Joel de Guzman wrote:
Giovanni Piero Deretta wrote:
No, that is not the "unlambda" semantics. There needs to be 2 function application there: 1) the one that substitutes the stl container for _1 (the leftmost _1) and 2) the one that substitutes the container's element for _1 (the rightmost _1). The key point here is that phx.lambda (and BLL protect), *returns a lambda-functor that returns a lambda-functor*. unlambda OTOH simply returns a plain functor (non-actor). A plain functor can only do one function application (bear in mind that this is phoenix for_each (lazy), not std::for_each (eager)).
Ok, follow me:
let 'f' me an unary function object, one suitable as a parameter to for_each: Am I right that the following calls should work and apply every element of some range to 'f'?
phx::for_each(_1, f)(some_range);
No. Not unless you bind f.
Ok, I think this is it! This is the main misunderstanding about phx::for_each and all phoenix functions that take in other functions. All phoenix expressions must be a legitimate phoenix actor (following the actor concept). That is obvious in other contexts. For example, given a plain function object f: struct f { int operator()(int i); }; you can't just write: f(_1) + val(123) // error f can't take a _1 argument you need to either: 1) bind it 2) make a phoenix function out of it So: phx::for_each(_1, f)(some_range); is an error. f is not an actor. Hence, make it one. Let's use bind: phx::for_each(_1, bind(f, _1))(some_range); now, that is still in error: 1) The placeholders are eagerly substituted. 2) After substituting _1 the result of the bind: f(some_range) has the wrong call. So, you'll want to protect it. In phoenix, we use "lambda": phx::for_each(_1, lambda[bind(f, _1)])(some_range); Assuming phoenix had "unlambda" (which BTW is very easy to do), We're back to square one. Phoenix does not allow arbitrary functions/function objects as the function argument to for_each (or any phoenix function that takes in a higher-order function. It must be a phoenix actor. Regards, -- Joel de Guzman http://www.boostpro.com http://spirit.sf.net