134 lines
5.4 KiB
C++
134 lines
5.4 KiB
C++
/** @file GCObjectConversion.hpp
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*
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* @author Roland Conybeare, Jan 2026
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**/
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#pragma once
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#include <xo/alloc2/GCObject.hpp>
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#include <xo/alloc2/Allocator.hpp>
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#include <xo/facet/FacetRegistry.hpp>
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#include <xo/facet/obj.hpp>
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#include <xo/indentlog/scope.hpp>
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namespace xo {
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namespace scm {
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class GCObjectConversionUtil {
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public:
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using AGCObject = xo::mm::AGCObject;
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using typeseq = xo::reflect::typeseq;
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/** helper method fro GCObjectConversion<..>::from_gco()
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* on conversion failure
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**/
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static void _from_gco_fail_aux(obj<AGCObject> gco,
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typeseq tseq,
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scope * p_log);
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};
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/** @brief compile-time conversion obj<AGCObject> <-> T
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*
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* Specialize for each T that participates in conversion.
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* Methods here aren't implemented
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**/
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template <typename T>
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class GCObjectConversion {
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public:
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using AGCObject = xo::mm::AGCObject;
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using AAllocator = xo::mm::AAllocator;
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/** find gc-aware representation for @p x.
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* If necessary allocate from @p mm, but may
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* refer to @p x in-place
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**/
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static obj<AGCObject> to_gco(obj<AAllocator> mm, const T & x);
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/** convert to native representation @tparam T from gc-aware
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* @p gco. If necessary allocate from @p mm, but
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* may instead refer to @p x in-place
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**/
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static T from_gco(obj<AAllocator> mm, obj<AGCObject> gco);
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};
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/** Motivating use-case for GCObjectConversion is to transform
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* primitive function arguments and results to/from gc-aware
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* representation.
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*
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* However: Schematika also supports runtime polymorphism
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* which leads to primitives that expect obj<AFacet> arguments.
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*
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* Also, Schematika expression parser needs representation for
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* expressions, before type unification.
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*
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* Consider a function like:
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* def fact = lambda (n : i64) { if (n <= 0) then 1 else (n * fact(n - 1)); }
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* During expression parsing the rhs argument to multiply has unknown type.
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* To construct an expression for input to unification will use polymorphic
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* binding for multiply primitive, relying on specialization here for
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* its implementation.
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**/
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template <typename AFacet, typename DRepr>
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struct GCObjectConversion<obj<AFacet,DRepr>> {
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using AGCObject = xo::mm::AGCObject;
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using AAllocator = xo::mm::AAllocator;
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using FacetRegistry = xo::facet::FacetRegistry;
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using DVariantPlaceholder = xo::facet::DVariantPlaceholder;
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static obj<AGCObject> to_gco(obj<AAllocator>,
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obj<AFacet,DRepr> gco) {
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if constexpr (std::is_same_v<AFacet, AGCObject>) {
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// trivial conversion!
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return gco;
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} else if constexpr (std::is_same_v<DRepr, DVariantPlaceholder>) {
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// runtime polymorphism
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return FacetRegistry::instance().variant<AGCObject,AFacet>(gco);
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} else /* DRepr != DVariantPlaceholder */ {
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// known content w/ fat object pointer
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return obj<AGCObject,DRepr>(gco.data());
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}
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}
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/** Several use cases here:
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* 1. runtime polymorphism
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* obj<AGCObject,DArray> v(DArray::make(..));
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* // from_gco() doesn't know v repr
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* auto gc = GCObjectConversion<ASequence,DArray>::from_gco(mm, v);
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*
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**/
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static obj<AFacet,DRepr> from_gco(obj<AAllocator>,
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obj<AGCObject> gco) {
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scope log(XO_DEBUG(false));
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if constexpr (std::is_same_v<AFacet, AGCObject>) {
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// Need accurate handling of DVariantPlaceholder.
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// runtime type must be some concrete type.
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// Only use obj<AFacet,DRepr>::from when DRepr is a concrete type
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if constexpr (std::is_same_v<DRepr,DVariantPlaceholder>) {
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// At comptime gco has unknown repr. At runtime
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// will have some known repr, which assignment here will transfer
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return gco;
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} else {
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// Runtime conversion to concrete type DRepr
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auto retval = obj<AFacet,DRepr>::from(gco);
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if (!retval) {
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GCObjectConversionUtil::_from_gco_fail_aux
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(gco, reflect::typeseq::id<DRepr>(), &log);
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}
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return retval;
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}
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} else {
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// both runtime and comptime polymorphism
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// use same path here, since representation of @p gco
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// is type-erased here
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return FacetRegistry::instance().variant<AFacet,AGCObject>(gco);
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}
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}
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};
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} /*namespace scm */
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} /*namespace xo*/
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/* end GCObjectConversion.hpp */
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