The map Function
The map function takes in a function and a list, and applies the given function to each element of that list. As we have seen how list comprehensions work in the previous chapter, we could define map as:
map :: (a -> b) -> [a] -> [b]
map f xs = [f x | x <- xs]
Note that the type variables a and b in the function definition could represent the same type, but this definition gives us flexibility so that a function passed in that takes in one type (a) can return another type (b), in which case we end up with a list of type [b]. For example, we can pass in our function squareGt100 to a list of numbers and end up with a list of booleans:
ghci> map squareGt100 [7..12]
[False, False, False, False, True, True]
Here are some other examples of using map with other pre-defined functions:
ghci> map (* 2) [1..5] -- multiply each number in the list by 2
[2, 4, 6, 8, 10]
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ghci> map not [True, False] -- not function reverses the boolean value
[False, True]
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ghci> map reverse ["Cardano", "ADA"] -- reverse a given list
["onadraC","ADA"]
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ghci> map ("Hi, " ++) ["Joe", "Jan"]
["Hi, Joe","Hi, Jan"]
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