how would I use recursion instead of foldl in STANDARD ML programming language?

For example the code would originally be

fun sum fname = let val file = TextIO.openIn fname val ints = valOf(TextIO.scanStream intListParser file) in TextIO.closeIn file; foldl op+ 0 ints end

i was trying to find another way to read through every chacter and line of the text without using a built in function.