return output invisibly for set* functions

Author: MichaelChirico

R/mergelist.R

@@ -9,7 +9,7 @@ cbindlist_impl_ = function(l, copy) {
 }
 
 cbindlist = function(l) cbindlist_impl_(l, copy=TRUE)
-setcbindlist = function(l) cbindlist_impl_(l, copy=FALSE)
+setcbindlist = function(l) invisible(cbindlist_impl_(l, copy=FALSE))
 
 # when 'on' is missing then use keys, used only for inner and full join
 onkeys = function(x, y) {
@@ -352,7 +352,7 @@ mergelist = function(l, on, cols=NULL, how=c("left", "inner", "full", "right", "
 setmergelist = function(l, on, cols=NULL, how=c("left", "inner", "full", "right", "semi", "anti", "cross"), mult, join.many=getOption("datatable.join.many")) {
   if (missing(how) || is.null(how))
     how = match.arg(how)
-  mergelist_impl_(l, on, cols, how, mult, join.many, copy=FALSE)
+  invisible(mergelist_impl_(l, on, cols, how, mult, join.many, copy=FALSE))
 }
 
 # Previously, we had a custom C implementation here, which is ~2x faster,

inst/tests/mergelist.Rraw

@@ -1314,3 +1314,9 @@ local({
   test(301.01, data.table(x=unique(dt$x)), ans) ## OK
   test(301.02, fdistinct(dt, on="x"), ans)      ## force sort=TRUE for the moment
 })
+
+# output from set* functions is returned invisibly
+l = list(data.table(a=1:3), data.table(b=2:4))
+test(302.1, capture.output(setcbindlist(l)), character()) # NB test(notOutput=) doesn't work since print() is explicitly run
+l = list(data.table(a=1:3), data.table(a=2:4))
+test(302.2, capture.output(setmergelist(l, on='a')), character())