We develop a nonanticipative calculus for functionals of a continuous semimartingale, using an extension of the Itô formula to pathdependent functionals which possess certain directional derivatives. The construction is based on a pathwise derivative, introduced by Dupire, for functionals on the space of right-continuous functions with left limits. We show that this functional derivative admits a suitable extension to the space of square-integrable martingales. This extension defines a weak derivative which is shown to be the inverse of the Itô integral and which may be viewed as a nonanticipative "lifting" of the Malliavin derivative.These results lead to a constructive martingale representation formula for Itô processes. By contrast with the Clark-Haussmann-Ocone formula, this representation only involves nonanticipative quantities which may be computed pathwise.

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