evalCall function

Evaluate a Function Call

Evaluates a function call after resolving potential argument conflicts.

evalCall(fn, argu, ..., envir = parent.frame(),
    checkdef=FALSE, checkpar=FALSE)

Arguments

• fn: R function
• argu: list of explicitly named arguments and their values to pass to fn.
• ...: additional arguments that a user might wish to pass to fn.
• envir: environment from which the call originates (currently has no use or effect).
• checkdef: logical: if TRUE, gather additional formal arguments from the functions default function.
• checkpar: logical: if TRUE, gather additional graphical arguments from the list object par.

Details

This function builds a call to the specified function and executes it. During the build, optional arguments (...) are checked for

(i) duplication with explicit arguments argu: if any are duplicated, the user-supplied arguments supersede the explicit ones;

(ii) availability as usable arguments in fn, fn.default if checkdef=TRUE, and par if checkpar=TRUE.

Note

Sometimes the user may wish to pass arguments into a function to be used by other functions within, but may not want all the arguments to be used, depending on the functions subsequently called. In this case, the user needs to create a list object called dots, which is passed to evalCall.

For instance, if the user passes lwd=4 but only wants this used in a call to lines but not in a call to points, the function might look like this:

myfunc = function(x=seq(0,360,5), ...) {
  pdots = ldots = list(...)
  pdots[["lwd"]] = NULL
  ldots[["col"]] = "cyan"
  xrad = x*pi/180
  plot(sin(xrad),type="n")
  evalCall(lines, argu=list(x=sin(xrad)), dots=ldots, checkpar=TRUE)
  evalCall(points,argu=list(x=sin(xrad)), dots=pdots, checkpar=TRUE)
}
myfunc(lwd=4,pch=20,col=" blue")

Returns

Invisibly returns the string expression of the function call that is passed to eval(parse(text=expr)).

Author(s)

Rowan Haigh, Pacific Biological Station, Fisheries and Oceans Canada, Nanaimo BC

See Also

doAction, plotAsp

Examples

local(envir=.PBSmodEnv,expr={
  oldpar = par(no.readonly=TRUE)
  # A user may have a function that calls other functions
  # using specific defaults (e.g., blue triangles)
  #------------------------------------------------------
  pbsfun = function(..., use.evalCall=TRUE) {
    plotAsp(0,0,type="n",xlim=c(-1.5,1.5),ylim=c(-1.5,1.5),
      axes=FALSE, frame.plot=TRUE, xlab="",ylab="")
    if (use.evalCall)
      evalCall(polygon, ...,
        argu=list(x=c(-1,1,0),y=c(1,1,-1), col="dodgerblue", border="grey"))
    else
      polygon(x=c(-1,1,0),y=c(1,1,-1),col="dodgerblue",border="grey",...) 
  }
  par(mfrow=c(2,1))
  pbsfun(lwd=4,use.evalCall=FALSE)
  #------------------------------------------------------
  # But what if the user wants pink triangles?
  pbsfun(col="pink",lwd=4,use.evalCall=TRUE,checkpar=TRUE)
  par(oldpar)
})

# Without 'evalCall' an error occurs due to duplicated arguments
## Not run: pbsfun(col="pink",lwd=4,use.evalCall=FALSE)