &lt;&lt; <span class="Constant">&quot; in '&quot;</span> &lt;&lt; to_string<span class="Delimiter">(</span>inst<span class="Delimiter">)</span> &lt;&lt; <span class="cSpecial">'\n'</span> &lt;&lt; end<span class="Delimiter">();</span>
    <span class="Identifier">break</span><span class="Delimiter">;</span>
  <span class="Delimiter">}</span>
  reagent variant = variant_type<span class="Delimiter">(</span>base<span class="Delimiter">,</span> offset<span class="Delimiter">.</span>value<span class="Delimiter">);</span>
  variant<span class="Delimiter">.</span>type = new type_tree<span class="Delimiter">(</span><span class="Constant">&quot;address&quot;</span><span class="Delimiter">,</span> get<span class="Delimiter">(</span>Type_ordinal<span class="Delimiter">,</span> <span class="Constant">&quot;address&quot;</span><span class="Delimiter">),</span> variant<span class="Delimiter">.</span>type<span class="Delimiter">);</span>
  if <span class="Delimiter">(</span>!types_coercible<span class="Delimiter">(</span>product<span class="Delimiter">,</span> variant<span class="Delimiter">))</span> <span class="Delimiter">{</span>
    raise_error &lt;&lt; maybe<span class="Delimiter">(</span>caller<span class="Delimiter">.</span>name<span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;'maybe-convert &quot;</span> &lt;&lt; base<span class="Delimiter">.</span>original_string &lt;&lt; <span class="Constant">&quot;, &quot;</span> &lt;&lt; inst<span class="Delimiter">.</span>ingredients<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Constant">1</span><span class="Delimiter">).</span>original_string &lt;&lt; <span class="Constant">&quot;' should write to &quot;</span> &lt;&lt; to_string<span class="Delimiter">(</span>variant<span class="Delimiter">.</span>type<span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot; but &quot;</span> &lt;&lt; product<span class="Delimiter">.</span>name &lt;&lt; <span class="Constant">&quot; has type &quot;</span> &lt;&lt; to_string<span class="Delimiter">(</span>product<span class="Delimiter">.</span>type<span class="Delimiter">)</span> &lt;&lt; <span class="cSpecial">'\n'</span> &lt;&lt; end<span class="Delimiter">();</span>
    <span class="Identifier">break</span><span class="Delimiter">;</span>
  <span class="Delimiter">}</span>
  <span class="Identifier">break</span><span class="Delimiter">;</span>
<span class="Delimiter">}</span>
<span class="Delimiter">:(before &quot;End Primitive Recipe Implementations&quot;)</span>
case MAYBE_CONVERT: <span class="Delimiter">{</span>
  reagent base = current_instruction<span class="Delimiter">().</span>ingredients<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Constant">0</span><span class="Delimiter">);</span>
  canonize<span class="Delimiter">(</span>base<span class="Delimiter">);</span>
  long long int base_address = base<span class="Delimiter">.</span>value<span class="Delimiter">;</span>
  if <span class="Delimiter">(</span>base_address == <span class="Constant">0</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
    raise_error &lt;&lt; maybe<span class="Delimiter">(</span>current_recipe_name<span class="Delimiter">())</span> &lt;&lt; <span class="Constant">&quot;tried to access location 0 in '&quot;</span> &lt;&lt; to_string<span class="Delimiter">(</span>current_instruction<span class="Delimiter">())</span> &lt;&lt; <span class="Constant">&quot;'</span><span class="cSpecial">\n</span><span class="Constant">&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
    <span class="Identifier">break</span><span class="Delimiter">;</span>
  <span class="Delimiter">}</span>
  long long int tag = current_instruction<span class="Delimiter">().</span>ingredients<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Constant">1</span><span class="Delimiter">).</span>value<span class="Delimiter">;</span>
  long long int result<span class="Delimiter">;</span>
  if <span class="Delimiter">(</span>tag == static_cast&lt;long long int&gt;<span class="Delimiter">(</span>get_or_insert<span class="Delimiter">(</span>Memory<span class="Delimiter">,</span> base_address<span class="Delimiter">)))</span> <span class="Delimiter">{</span>
    result = base_address+<span class="Constant">1</span><span class="Delimiter">;</span>
  <span class="Delimiter">}</span>
  else <span class="Delimiter">{</span>
    result = <span class="Constant">0</span><span class="Delimiter">;</span>
  <span class="Delimiter">}</span>
  products<span class="Delimiter">.</span>resize<span class="Delimiter">(</span><span class="Constant">1</span><span class="Delimiter">);</span>
  products<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Constant">0</span><span class="Delimiter">).</span>push_back<span class="Delimiter">(</span>result<span class="Delimiter">);</span>
  <span class="Identifier">break</span><span class="Delimiter">;</span>
<span class="Delimiter">}</span>

<span class="Delimiter">:(code)</span>
const reagent variant_type<span class="Delimiter">(</span>const reagent&amp; canonized_base<span class="Delimiter">,</span> long long int tag<span class="Delimiter">)</span> <span class="Delimiter">{</span>
  assert<span class="Delimiter">(</span>tag &gt;= <span class="Constant">0</span><span class="Delimiter">);</span>
  assert<span class="Delimiter">(</span>contains_key<span class="Delimiter">(</span>Type<span class="Delimiter">,</span> canonized_base<span class="Delimiter">.</span>type<span class="Delimiter">-&gt;</span>value<span class="Delimiter">));</span>
  assert<span class="Delimiter">(</span>!get<span class="Delimiter">(</span>Type<span class="Delimiter">,</span> canonized_base<span class="Delimiter">.</span>type<span class="Delimiter">-&gt;</span>value<span class="Delimiter">).</span>name<span class="Delimiter">.</span>empty<span class="Delimiter">());</span>
  const type_info&amp; info = get<span class="Delimiter">(</span>Type<span class="Delimiter">,</span> canonized_base<span class="Delimiter">.</span>type<span class="Delimiter">-&gt;</span>value<span class="Delimiter">);</span>
  assert<span class="Delimiter">(</span>info<span class="Delimiter">.</span>kind == EXCLUSIVE_CONTAINER<span class="Delimiter">);</span>
  reagent element = info<span class="Delimiter">.</span>elements<span class="Delimiter">.</span>at<span class="Delimiter">(</span>tag<span class="Delimiter">);</span>
  <span class="Comment">// End variant_type Special-cases</span>
  <span class="Identifier">return</span> element<span class="Delimiter">;</span>
<span class="Delimiter">}</span>

<span class="Delimiter">:(scenario maybe_convert_product_type_mismatch)</span>
<span class="Special">% Hide_errors = true;</span>
recipe main [
  <span class="Constant">12</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">1</span>
  <span class="Constant">13</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">35</span>
  <span class="Constant">14</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">36</span>
  <span class="Constant">20</span>:address:number<span class="Special"> &lt;- </span>maybe-convert <span class="Constant">12</span>:number-or-point/unsafe<span class="Delimiter">,</span> <span class="Constant">1</span>:variant
]
<span class="traceContains">+error: main: 'maybe-convert 12:number-or-point/unsafe, 1:variant' should write to (address point) but 20 has type (address number)</span>

<span class="SalientComment">//:: Allow exclusive containers to be defined in mu code.</span>

<span class="Delimiter">:(scenario exclusive_container)</span>
exclusive-container foo [
  x:number
  y:number
]
<span class="traceContains">+parse: --- defining exclusive-container foo</span>
<span class="traceContains">+parse: element: x: &quot;number&quot;</span>
<span class="traceContains">+parse: element: y: &quot;number&quot;</span>

<span class="Delimiter">:(before &quot;End Command Handlers&quot;)</span>
else if <span class="Delimiter">(</span>command == <span class="Constant">&quot;exclusive-container&quot;</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
  insert_container<span class="Delimiter">(</span>command<span class="Delimiter">,</span> EXCLUSIVE_CONTAINER<span class="Delimiter">,</span> in<span class="Delimiter">);</span>
<span class="Delimiter">}</span>

<span class="Comment">//: arrays are disallowed inside exclusive containers unless their length is</span>
<span class="Comment">//: fixed in advance</span>

<span class="Delimiter">:(scenario exclusive_container_contains_array)</span>
exclusive-container foo [
  x:array:number:<span class="Constant">3</span>
]
$error: <span class="Constant">0</span>

<span class="Delimiter">:(scenario exclusive_container_disallows_dynamic_array_element)</span>
<span class="Special">% Hide_errors = true;</span>
exclusive-container foo [
  x:array:number
]
<span class="traceContains">+error: container 'foo' cannot determine size of element x</span>

<span class="SalientComment">//:: To construct exclusive containers out of variant types, use 'merge'.</span>
<span class="Delimiter">:(scenario lift_to_exclusive_container)</span>
exclusive-container foo [
  x:number
  y:number
]

recipe main [
  <span class="Constant">1</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">34</span>
  <span class="Constant">2</span>:foo<span class="Special"> &lt;- </span>merge <span class="Constant">0</span>/x<span class="Delimiter">,</span> <span class="Constant">1</span>:number  <span class="Comment"># tag must be a literal when merging exclusive containers</span>
  <span class="Constant">4</span>:foo<span class="Special"> &lt;- </span>merge <span class="Constant">1</span>/y<span class="Delimiter">,</span> <span class="Constant">1</span>:number
]
<span class="traceContains">+mem: storing 0 in location 2</span>
<span class="traceContains">+mem: storing 34 in location 3</span>
<span class="traceContains">+mem: storing 1 in location 4</span>
<span class="traceContains">+mem: storing 34 in location 5</span>

<span class="Comment">//: type-checking for 'merge' on exclusive containers</span>

<span class="Delimiter">:(scenario merge_handles_exclusive_container)</span>
exclusive-container foo [
  x:number
  y:bar
]
container bar [
  z:number
]
recipe main [
  <span class="Constant">1</span>:foo<span class="Special"> &lt;- </span>merge <span class="Constant">0</span>/x<span class="Delimiter">,</span> <span class="Constant">34</span>
]
<span class="traceContains">+mem: storing 0 in location 1</span>
<span class="traceContains">+mem: storing 34 in location 2</span>
$error: <span class="Constant">0</span>

<span class="Delimiter">:(scenario merge_requires_literal_tag_for_exclusive_container)</span>
<span class="Special">% Hide_errors = true;</span>
exclusive-container foo [
  x:number
  y:bar
]
container bar [
  z:number
]
recipe main [
  local-scope
  <span class="Constant">1</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">0</span>
  <span class="Constant">2</span>:foo<span class="Special"> &lt;- </span>merge <span class="Constant">1</span>:number<span class="Delimiter">,</span> <span class="Constant">34</span>
]
<span class="traceContains">+error: main: ingredient 0 of 'merge' should be a literal, for the tag of exclusive-container foo</span>

<span class="Delimiter">:(before &quot;End valid_merge Cases&quot;)</span>
case EXCLUSIVE_CONTAINER: <span class="Delimiter">{</span>
  assert<span class="Delimiter">(</span>state<span class="Delimiter">.</span>data<span class="Delimiter">.</span>top<span class="Delimiter">().</span>container_element_index == <span class="Constant">0</span><span class="Delimiter">);</span>
  trace<span class="Delimiter">(</span><span class="Constant">9999</span><span class="Delimiter">,</span> <span class="Constant">&quot;transform&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;checking exclusive container &quot;</span> &lt;&lt; to_string<span class="Delimiter">(</span>container<span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot; vs ingredient &quot;</span> &lt;&lt; ingredient_index &lt;&lt; end<span class="Delimiter">();</span>
  if <span class="Delimiter">(</span>!is_literal<span class="Delimiter">(</span>ingredients<span class="Delimiter">.</span>at<span class="Delimiter">(</span>ingredient_index<span class="Delimiter">)))</span> <span class="Delimiter">{</span>
    raise_error &lt;&lt; maybe<span class="Delimiter">(</span>caller<span class="Delimiter">.</span>name<span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;ingredient &quot;</span> &lt;&lt; ingredient_index &lt;&lt; <span class="Constant">&quot; of 'merge' should be a literal, for the tag of exclusive-container &quot;</span> &lt;&lt; container_info<span class="Delimiter">.</span>name &lt;&lt; <span class="cSpecial">'\n'</span> &lt;&lt; end<span class="Delimiter">();</span>
    <span class="Identifier">return</span><span class="Delimiter">;</span>
  <span class="Delimiter">}</span>
  reagent ingredient = ingredients<span class="Delimiter">.</span>at<span class="Delimiter">(</span>ingredient_index<span class="Delimiter">);</span>  <span class="Comment">// unnecessary copy just to keep this function from modifying caller</span>
  populate_value<span class="Delimiter">(</span>ingredient<span class="Delimiter">);</span>
  if <span class="Delimiter">(</span>ingredient<span class="Delimiter">.</span>value &gt;= SIZE<span class="Delimiter">(</span>container_info<span class="Delimiter">.</span>elements<span class="Delimiter">))</span> <span class="Delimiter">{</span>
    raise_error &lt;&lt; maybe<span class="Delimiter">(</span>caller<span class="Delimiter">.</span>name<span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;invalid tag at &quot;</span> &lt;&lt; ingredient_index &lt;&lt; <span class="Constant">&quot; for &quot;</span> &lt;&lt; container_info<span class="Delimiter">.</span>name &lt;&lt; <span class="Constant">&quot; in '&quot;</span> &lt;&lt; to_string<span class="Delimiter">(</span>inst<span class="Delimiter">)</span> &lt;&lt; <span class="cSpecial">'\n'</span> &lt;&lt; end<span class="Delimiter">();</span>
    <span class="Identifier">return</span><span class="Delimiter">;</span>
  <span class="Delimiter">}</span>
  reagent variant = variant_type<span class="Delimiter">(</span>container<span class="Delimiter">,</span> ingredient<span class="Delimiter">.</span>value<span class="Delimiter">);</span>
  trace<span class="Delimiter">(</span><span class="Constant">9999</span><span class="Delimiter">,</span> <span class="Constant">&quot;transform&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;tag: &quot;</span> &lt;&lt; ingredient<span class="Delimiter">.</span>value &lt;&lt; end<span class="Delimiter">();</span>
  <span class="Comment">// replace union with its variant</span>
  state<span class="Delimiter">.</span>data<span class="Delimiter">.</span>pop<span class="Delimiter">();</span>
  state<span class="Delimiter">.</span>data<span class="Delimiter">.</span>push<span class="Delimiter">(</span>merge_check_point<span class="Delimiter">(</span>variant<span class="Delimiter">,</span> <span class="Constant">0</span><span class="Delimiter">));</span>
  ++ingredient_index<span class="Delimiter">;</span>
  <span class="Identifier">break</span><span class="Delimiter">;</span>
<span class="Delimiter">}</span>

<span class="Delimiter">:(scenario merge_check_container_containing_exclusive_container)</span>
container foo [
  x:number
  y:bar
]
exclusive-container bar [
  x:number
  y:number
]
recipe main [
  <span class="Constant">1</span>:foo<span class="Special"> &lt;- </span>merge <span class="Constant">23</span><span class="Delimiter">,</span> <span class="Constant">1</span>/y<span class="Delimiter">,</span> <span class="Constant">34</span>
]
<span class="traceContains">+mem: storing 23 in location 1</span>
<span class="traceContains">+mem: storing 1 in location 2</span>
<span class="traceContains">+mem: storing 34 in location 3</span>
$error: <span class="Constant">0</span>

<span class="Delimiter">:(scenario merge_check_container_containing_exclusive_container_2)</span>
<span class="Special">% Hide_errors = true;</span>
container foo [
  x:number
  y:bar
]
exclusive-container bar [
  x:number
  y:number
]
recipe main [
  <span class="Constant">1</span>:foo<span class="Special"> &lt;- </span>merge <span class="Constant">23</span><span class="Delimiter">,</span> <span class="Constant">1</span>/y<span class="Delimiter">,</span> <span class="Constant">34</span><span class="Delimiter">,</span> <span class="Constant">35</span>
]
<span class="traceContains">+error: main: too many ingredients in '1:foo &lt;- merge 23, 1/y, 34, 35'</span>

<span class="Delimiter">:(scenario merge_check_exclusive_container_containing_container)</span>
exclusive-container foo [
  x:number
  y:bar
]
container bar [
  x:number
  y:number
]
recipe main [
  <span class="Constant">1</span>:foo<span class="Special"> &lt;- </span>merge <span class="Constant">1</span>/y<span class="Delimiter">,</span> <span class="Constant">23</span><span class="Delimiter">,</span> <span class="Constant">34</span>
]
<span class="traceContains">+mem: storing 1 in location 1</span>
<span class="traceContains">+mem: storing 23 in location 2</span>
<span class="traceContains">+mem: storing 34 in location 3</span>
$error: <span class="Constant">0</span>

<span class="Delimiter">:(scenario merge_check_exclusive_container_containing_container_2)</span>
exclusive-container foo [
  x:number
  y:bar
]
container bar [
  x:number
  y:number
]
recipe main [
  <span class="Constant">1</span>:foo<span class="Special"> &lt;- </span>merge <span class="Constant">0</span>/x<span class="Delimiter">,</span> <span class="Constant">23</span>
]
$error: <span class="Constant">0</span>

<span class="Delimiter">:(scenario merge_check_exclusive_container_containing_container_3)</span>
<span class="Special">% Hide_errors = true;</span>
exclusive-container foo [
  x:number
  y:bar
]
container bar [
  x:number
  y:number
]
recipe main [
  <span class="Constant">1</span>:foo<span class="Special"> &lt;- </span>merge <span class="Constant">1</span>/y<span class="Delimiter">,</span> <span class="Constant">23</span>
]
<span class="traceContains">+error: main: too few ingredients in '1:foo &lt;- merge 1/y, 23'</span>

<span class="Comment">//: Since the different variants of an exclusive-container might have</span>
<span class="Comment">//: different sizes, relax the size mismatch check for 'merge' instructions.</span>
<span class="Delimiter">:(before &quot;End size_mismatch(x) Cases&quot;)</span>
if <span class="Delimiter">(</span>current_instruction<span class="Delimiter">().</span>operation == MERGE
    &amp;&amp; !current_instruction<span class="Delimiter">().</span>products<span class="Delimiter">.</span>empty<span class="Delimiter">()</span>
    &amp;&amp; current_instruction<span class="Delimiter">().</span>products<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Constant">0</span><span class="Delimiter">).</span>type<span class="Delimiter">)</span> <span class="Delimiter">{</span>
  reagent x = current_instruction<span class="Delimiter">().</span>products<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Constant">0</span><span class="Delimiter">);</span>
  canonize<span class="Delimiter">(</span>x<span class="Delimiter">);</span>
  if <span class="Delimiter">(</span>get<span class="Delimiter">(</span>Type<span class="Delimiter">,</span> x<span class="Delimiter">.</span>type<span class="Delimiter">-&gt;</span>value<span class="Delimiter">).</span>kind == EXCLUSIVE_CONTAINER<span class="Delimiter">)</span> <span class="Delimiter">{</span>
    <span class="Identifier">return</span> size_of<span class="Delimiter">(</span>x<span class="Delimiter">)</span> &lt; SIZE<span class="Delimiter">(</span>data<span class="Delimiter">);</span>
  <span class="Delimiter">}</span>
<span class="Delimiter">}</span>

<span class="Delimiter">:(scenario merge_exclusive_container_with_mismatched_sizes)</span>
container foo [
  x:number
  y:number
]

exclusive-container bar [
  x:number
  y:foo
]

recipe main [
  <span class="Constant">1</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">34</span>
  <span class="Constant">2</span>:number<span class="Special"> &lt;- </span>copy <span class="Constant">35</span>
  <span class="Constant">3</span>:bar<span class="Special"> &lt;- </span>merge <span class="Constant">0</span>/x<span class="Delimiter">,</span> <span class="Constant">1</span>:number
  <span class="Constant">6</span>:bar<span class="Special"> &lt;- </span>merge <span class="Constant">1</span>/foo<span class="Delimiter">,</span> <span class="Constant">1</span>:number<span class="Delimiter">,</span> <span class="Constant">2</span>:number
]
<span class="traceContains">+mem: storing 0 in location 3</span>
<span class="traceContains">+mem: storing 34 in location 4</span>
<span class="Comment"># bar is always 3 large so location 5 is skipped</span>
<span class="traceContains">+mem: storing 1 in location 6</span>
<span class="traceContains">+mem: storing 34 in location 7</span>
<span class="traceContains">+mem: storing 35 in location 8</span>
</pre>
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