fact n = do
let n' = n - 1
if n <= 1 then 1 else n * fact n'
Here you go. Not sure about Haskell but in PureScript it compiles. Use "<-" for functions which return a value in IO type constructor, otherwise use "let".
That's kinda the point though, right? There's something different about this `IO` thing that uses special syntax. You might be able to guess something and get it to compile, but should you?
fact n = do
let n' = do n - 1
if do n <= 1 then do 1 else do n * fact n'