Given a list of fractional numbers, produce a smoothed list, where each value is averaged with its immediate neighbors (except the first and last, which fail to have both neighbors), as shown below.

[0.0,4.0,2.0,6.0,1.0,2.0]

[    2.0,4.0,3.0,3.0    ]

It seems natural to my eye to express this as, "A list with at least three elements contributes a value to the result", as in:

smooth :: Fractional n => [n] -> [n]

smooth (a:z@(b:c:_)) = (a + b + c) / 3 : smooth z

smooth _             = []