Fun with pipe and armed expressions

arm

#' Arm a value with a function which fires when evaluated.
#'
#' @param expr an expression
#' @param fun a function of one argument.
#' 
#' @return an expression which when evaluated returns the result of expr,
#'   after evaluating fun(expr).
arm <- function(expr, fun)
{
	substitute({
 	  expr %T>% fun
	}, 
		list(
			fun  = substitute(fun), 
			expr = substitute(expr)
		)
	)
}

#' replace pipe_with without messing with magrittr:::pipe/`%>%` itself.
#' 
#' @param expr som expression, e.g. using compose, %,%, or simply a function.
pipe_with <- function(expr) function(lhs, rhs)
{
	parent <- parent.frame()
	lhs    <- eval(call("arm", substitute(lhs), expr), parent, parent)
	eval(call("%>%", lhs, rhs), parent, parent)
}

#' Construct an armed pipe
`%a>%` <- pipe_with(
	sum(., na = TRUE) %,% 
	cat("SUM:", ., "\n")
)

#' Try it out:
1:10 %a>% sin %a>% cos

I think

eval(call("%>%", lhs, rhs), parent, parent)

should be:

eval(call("%>%", lhs, substitute(rhs)), parent, parent)

At least it didn't work properly for me when the rhs had more than one argument until I changed this.