From 2345d761386eb095cfb42ee03fa3d798be8c8e44 Mon Sep 17 00:00:00 2001
From: Ichthyostega <prg@ichthyostega.de>
Date: Sun, 19 Nov 2017 00:07:47 +0100
Subject: [PATCH] Research: how to detect that a type in question exposes a
 free function extension point

The key trick is to form an expression with the free function, using a declval of the type to probe.
What is somewhat tricky is the fact that functions can be void, so we need just to pick up
the type and use it in another type expression
---
 research/try.cpp                          | 76 +++++------------------
 tests/library/iter-tree-explorer-test.cpp |  6 ++
 2 files changed, 21 insertions(+), 61 deletions(-)

diff --git a/research/try.cpp b/research/try.cpp
index 8371cc4f0..6a18af5c0 100644
--- a/research/try.cpp
+++ b/research/try.cpp
@@ -33,18 +33,11 @@
 // 1/16  - build tuple from runtime-typed variant container
 // 3/17  - generic function signature traits, including support for Lambdas
 // 9/17  - manipulate variadic templates to treat varargs in several chunks
+// 11/17 - metaprogramming to detect the presence of extension points
 
 
 /** @file try.cpp
- ** Metaprogramming: manipulations on variadic template argument packs.
- ** Investigation how to transform a parameter pack such as to forward a fixed chunk
- ** to one sub-ctor and delegate the rest to a tail recursive call.
- ** 
- ** This investigation was spurred by an attempt to create an inline storage layout
- ** with possible heap-based extension. Being able to build such an object layout
- ** would enable several possibly interesting optimisations when most usage scenarios
- ** of a class will only use a small element count, while some usages still require
- ** an essentially unlimited open number of elements.
+ ** Metaprogramming: how to detect that a type in question exposes a free function extension point.
  ** 
  */
 
@@ -53,15 +46,13 @@ typedef unsigned int uint;
 #include "lib/format-cout.hpp"
 #include "lib/format-util.hpp"
 //#include "lib/meta/function.hpp"
-#include "lib/meta/variadic-helper.hpp"
 #include "lib/test/test-helper.hpp"
 
 #include <functional>
 #include <utility>
 #include <string>
-#include <array>
 
-using lib::meta::_Fun;
+//using lib::meta::_Fun;
 using lib::test::showSizeof;
 
 using std::function;
@@ -76,11 +67,11 @@ using std::string;
     cout << "typeof( " << STRINGIFY(_TY_) << " )= " << lib::meta::typeStr<_TY_>() <<endl;
 
 
-template<typename...ARGS>
-void
-fun2 (ARGS... is)
+int
+fun1 (long)
   {
-    cout << "FUN-"<<sizeof...(is)<<": "<<util::join({is...}, " ") <<endl;
+    cout << "long FUN" <<endl;
+    return 12;
   }
 
 void
@@ -90,59 +81,22 @@ fun2 ()
   }
 
 
-using lib::meta::IndexSeq;
-using lib::meta::BuildIndexSeq;
-using lib::meta::BuildIdxIter;
-using lib::meta::pickArg;
-using lib::meta::pickInit;
-
-
-using Arr = std::array<int, 3>;
-
-
-template<size_t...idx1, size_t...idx2, typename...ARGS>
-Arr
-dispatch_ (IndexSeq<idx1...>,IndexSeq<idx2...>, ARGS...args)
-  {
-    Arr arr{pickInit<idx1,int> (args...) ...};
-    fun2 (pickArg<idx2> (args...) ...);
-    return arr;
-  }
-
-template<typename...ARGS>
-Arr
-dispatch (ARGS...args)
-  {
-    enum {SIZ = sizeof...(args)};
-    
-    using First = typename BuildIndexSeq<3>::Ascending;
-    using Next  = typename BuildIdxIter<ARGS...>::template After<3>;
-    
-    return dispatch_ (First(),Next(), args...);
-  }
+template<typename X, typename SEL =decltype(fun(std::declval<X>()))>
+struct Probe
+  { };
 
 
 int
 main (int, char**)
   {
-    fun2 (1,2,3,4);
-    fun2 (5,6);
+    fun1 (23);
     fun2 ();
     
-    auto arr = dispatch (2,3,4,5,6,7,8);
-    cout << util::join(arr) << "| " << showSizeof(arr) <<endl;
+    using Ty1 = decltype (fun1(12));
+    using Ty2 = decltype (fun2());
     
-    arr = dispatch (5,6,7,8);
-    cout << util::join(arr) <<endl;
-    
-    arr = dispatch (6,7,8);
-    cout << util::join(arr) <<endl;
-    
-    arr = dispatch (7,8);
-    cout << util::join(arr) <<endl;
-    
-    arr = dispatch ();
-    cout << util::join(arr) <<endl;
+    cout << "siz-1="<< sizeof(Ty1)<<endl;
+    cout << "siz-2="<< sizeof(Ty2)<<endl;
     
     cout <<  "\n.gulp.\n";
     
diff --git a/tests/library/iter-tree-explorer-test.cpp b/tests/library/iter-tree-explorer-test.cpp
index dd31982ee..a2402a39c 100644
--- a/tests/library/iter-tree-explorer-test.cpp
+++ b/tests/library/iter-tree-explorer-test.cpp
@@ -248,6 +248,7 @@ namespace test{
       void
       verify_wrappedState()
         {
+#if false /////////////////////////////////////////////////////////////////////////////////////////////////////////////UNIMPLEMENTED :: TICKET #1117
           auto ii = treeExplore (State{1,5});
           CHECK (!isnil (ii));
           CHECK (1 == *ii);
@@ -267,6 +268,7 @@ namespace test{
           ii = treeExplore (State{1,0});
           CHECK ( isnil (ii));
           CHECK (!ii);
+#endif    /////////////////////////////////////////////////////////////////////////////////////////////////////////////UNIMPLEMENTED :: TICKET #1117
         }
       
       
@@ -274,6 +276,7 @@ namespace test{
       void
       verify_wrappedIterator()
         {
+#if false /////////////////////////////////////////////////////////////////////////////////////////////////////////////UNIMPLEMENTED :: TICKET #1117
           vector<int> numz{1,-2,3,-5,8,-13};
           auto ii = eachElm(numz);
           CHECK (!isnil (ii));
@@ -292,6 +295,7 @@ namespace test{
 
           CHECK (materialise(ii) == "-2-3--5-8--13");
           CHECK (materialise(jj) ==   "-3--5-8--13");
+#endif    /////////////////////////////////////////////////////////////////////////////////////////////////////////////UNIMPLEMENTED :: TICKET #1117
         }
       
       
@@ -302,6 +306,8 @@ namespace test{
       verify_mapOperation()
         {
           UNIMPLEMENTED("map function onto the results");
+#if false /////////////////////////////////////////////////////////////////////////////////////////////////////////////UNIMPLEMENTED :: TICKET #1117
+#endif    /////////////////////////////////////////////////////////////////////////////////////////////////////////////UNIMPLEMENTED :: TICKET #1117
         }