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Naive Map Isn't So Naive

Posted on October 1, 2013
Tags: haskell

One of the most beloved functions in functional programming languages is map. It can be defined like this:

    map :: (a -> b) -> [a] -> [b]
    map f (x:xs) = f x : map f xs
    map _ []     = []

However in a lot of languages, writing map like this is a no-no. It’s not tail recursive! For example in OCaml

    # let rec my_map f = function
       | [] -> []
       | x :: xs -> f x :: my_map f xs

    # my_map ((+) 1) list_from_0_to_5000000
    ... Wait a bit ...
    Error: Stackoverflow

Urk! That’s annoying. The problem is that we have to make a recursive function call that can’t be compiled down to a loop (tail recursion). Well, let’s look at how map is defined in Haskell to avoid this problem

    -- In Base
    map :: (a -> b) -> [a] -> [b]
    map f (x:xs) = f x : map f xs
    map _ []     = []

Wait, isn’t this bad? We just saw how this isn’t tail recursive!

The thing is, in Haskell things are lazy. map (+1) [1..10000] returns a thunk. Inside that thunk is something like this

    (:) 1 {thunk to get rest of list}

So this takes constant space! After all, none of those extra stack frames are used because : doesn’t evaluate its arguments. This means that a lot of not tail recursive functions in Haskell still take constant space, however, you have to be careful about consuming the results.

Take for example sum.

    sum :: [Integer] -> Integer
    sum (x:xs) = x + sum xs
    sum []     = 0

Now this also isn’t tail recursive, but there’s a problem: + is strict. By this I mean that to evaluate a+b you must first evaluate a and then b. This means that to evaluate x + sum xs we have to evaluate sum xs.

Urk, now we’re building up a big pile of expressions, something like

    a + sum (b:c:d:e:[])
    a + (b + sum (c:d:e:[]))
    a + (b + (c + sum (d:e:[])))
    a + (b + (c + (d + sum (e:[]))))
    a + (b + (c + (d + (e + 0))))

Now we can see why this will blow up, it’s building up a huge expression before we can evaluate anything. Hi stack overflow.

Now this is when we do want the tail recursive function like foldl' to keep things in constant space.


When you construct something with : for example, it’s possible to evaluate the head without evaluating the tail. Similarly with most constructors. With things like this, it’s possible to keep naive recursion in constant space.

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