.. highlight:: c Transform CLEF expressions into functions =============================================== Clef expressions are **NOT** functions. In short, * clef expressions are *evaluated* (the order of argument *does not* matter) :: eval( expr, x_=1, y_=2, ...); * while functions are *called*, as usual (the order of argument *does* matter !) :: f(1,2) It is however possible to transform expressions into functions, *as soon as you specify the order of the placeholders*. (the opposite is true, if the function accept lazy arguments, cf :ref:`overload_function`). make_function --------------- Given any expression with placeholder `x_`, `y_`, `z_`, ..., `make_function` transform them into a regular function. If we say :: auto f = make_function( clef_expression, placeholder_1, placeholder_2, placeholder_3, ...) then f is :: a function (x1,x2,x3) --> RESULT where RESULT is: * the result of the complete evaluation of the expression if the list of placeholder exhausts the placeholders of the expression. * otherwise a clef_expression of the remaining placeholders, returning a **function**. Short notation with >> operator ..................................... For function of *one* variable, the make_function notation can be simplified into :: // same auto f = make_function( 2*x_ + y_ + 1, x_); auto f = x_ >> 2*x_ + y_ + 1; // same auto f = make_function ( make_function( 2*x_ + y_ + 1, y_), x_); auto f = x_ >> (y_ >> 2*x_ + y_ + 1) ; .. warning:: The notation :: `x_` >> `y_` >> expression is banned because it conflicts with the standard priority of >>. Use parenthesis. clef::function -------------------------- The class nda::clef::function stored a function of a given signature It is similar to std::function but it can be constructed from an expression and an ordered list of placeholders. clef::function can be assigned with the = operator, Cf example below. .. note:: Like std::function, it stores the expression polymorphically, by erasing its type. This might lead to some performance penalty in some case, even though tests do not show that at present... Examples --------- .. literalinclude:: /documentation/manual/triqs/clef/function_0.cpp :language: cpp .. literalinclude:: /documentation/manual/triqs/clef/function_0.output