Preheat oven to 350 degrees F.
Place sweet potatoes, butter, pineapple, and maple syrup in a 9 by 13-inch glass baking dish. Combine well, using a potato masher. Place in oven, and bake until bubbling, about 45 minutes. Top with marshmallows, and continue to bake until marshmallows are soft but not completely melted, about 15 minutes.
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