But if I chuck it in a source file and attempt to runhaskell or compile

it, this fails with a very nasty error. Oh, what fun! To make it work ni
a source file, you need a type signature in there somewhere. E.g.:

main = mapM_ (\n -> printf "%d %d" (n :: Int) (n^2)) [1..100]

This works fine. This is indeed interesting! Someone care to
additionally make it enlightening as well?