Sorry this was my error, I still can't figure out how to use this type though. If I do

latexWriterOptions = def { writerTableOfContents = True, writerVariables = Context (M.fromList [("geometry",MapVal (Context $ M.fromList [("margin",SimpleVal "3cm")])), ("fontsize", (SimpleVal "10")), ("linkcolor",SimpleVal "blue")]) }

then it type checks but I get this now

user error ("! Package keyval Error: true undefined.\n\nSee the keyval package documentation for explanation.\nType  H <return>  for immediate help.\n ...                                              \n                                                  \nl.1005 \\ProcessOptionsKV[p]{Gm}\n")