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    Use `andThen` to transform data while decoding JSON into Elm

    Some data transformations can be obtained by "tagging on" a function to a decoder with map. But if the transformation may fail, then we need a different approach: express failure in terms of decoding (if the transformation fails, decoding fails), and then tag this Decoder-returning function on to the previous Decoder with andThen. This is functional programming's good old bind, particularized for the Decoder type.

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