To apply a function to a list in Haskell, you can use the map function. The map function takes a function as its first argument and a list as its second argument, and it applies the function to each element of the list, returning a new list with the modified elements. For example, if you have a function double that doubles a number, you can apply this function to a list of numbers by using map double [1, 2, 3], which will result in [2, 4, 6].