dft_tools/triqs/gfs/gf.hpp

/*******************************************************************************
 *
 * TRIQS: a Toolbox for Research in Interacting Quantum Systems
 *
 * Copyright (C) 2012-2013 by M. Ferrero, O. Parcollet
 *
 * TRIQS is free software: you can redistribute it and/or modify it under the
 * terms of the GNU General Public License as published by the Free Software
 * Foundation, either version 3 of the License, or (at your option) any later
 * version.
 *
 * TRIQS is distributed in the hope that it will be useful, but WITHOUT ANY
 * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
 * details.
 *
 * You should have received a copy of the GNU General Public License along with
 * TRIQS. If not, see <http://www.gnu.org/licenses/>.
 *
 ******************************************************************************/
#ifndef TRIQS_GF_GFBASECLASS_H
#define TRIQS_GF_GFBASECLASS_H
#include <triqs/utility/first_include.hpp>
#include <triqs/utility/std_vector_expr_template.hpp>
#include <triqs/utility/factory.hpp>
#include <triqs/utility/tuple_tools.hpp>
#include <triqs/arrays/h5.hpp>
#include <vector>
#include "./tools.hpp"
#include "./data_proxies.hpp"

namespace triqs { namespace gfs {
 using utility::factory;
 using arrays::make_shape;

 template<typename T> long get_shape(std::vector<T> const & x) {return x.size();}

 template<typename Variable, typename Opt=void> struct gf_mesh;
 template<typename Variable, typename Target=matrix_valued, typename Opt=void> class gf;         // the regular type
 template<typename Variable, typename Target=matrix_valued, typename Opt=void> class gf_view;    // the view type
 template<typename Variable, typename Target, typename Opt, bool IsView> class gf_impl;

 // various implementation traits
 namespace gfs_implementation { // never use using of this...

  // evaluator regroup functions to evaluate the function.
  template<typename Variable, typename Target, typename Opt> struct evaluator{ static constexpr int arity = 0;};

  // closest_point mechanism
  template<typename Variable, typename Target, typename Opt> struct get_closest_point;

  // singularity
  template<typename Variable, typename Target, typename Opt> struct singularity { typedef nothing type;};

  // symmetry
  template<typename Variable, typename Target, typename Opt> struct symmetry { typedef nothing type;};

  // data_proxy contains function to manipulate the data array, but no data itself.
  // this is used to specialize this part of the code to array of dim 3 (matrix gf), dim 1 (scalar gf) and vector (e.g. block gf, ...)
  template<typename Variable, typename Target, typename Opt, typename Enable = void> struct data_proxy;

  // This trait contains functions to read/write in hdf5 files. Can be specialized for some case (Cf block)
  template <typename Variable, typename Target, typename Opt> struct h5_name; // value is a const char *

  template<typename Variable, typename Opt> struct h5_name<Variable,scalar_valued,Opt>      { static std::string invoke(){ return h5_name<Variable,matrix_valued,Opt>::invoke() + "_s";}};
 
  // the h5 write and read of gf members, so that we can specialize it e.g. for block gf
  template <typename Variable, typename Target, typename Opt, bool IsView> struct h5_rw {

   static void write (h5::group gr, gf_impl<Variable,Target,Opt,IsView> const & g) {
    h5_write(gr,"data",g._data);
    h5_write(gr,"singularity",g._singularity);
    h5_write(gr,"mesh",g._mesh);
    h5_write(gr,"symmetry",g._symmetry);
   }

   static void read (h5::group gr, gf_impl<Variable,Target,Opt,IsView> & g) {
    h5_read(gr,"data",g._data);
    h5_read(gr,"singularity",g._singularity);
    h5_read(gr,"mesh",g._mesh);
    h5_read(gr,"symmetry",g._symmetry);
   }
  };

  // factories regroup all factories (constructors..) for all types of gf.
  template <typename Variable, typename Target, typename Opt> struct factories;

  template<int R, typename Var, typename Opt> struct factories<Var,tensor_valued<R>,Opt> { 
   typedef gf<Var,tensor_valued<R>,Opt> gf_t;
   typedef tqa::mini_vector<size_t,R> target_shape_t;
   typedef typename gf_t::mesh_t mesh_t;

   static typename gf_t::data_t make_data(mesh_t const & m, target_shape_t shape) { 
    typename gf_t::data_t A(shape.front_append(m.size())); 
    A() =0;
    return A;
   }

   static typename gf_t::singularity_t make_singularity(mesh_t const & m, target_shape_t shape) { return typename gf_t::singularity_t(shape); }
  };

  template<typename Var, typename Opt> struct factories<Var,matrix_valued,Opt> { 
   typedef gf<Var,matrix_valued,Opt> gf_t;
   typedef tqa::mini_vector<size_t,2> target_shape_t;
   typedef typename gf_t::mesh_t mesh_t;

   static typename gf_t::data_t make_data(mesh_t const & m, target_shape_t shape) { 
    typename gf_t::data_t A(shape.front_append(m.size())); 
    A() =0;
    return A;
   }

   static typename gf_t::singularity_t make_singularity(mesh_t const & m, target_shape_t shape) { return typename gf_t::singularity_t(shape); }
  };
  
  template<typename Var, typename Opt> struct factories<Var,scalar_valued,Opt> { 
   typedef gf<Var,scalar_valued,Opt> gf_t;
   struct target_shape_t {};
   typedef typename gf_t::mesh_t mesh_t;

   static typename gf_t::data_t make_data(mesh_t const & m, target_shape_t shape) { 
    typename gf_t::data_t A(m.size()); 
    A() =0;
    return A;
   }
   static typename gf_t::singularity_t make_singularity(mesh_t const & m, target_shape_t shape) { return typename gf_t::singularity_t {1,1} ; }

  };

 } // gfs_implementation

 // make_gf and make_gf_view forward any args to them
 template <typename Variable, typename Target=matrix_valued, typename Opt=void, typename ... U> 
  gf<Variable,Target,Opt> make_gf(gf_mesh<Variable,Opt> m, U && ... x) 
  { return gfs_implementation::factories<Variable,Target,Opt>::make_gf(std::move(m),std::forward<U>(x)...);}

 template <typename Variable, typename Target=matrix_valued, typename Opt=void, typename ... U> 
  gf<Variable,Target,Opt> make_gf(U && ... x) { return gfs_implementation::factories<Variable,Target,Opt>::make_gf(std::forward<U>(x)...);}

 template <typename Variable, typename Target=matrix_valued, typename Opt=void, typename ... U> 
  gf_view<Variable,Target,Opt> make_gf_view(U && ... x) { return gfs_implementation::factories<Variable,Target,Opt>::make_gf_view(std::forward<U>(x)...);}

 // The trait that "marks" the Green function
 TRIQS_DEFINE_CONCEPT_AND_ASSOCIATED_TRAIT(ImmutableGreenFunction);

 template<typename G>
  auto get_gf_data_shape(G const & g) DECL_AND_RETURN(g.get_data_shape());

 // ---------------------- implementation --------------------------------

 /// A common implementation class for gf and gf_view. They will only redefine contructor and = ...
 template<typename Variable, typename Target, typename Opt, bool IsView> class gf_impl : 
  TRIQS_CONCEPT_TAG_NAME(ImmutableGreenFunction){
   public :
    typedef gf_view<Variable,Target,Opt> view_type;
    typedef gf<Variable,Target,Opt>      regular_type;

    typedef Variable variable_t;
    typedef Target target_t;
    typedef Opt option_t;

    typedef gf_mesh<Variable,Opt>                                                               mesh_t;
    typedef typename mesh_t::domain_t                                                           domain_t;
    typedef typename mesh_t::mesh_point_t                                                       mesh_point_t;
    typedef typename mesh_t::index_t                                                            mesh_index_t;
    typedef typename gfs_implementation::symmetry<Variable,Target,Opt>::type                    symmetry_t;
    typedef gfs_implementation::evaluator<Variable,Target,Opt>                                  evaluator_t;

    typedef gfs_implementation::data_proxy<Variable,Target,Opt>                                 data_proxy_t;
    typedef typename data_proxy_t::storage_t                                                    data_regular_t;
    typedef typename data_proxy_t::storage_view_t                                               data_view_t;
    typedef typename std::conditional<IsView, data_view_t, data_regular_t>::type                data_t;

    typedef typename gfs_implementation::singularity<Variable,Target,Opt>::type                 singularity_non_view_t;
    typedef typename view_type_if_exists_else_type<singularity_non_view_t>::type                singularity_view_t;
    typedef typename std::conditional<IsView, singularity_view_t, singularity_non_view_t>::type singularity_t;

    mesh_t const &        mesh()          const { return _mesh;}
    domain_t const &      domain()        const { return _mesh.domain();}
    data_t &              data()                { return _data;}
    data_t const &        data()          const { return _data;}
    singularity_t &       singularity()         { return _singularity;}
    singularity_t const & singularity()   const { return _singularity;}
    symmetry_t const &    symmetry()      const { return _symmetry;}
    evaluator_t const &   get_evaluator() const { return _evaluator;}

    auto get_data_shape() const DECL_AND_RETURN(get_shape(this->data()));

   protected:
    mesh_t        _mesh;
    data_t        _data;
    singularity_t _singularity;
    symmetry_t    _symmetry;
    evaluator_t   _evaluator;
    data_proxy_t  _data_proxy;

    // --------------------------------Constructors -----------------------------------------------
    // all protected but one, this is an implementation class, see gf/gf_view later for public one
    gf_impl() {} // all arrays of zero size (empty)

   public : //everyone can make a copy (for clef lib in particular, this one needs to be public)

    gf_impl(gf_impl const & x) : _mesh(x.mesh()), _data(factory<data_t>(x.data())),
    _singularity(factory<singularity_t>(x.singularity())), _symmetry(x.symmetry()), _evaluator(x._evaluator){}

    gf_impl(gf_impl &&) = default;

   protected:

    gf_impl(gf_impl<Variable,Target,Opt,!IsView> const & x): _mesh(x.mesh()), _data(factory<data_t>(x.data())),
    _singularity(factory<singularity_t>(x.singularity())), _symmetry(x.symmetry()), _evaluator(x.get_evaluator()){}

    template<typename M, typename D, typename S, typename SY, typename EV> 
     gf_impl(M && m, D && dat, S && sing, SY && sy, EV && ev) : 
      _mesh(std::forward<M>(m)), _data(std::forward<D>(dat)), _singularity(std::forward<S>(sing)),_symmetry(std::forward<SY>(sy)), _evaluator(std::forward<EV>(ev)){}

    void operator = (gf_impl const & rhs) = delete; // done in derived class.

    void swap_impl (gf_impl & b) noexcept {
     using std::swap;
     swap(this->_mesh, b._mesh); swap(this->_data, b._data); swap (this->_singularity,b._singularity); swap(this->_symmetry,b._symmetry); swap(this->_evaluator,b._evaluator);
    }

   public:
    // ------------- All the call operators -----------------------------

    // First, a simple () returns a view, like for an array...
    view_type operator()() const { return *this;}

    /// Calls are (perfectly) forwarded to the evaluator::operator(), except mesh_point_t and when
    /// there is at least one lazy argument ...
    template<typename Arg0, typename... Args >    // match any argument list, picking out the first type : () is not permitted
     typename std::add_const<
     typename boost::lazy_disable_if<  // disable the template if one the following conditions it true
     boost::mpl::or_< // starting condition [OR]
     clef::is_any_lazy<Arg0, Args...>                          // One of Args is a lazy expression
     , boost::mpl::bool_<(sizeof...(Args)!= evaluator_t::arity -1 ) && (evaluator_t::arity !=-1)> // if -1 : no check
     >,                                                       // end of OR
     std::result_of<evaluator_t(gf_impl*,Arg0, Args...)> // what is the result type of call
      >::type     // end of lazy_disable_if
      >::type // end of add_Const
      operator() (Arg0&& arg0, Args&&... args) const { return _evaluator(this,std::forward<Arg0>( arg0), std::forward<Args>(args)...); }

    // WHY SEPARATE ARg0 ?
    template<typename Arg0, typename ...Args>
     typename clef::_result_of::make_expr_call<gf_impl &,Arg0, Args...>::type
     operator()(Arg0 &&arg0, Args&&... args) & {
      return clef::make_expr_call(*this,std::forward<Arg0>(arg0), std::forward<Args>(args)...);
     }

    template<typename Arg0, typename ...Args>
     typename clef::_result_of::make_expr_call<gf_impl const &,Arg0, Args...>::type
     operator()(Arg0 &&arg0, Args&&... args) const & {
      return clef::make_expr_call(*this,std::forward<Arg0>(arg0), std::forward<Args>(args)...);
     }

    template<typename Arg0, typename ...Args>
     typename clef::_result_of::make_expr_call<gf_impl,Arg0, Args...>::type
     operator()(Arg0 &&arg0, Args&&... args) && {
      return clef::make_expr_call(std::move(*this),std::forward<Arg0>(arg0), std::forward<Args>(args)...);
     }

    /*
    // on mesh component for composite meshes
    // enable iif the first arg is a mesh_point_t for the first component of the mesh_t
    template<typename Arg0, typename ... Args, bool MeshIsComposite = std::is_base_of<tag::composite, mesh_t>::value >
    typename std::enable_if< MeshIsComposite && std::is_base_of< tag::mesh_point, Arg0>::value,  r_type>::type
    operator() (Arg0 const & arg0, Args const & ... args)
    { return _data_proxy(_data, _mesh.mesh_pt_components_to_linear(arg0, args...));}

    template<typename Arg0, typename ... Args, bool MeshIsComposite = std::is_base_of<tag::composite, mesh_t>::value >
    typename std::enable_if< MeshIsComposite && std::is_base_of< tag::mesh_point, Arg0>::value,  cr_type>::type
    operator() (Arg0 const & arg0, Args const & ... args) const
    { return _data_proxy(_data, _mesh.mesh_pt_components_to_linear(arg0, args...));}
    */

    //// [] and access to the grid point
    typedef typename std::result_of<data_proxy_t(data_t       &,size_t)>::type r_type;
    typedef typename std::result_of<data_proxy_t(data_t const &,size_t)>::type cr_type;

    r_type  operator[] (mesh_index_t const & arg)       { return _data_proxy(_data,_mesh.index_to_linear(arg));}
    cr_type operator[] (mesh_index_t const & arg) const { return _data_proxy(_data,_mesh.index_to_linear(arg));}

    r_type  operator[] (mesh_point_t const & x)       { return _data_proxy(_data, x.linear_index());}
    cr_type operator[] (mesh_point_t const & x) const { return _data_proxy(_data, x.linear_index());}

    template<typename ... U>
     r_type  operator[] (closest_pt_wrap<U...> const & p)       { return _data_proxy(_data, _mesh.index_to_linear( gfs_implementation::get_closest_point<Variable,Target,Opt>::invoke(this,p)));}
    template<typename ... U>
     cr_type operator[] (closest_pt_wrap<U...> const & p) const { return _data_proxy(_data, _mesh.index_to_linear( gfs_implementation::get_closest_point<Variable,Target,Opt>::invoke(this,p)));}

    template<typename Arg>
     typename clef::_result_of::make_expr_subscript<gf_impl const &,Arg>::type
     operator[](Arg && arg) const & { return clef::make_expr_subscript(*this,std::forward<Arg>(arg));}

    template<typename Arg>
     typename clef::_result_of::make_expr_subscript<gf_impl &,Arg>::type
     operator[](Arg && arg) & { return clef::make_expr_subscript(*this,std::forward<Arg>(arg));}

    template<typename Arg>
     typename clef::_result_of::make_expr_subscript<gf_impl,Arg>::type
     operator[](Arg && arg) && { return clef::make_expr_subscript(std::move(*this),std::forward<Arg>(arg));}

    /// A direct access to the grid point
    template<typename... Args>
     r_type on_mesh (Args&&... args) { return _data_proxy(_data,_mesh.index_to_linear(mesh_index_t(std::forward<Args>(args)...)));}

    template<typename... Args>
     cr_type on_mesh (Args&&... args) const { return _data_proxy(_data,_mesh.index_to_linear(mesh_index_t(std::forward<Args>(args)...)));}

   private:
    struct _on_mesh_wrapper_const {
     gf_impl const & f; _on_mesh_wrapper_const (gf_impl const & _f) : f(_f) {}
     template <typename... Args> cr_type operator ()(Args && ... args) const { return f.on_mesh(std::forward<Args>(args)...);}
    };
    struct _on_mesh_wrapper {
     gf_impl & f; _on_mesh_wrapper (gf_impl & _f) : f(_f) {}
     template <typename... Args> r_type  operator ()(Args && ... args) const { return f.on_mesh(std::forward<Args>(args)...);}
    };
    _on_mesh_wrapper_const friend on_mesh(gf_impl const & f) { return f;}
    _on_mesh_wrapper friend on_mesh(gf_impl & f) { return f;}
   public:

    //----------------------------- HDF5 -----------------------------

    friend std::string get_triqs_hdf5_data_scheme(gf_impl const & g) { return "Gf" + gfs_implementation::h5_name<Variable,Target,Opt>::invoke();}

    friend class gfs_implementation::h5_rw<Variable,Target,Opt,IsView>;

    /// Write into HDF5
    friend void h5_write (h5::group fg, std::string subgroup_name, gf_impl const & g) {
     auto gr =  fg.create_group(subgroup_name);
     gr.write_triqs_hdf5_data_scheme(g); 
     gfs_implementation::h5_rw<Variable,Target,Opt,IsView>::write(gr, g);
    }

    /// Read from HDF5
    friend void h5_read (h5::group fg, std::string subgroup_name, gf_impl & g) {
     auto gr = fg.open_group(subgroup_name);
     // Check the attribute or throw
     auto tag_file = gr.read_triqs_hdf5_data_scheme();
     auto tag_expected= get_triqs_hdf5_data_scheme(g);
     if (tag_file != tag_expected) 
      TRIQS_RUNTIME_ERROR<< "h5_read : mismatch of the tag TRIQS_HDF5_data_scheme tag in the h5 group : found "<<tag_file << " while I expected "<< tag_expected; 
     gfs_implementation::h5_rw<Variable,Target,Opt,IsView>::read(gr, g);
    }

    //-----------------------------  BOOST Serialization -----------------------------
    friend class boost::serialization::access;
    template<class Archive>
     void serialize(Archive & ar, const unsigned int version) {
      ar & boost::serialization::make_nvp("data",_data);
      ar & boost::serialization::make_nvp("singularity",_singularity);
      ar & boost::serialization::make_nvp("mesh",_mesh);
      ar & boost::serialization::make_nvp("symmetry",_symmetry);
     }

    /// print
    friend std::ostream & operator << (std::ostream & out, gf_impl const & x) { return out<<(IsView ? "gf_view": "gf");}
    friend std::ostream & triqs_nvl_formal_print(std::ostream & out, gf_impl const & x) { return out<<(IsView ? "gf_view": "gf");}

    // Interaction with the CLEF library : auto assignment of the gf (gf(om_) << expression fills the functions by evaluation of expression)
    template<typename RHS> friend void triqs_clef_auto_assign (gf_impl & g, RHS const & rhs) {
     // access to the data . Beware, we view it as a *matrix* NOT an array... (crucial for assignment to scalars !)
     g.triqs_clef_auto_assign_impl(rhs, typename std::is_base_of<tag::composite,mesh_t>::type());
     assign_from_expression(g.singularity(),rhs);
     // if f is an expression, replace the placeholder with a simple tail. If f is a function callable on freq_infty,
     // it uses the fact that tail_non_view_t can be casted into freq_infty
    }

    // enable the writing g[om_] << .... also 
    template<typename RHS> friend void triqs_clef_auto_assign_subscript (gf_impl & g, RHS rhs) { triqs_clef_auto_assign(g,rhs);}

   private:
    template<typename RHS> void triqs_clef_auto_assign_impl (RHS const & rhs, std::integral_constant<bool,false>) {
     for (auto const & w: this->mesh()) (*this)[w] = rhs(w);
     //for (auto const & w: this->mesh()) (*this)[w] = rhs(typename B::mesh_t::mesh_point_t::cast_t(w));
    }
    template<typename RHS> void triqs_clef_auto_assign_impl (RHS const & rhs, std::integral_constant<bool,true>) {
     for (auto const & w: this->mesh())  {
      //std::cout  << "abot "<<w.components_tuple()<<std::endl ;
      //std::cout  << "eefef "<< triqs::tuple::apply(rhs,w.components_tuple()) << std::endl ; 
      (*this)[w] = triqs::tuple::apply(rhs,w.components_tuple());
      //std::cout  << "done "<<std::endl ;
     }
     //for (auto w: this->mesh()) triqs::tuple::apply(*this,w.components_tuple()) = triqs::tuple::apply(rhs,w.components_tuple());
    }
  };

 // ---------------------------------------------------------------------------------
 ///The regular class of GF
 template<typename Variable, typename Target, typename Opt> class gf : public gf_impl<Variable,Target,Opt,false> {
  typedef gf_impl<Variable,Target,Opt,false> B;
  typedef gfs_implementation::factories<Variable,Target,Opt> factory;
  public :

  gf():B() {}
  gf(gf const & g): B(g){}
  gf(gf && g) noexcept : B(std::move(g)){}
  gf(gf_view<Variable,Target,Opt> const & g): B(g){}
  template<typename GfType> gf(GfType const & x,typename std::enable_if<ImmutableGreenFunction<GfType>::value>::type *dummy =0 ): B() { *this = x;}

  gf(typename B::mesh_t m,
    typename B::data_t dat,
    typename B::singularity_view_t const & si,
    typename B::symmetry_t const & s ,
    typename B::evaluator_t const & eval = typename B::evaluator_t ()
    ) :
   B(std::move(m),std::move(dat), si,s,eval) {}

  typedef typename factory::target_shape_t target_shape_t;

  gf(typename B::mesh_t m, target_shape_t shape = target_shape_t{} ):
   B(std::move(m), factory::make_data(m,shape), factory::make_singularity(m,shape), typename B::symmetry_t {}, typename B::evaluator_t{}) {} 

  friend void swap (gf & a, gf & b) noexcept { a.swap_impl (b);}

  gf & operator = (gf const & rhs)     { *this = gf(rhs); return *this;} // use move =
  gf & operator = (gf & rhs)           { *this = gf(rhs); return *this;} // use move =
  gf & operator = (gf && rhs) noexcept { swap(*this,rhs); return *this;}

  template<typename RHS> void operator = (RHS && rhs) {
   this->_mesh = rhs.mesh();
   this->_data.resize(get_gf_data_shape(rhs));
   for (auto const & w: this->mesh())  { (*this)[w] = rhs[w]; }
   this->_singularity = rhs.singularity();
   // to be implemented : there is none in the gf_expr in particular....
   //this->_symmetry = rhs.symmetry(); 
  }

 };

 // ---------------------------------------------------------------------------------
 ///The View class of GF
 template<typename Variable, typename Target, typename Opt> class gf_view : public gf_impl<Variable,Target,Opt,true> {
  typedef gf_impl<Variable,Target,Opt,true> B;
  public :
  gf_view(gf_view const & g): B(g){}
  gf_view(gf_view && g) noexcept : B(std::move(g)){}

  template<bool V> gf_view(gf_impl<Variable,Target,Opt,V> const & g): B(g){}

  template<typename D>
   gf_view (typename B::mesh_t const & m,
     D const & dat,typename B::singularity_view_t const & t,typename B::symmetry_t const & s,
     typename B::evaluator_t const &e = typename B::evaluator_t ()  ) :
    B(m,factory<typename B::data_t>(dat),t,s,e) {}

  friend void swap (gf_view & a, gf_view & b) noexcept { a.swap_impl (b);}

  void rebind (gf_view const &X) noexcept {
   this->_mesh = X._mesh; this->_symmetry = X._symmetry; 
   this->_data_proxy.rebind(this->_data,X); 
   this->_singularity.rebind(X._singularity);
  }

  gf_view & operator = (gf_view const & rhs)  { triqs_gf_view_assign_delegation(*this,rhs); return *this;}

  template<typename RHS> gf_view & operator = (RHS const & rhs) { triqs_gf_view_assign_delegation(*this,rhs); return *this;}
 }; // class gf_view

 // delegate = so that I can overload it for specific RHS...
 template<typename Variable, typename Target, typename Opt, typename RHS>
  DISABLE_IF(arrays::is_scalar<RHS>) triqs_gf_view_assign_delegation( gf_view<Variable,Target,Opt> g, RHS const & rhs) {
   if (!(g.mesh() == rhs.mesh()))  TRIQS_RUNTIME_ERROR<<"Gf Assignment in View : incompatible mesh";
   for (auto const & w: g.mesh()) g[w] = rhs[w];
   g.singularity() = rhs.singularity();
  }

 template<typename Variable, typename Target, typename Opt, typename T>
  ENABLE_IF(arrays::is_scalar<T>) triqs_gf_view_assign_delegation( gf_view<Variable,Target,Opt> g, T const & x) {
   gf_view<Variable,Target,Opt>::data_proxy_t::assign_to_scalar(g.data(), x); 
   g.singularity() = x;
  }

 // tool for lazy transformation
 template<typename Tag, typename D, typename Target = matrix_valued> struct gf_keeper{ gf_view<D,Target> g; gf_keeper (gf_view<D,Target> const & g_) : g(g_) {} };

 // ---------------------------------- slicing ------------------------------------

 //slice
 template<typename Variable, typename Target, typename Opt, typename... Args>
  gf_view<Variable,matrix_valued,Opt> slice_target (gf_view<Variable,Target,Opt> g, Args&& ... args) {
   static_assert(std::is_same<Target,matrix_valued>::value, "slice_target only for matrix_valued GF's");
   using arrays::range;
   //auto sg=slice_target (g.singularity(),range(args,args+1)...);
   return gf_view<Variable,matrix_valued,Opt>(g.mesh(), g.data()(range(), std::forward<Args>(args)... ), slice_target (g.singularity(), std::forward<Args>(args)...) , g.symmetry());
  }

 template<typename Variable, typename Target, typename Opt, typename... Args>
  gf_view<Variable,scalar_valued,Opt> slice_target_to_scalar (gf_view<Variable,Target,Opt> g, Args&& ... args) {
   static_assert(std::is_same<Target,matrix_valued>::value, "slice_target only for matrix_valued GF's");
   using arrays::range;
   auto sg=slice_target (g.singularity(),range(args,args+1)...);
   return gf_view<Variable,scalar_valued,Opt>(g.mesh(), g.data()(range(), std::forward<Args>(args)... ), sg, g.symmetry());
  }

 // a scalar_valued gf can be viewed as a 1x1 matrix
 template<typename Variable, typename Opt, typename... Args>
  gf_view<Variable,matrix_valued,Opt> reinterpret_scalar_valued_gf_as_matrix_valued (gf_view<Variable,scalar_valued,Opt> g) {
   typedef typename gf_view<Variable,matrix_valued,Opt>::data_view_t a_t;
   auto a = a_t {typename a_t::indexmap_type (join(g.data().shape(),make_shape(1,1))), g.data().storage()};
   return gf_view<Variable,matrix_valued,Opt>(g.mesh(), a, g.singularity(), g.symmetry());
  }

 /*
    template<typename Variable1,typename Variable2, typename Target, typename Opt, bool V, typename... Args>
    gf_view<Variable2,Target,Opt> slice_mesh (gf_impl<Variable1,Target,Opt,V> const & g, Args... args) {
    return gf_view<Variable2,Target,Opt>(g.mesh().slice(args...), g.data()(g.mesh().slice_get_range(args...),arrays::ellipsis()), g.singularity(), g.symmetry());
    }*/

}}

#include "./gf_expr.hpp"
#endif