Explore unusual behaviour of a factor returned by melt.data.frame

vanishingFactorLevels.r

library(testthat)
library(reshape2) # installed from github
                  # SHA 096afb80192f163ced5d001cc416acc1d9a85d12

## working with example from test-melt.r line 132 -->

df <- data.frame(
  id=1:2,
  f1=factor(c("a", "b")),
  f2=factor(c("b", "a"))
)

## the levels, 'a' and 'b', appear in the Console
df

## now melt and preserve the factor status and levels
m2 <- melt(df, 1, factorsAsStrings=FALSE)

## the __integers__ 1 and 2 appear in the Console
m2

## yes value IS a factor
expect_identical( class(m2$value), "factor" )

## yes value has the right levels
expect_identical( levels(m2$value), c("a", "b") )

## so why do the factor levels not display normally?
## they aren't written to file in the usual way either ...
write.table(m2, "yo.txt")
cat(readLines("yo.txt"), sep = "\n")