Use withInvisible() to track visibility of expression

R/swirl.R

@@ -370,15 +370,21 @@ resume.default <- function(e, ...){
       }
       e$expr <- parse(text=correctAns)[[1]]
       ce <- cleanEnv(e$snapshot)
-      e$val <- suppressMessages(suppressWarnings(eval(e$expr, ce)))
+      # evaluate e$expr keeping value and visibility information
+      # store the result in temporary object evaluation in order
+      # to avoid double potentially time consuming eval call
+      evaluation <- withVisible(eval(e$expr, ce))
+      e$vis <- evaluation$visible
+      e$val <- suppressMessages(suppressWarnings(evaluation$value))
       xfer(ce, globalenv())
       ce <- as.list(ce)
 
       # Inform the user and expose the correct answer
       swirl_out("Entering the following correct answer for you...",
                 skip_after=TRUE)
       message("> ", e$current.row[, "CorrectAnswer"])
-      if(e$vis && !is.null(e$val)) {
+
+      if(e$vis & !is.null(e$val)) {
         print(e$val)
       }
     }