Error using calc() function: cannot use this function using R?

Question

I need to run calculations on large multi-band rasters and export a RasterBrick, and am trying to do so using the calc() function in the raster package, for the purpose of memory efficiency. The function runs fine on its own, but when I try to include it in calc(), I keep getting this error:

Error in .calcTest(x[1:5], fun, na.rm, forcefun, forceapply) : cannot use this function

How can I make this work?

Simplified code:

fn = system.file("external/test.grd", package="raster")
s = stack(fn, fn, fn, fn)

out = calc(s, fun = function(x){
  for (i in 1:nlayers(x)){
    x[[i]] = x[[i]] - cellStats(x[[i]], "min")
    x[[i]] = x[[i]]* 5
  }
  list = unstack(x)
  out = brick(list)
  return(out)
}
)

Error in .calcTest(x[1:5], fun, na.rm, forcefun, forceapply) : 
  cannot use this function

Answer 1

calc iterates through layers of a Raster Stack on its own, so a for loop and indexing is not necessary here. Removing them, as well as the unstack and brick fixes the problem.