Add exceptional return to func_invoke in embedding doc (WebAssembly#268)

Embedding:
* Adds an auxiliary syntactic class to represent exception throwing/propagation
* Allows `func_invoke` to propagate exceptions
* Adds `tag_type` and `exn_alloc` to the embedding interface

core/exec:
* Refactors exception allocation into `alloc-exception` in modules.rst to expose it to the embedder

document/core/appendix/embedding.rst

@@ -26,10 +26,19 @@ For numeric parameters, notation like :math:`n:\u32` is used to specify a symbol
 
 .. _embed-error:
 
-Errors
-~~~~~~
+Exceptions and Errors
+~~~~~~~~~~~~~~~~~~~~~
 
-Failure of an interface operation is indicated by an auxiliary syntactic class:
+Invoking an exported function may throw or propagate exceptions, expressed by an auxiliary syntactic class:
+
+.. math::
+   \begin{array}{llll}
+   \production{exception} & \exception &::=& \ETHROW ~ \exnaddr \\
+   \end{array}
+
+The exception address :math:`exnaddr` identifies the exception thrown.
+
+Failure of an interface operation is also indicated by an auxiliary syntactic class:
 
 .. math::
    \begin{array}{llll}
@@ -43,6 +52,8 @@ In addition to the error conditions specified explicitly in this section, implem
    Implementations can refine it to carry suitable classifications and diagnostic messages.
 
 
+
+
 Pre- and Post-Conditions
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -293,21 +304,24 @@ Functions
 .. index:: invocation, value, result
 .. _embed-func-invoke:
 
-:math:`\F{func\_invoke}(\store, \funcaddr, \val^\ast) : (\store, \val^\ast ~|~ \error)`
-........................................................................................
+:math:`\F{func\_invoke}(\store, \funcaddr, \val^\ast) : (\store, \val^\ast ~|~ \exception ~|~ \error)`
+......................................................................................................
 
 1. Try :ref:`invoking <exec-invocation>` the function :math:`\funcaddr` in :math:`\store` with :ref:`values <syntax-val>` :math:`\val^\ast` as arguments:
 
   a. If it succeeds with :ref:`values <syntax-val>` :math:`{\val'}^\ast` as results, then let :math:`\X{result}` be :math:`{\val'}^\ast`.
 
-  b. Else it has trapped, hence let :math:`\X{result}` be :math:`\ERROR`.
+  b. Else if the outcome is an exception with a thrown :ref:`exception <exec-throw_ref>` :math:`\REFEXNADDR~\exnaddr` as the result, then let :math:`\X{result}` be :math:`\ETHROW~\exnaddr`
+
+  c. Else it has trapped, hence let :math:`\X{result}` be :math:`\ERROR`.
 
 2. Return the new store paired with :math:`\X{result}`.
 
 .. math::
    ~ \\
    \begin{array}{lclll}
    \F{func\_invoke}(S, a, v^\ast) &=& (S', {v'}^\ast) && (\iff \invoke(S, a, v^\ast) \stepto^\ast S'; F; {v'}^\ast) \\
+   \F{func\_invoke}(S, a, v^\ast) &=& (S', \ETHROW~a') && (\iff \invoke(S, a, v^\ast) \stepto^\ast S'; F; \XT[(\REFEXNADDR~a')~\THROWREF] \\
    \F{func\_invoke}(S, a, v^\ast) &=& (S', \ERROR) && (\iff \invoke(S, a, v^\ast) \stepto^\ast S'; F; \TRAP) \\
    \end{array}
 
@@ -562,6 +576,59 @@ Tags
    \end{array}
 
 
+.. _embed-tag-type:
+
+:math:`\F{tag\_type}(\store, \tagaddr) : \tagtype`
+..................................................
+
+1. Return :math:`S.\STAGS[a].\TAGITYPE`.
+
+2. Post-condition: the returned :ref:`tag type <syntax-tagtype>` is :ref:`valid  <valid-tagtype>`.
+
+.. math::
+   \begin{array}{lclll}
+   \F{tag\_type}(S, a) &=& S.\STAGS[a].\TAGITYPE \\
+   \end{array}
+
+
+.. index:: exception, exception address, store, exception instance, exception type
+.. _embed-exception:
+
+Exceptions
+~~~~~~~~~~
+
+.. _embed-exn-alloc:
+
+:math:`\F{exn\_alloc}(\store, \tagaddr, \val^\ast) : (\store, \exnaddr)`
+........................................................................
+
+1. Pre-condition: :math:`\tagaddr` is an allocated :ref:`tag address <syntax-tagaddr>`.
+
+2. Let :math:`\exnaddr` be the result of :ref:`allocating an exception <alloc-exception>` in :math:`\store` with :ref:`tag address <syntax-tagaddr>` :math:`\tagaddr` and initialization values :math:`\val^\ast`.
+
+3. Return the new store paired with :math:`\exnaddr`.
+
+.. math::
+   \begin{array}{lclll}
+   \F{exn\_alloc}(S, \tagaddr, \val^\ast) &=& (S', a) && (\iff \allocexn(S, \tagaddr, \val^\ast) = S', a) \\
+   \end{array}
+
+
+.. _embed-exn-read:
+
+:math:`\F{exn\_read}(\store, \exnaddr) : (\tagaddr, \val^\ast)`
+...............................................................
+
+1. Let :math:`\X{ei}` be the :ref:`exception instance <syntax-exninst>` :math:`\store.\SEXNS[\exnaddr]`.
+
+2. Return the :ref:`tag address <syntax-tagaddr>` :math:`\X{ei}.\EITAG~\tagaddr` paired with :ref:`values <syntax-val>` :math:`\X{ei}.\EIFIELDS~\val^\ast`.
+
+.. math::
+   \begin{array}{lcll}
+   \F{exn\_read}(S, a) &=& (a', v^\ast) \\
+   \end{array}
+
+
 .. index:: global, global address, store, global instance, global type, value
 .. _embed-global:
 

document/core/exec/instructions.rst

@@ -2719,27 +2719,25 @@ Control Instructions
 
 2. Assert: due to :ref:`validation <valid-throw>`, :math:`F.\AMODULE.\MITAGS[x]` exists.
 
-3. Let :math:`a` be the :ref:`tag address <syntax-tagaddr>` :math:`F.\AMODULE.\MITAGS[x]`.
+3. Let :math:`ta` be the :ref:`tag address <syntax-tagaddr>` :math:`F.\AMODULE.\MITAGS[x]`.
 
-4. Assert: due to :ref:`validation <valid-throw>`, :math:`S.\STAGS[a]` exists.
+4. Assert: due to :ref:`validation <valid-throw>`, :math:`S.\STAGS[ta]` exists.
 
-5. Let :math:`\X{ti}` be the :ref:`tag instance <syntax-taginst>` :math:`S.\STAGS[a]`.
+5. Let :math:`\X{ti}` be the :ref:`tag instance <syntax-taginst>` :math:`S.\STAGS[ta]`.
 
 6. Let :math:`[t^n] \toF [{t'}^\ast]` be the :ref:`tag type <syntax-tagtype>` :math:`\X{ti}.\TAGITYPE`.
 
 7. Assert: due to :ref:`validation <valid-throw>`, there are at least :math:`n` values on the top of the stack.
 
 8. Pop the :math:`n` values :math:`\val^n` from the stack.
 
-9. Let :math:`\X{exn}` be the :ref:`exception instance <syntax-exninst>` :math:`\{ \EITAG~a, \EIFIELDS~\val^n \}`.
+9. Let :math:`\X{ea}` be the :ref:`exception address <syntax-exnaddr>` resulting from :ref:`allocating <alloc-exception>` an exception instance with tag address :math:`ta` and initializer values :math:`\val^n`.
 
-10. Let :math:`\X{ea}` be the length of :math:`S.\SEXNS`.
+10. Let :math:`\X{exn}` be :math:`S.\SEXNS[ea]`
 
-11. Append :math:`\X{exn}` to :math:`S.\SEXNS`.
+11. Push the value :math:`\REFEXNADDR~\X{ea}` to the stack.
 
-12. Push the value :math:`\REFEXNADDR~\X{ea}` to the stack.
-
-13. Execute the instruction |THROWREF|.
+12. Execute the instruction |THROWREF|.
 
 .. math::
    ~\\[-1ex]

document/core/exec/modules.rst

@@ -190,7 +190,7 @@ The following auxiliary typing rules specify this typing relation relative to a
 Allocation
 ~~~~~~~~~~
 
-New instances of :ref:`functions <syntax-funcinst>`, :ref:`tables <syntax-tableinst>`, :ref:`memories <syntax-meminst>`, :ref:`tags <syntax-taginst>`, and :ref:`globals <syntax-globalinst>` are *allocated* in a :ref:`store <syntax-store>` :math:`S`, as defined by the following auxiliary functions.
+New instances of :ref:`functions <syntax-funcinst>`, :ref:`tables <syntax-tableinst>`, :ref:`memories <syntax-meminst>`, :ref:`tags <syntax-taginst>`, :ref:`exceptions <syntax-exninst>`, and :ref:`globals <syntax-globalinst>` are *allocated* in a :ref:`store <syntax-store>` :math:`S`, as defined by the following auxiliary functions.
 
 
 .. index:: function, function instance, function address, module instance, function type
@@ -338,6 +338,32 @@ New instances of :ref:`functions <syntax-funcinst>`, :ref:`tables <syntax-tablei
    \end{array}
 
 
+.. index:: exception, exception instance, exception address, tag address
+.. _alloc-exception:
+
+:ref:`Exceptions <syntax-exninst>`
+..................................
+
+1. Let :math:`ta` be the :ref:`tag address <syntax-tagaddr>` associated with the exception to allocate and :math:`\EIFIELDS~\val^\ast` be the values to initialize the exception with.
+
+2. Let :math:`a` be the first free :ref:`exception address <syntax-exnaddr>` in :math:`S`.
+
+3. Let :math:`\exninst` be the :ref:`exception instance <syntax-exninst>` :math:`\{ \EITAG~ta, \EIFIELDS~\val^\ast \}`.
+
+4. Append :math:`\exninst` to the |SEXNS| of :math:`S`.
+
+5. Return :math:`a`.
+
+.. math::
+   \begin{array}{rlll}
+   \allocexn(S, \tagaddr, \val^\ast) &=& S', \exnaddr \\[1ex]
+   \mbox{where:} \hfill \\
+   \exnaddr &=& |S.\SEXNS| \\
+   \exninst &=& \{ \EITAG~\tagaddr, \EIFIELDS~\val^\ast \} \\
+   S' &=& S \compose \{\SEXNS~\exninst\} \\
+   \end{array}
+
+
 .. index:: global, global instance, global address, global type, value type, mutability, value
 .. _alloc-global:
 

document/core/util/macros.def

@@ -981,6 +981,7 @@
 .. |alloctable| mathdef:: \xref{exec/modules}{alloc-table}{\F{alloctable}}
 .. |allocmem| mathdef:: \xref{exec/modules}{alloc-mem}{\F{allocmem}}
 .. |alloctag| mathdef:: \xref{exec/modules}{alloc-tag}{\F{alloctag}}
+.. |allocexn| mathdef:: \xref{exec/modules}{alloc-exception}{\F{allocexn}}
 .. |allocglobal| mathdef:: \xref{exec/modules}{alloc-global}{\F{allocglobal}}
 .. |allocelem| mathdef:: \xref{exec/modules}{alloc-elem}{\F{allocelem}}
 .. |allocdata| mathdef:: \xref{exec/modules}{alloc-data}{\F{allocdata}}
@@ -1335,3 +1336,5 @@
 
 .. |error| mathdef:: \xref{appendix/embedding}{embed-error}{\X{error}}
 .. |ERROR| mathdef:: \xref{appendix/embedding}{embed-error}{\K{error}}
+.. |exception| mathdef:: \xref{appendix/embedding}{embed-error}{\X{exception}}
+.. |ETHROW| mathdef:: \xref{appendix/embedding}{embed-error}{\K{THROW}}