Pure variants like this one are desugared into a case:

compress x = case x of
    []       -> []
    [a]      -> [a]             -- singleton list
    (x:y:xs) -> (if x == y then [] else [x]) ++ compress (y:xs)
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And can do no monadic monkey business because compress is pure. In this they are simply a more convenient way of spelling out some branches. You wouldn't say if is a sneaky way of doing sequential dependencies, would you?

Sure, with a monadic function you can also have pattern guards that do monadic stuff, but I don't think you can bind without noticing, and in any case the function type will tell you it's monadic.