Haskell/Higher-order functions and Currying
Higher-order functions are functions that take other functions as arguments. We have already met some of them, such as
map, so there isn't anything really frightening or unfamiliar about them. They offer a form of abstraction that is unique to the functional programming style. In functional programming languages like Haskell, functions are just like any other value, so it doesn't get any harder to deal with higher-order functions.
Higher order functions have a separate section in this book, not because they are particularly difficult – we've already worked with them, after all – but because they are powerful enough to draw special attention to them. We will see in this section how much we can do if we can pass around functions as values. Generally speaking, it is a good idea to abstract over a functionality whenever we can. Besides, Haskell without higher order functions wouldn't be quite as much fun.
The Quickest Sorting Algorithm In Town
Don't get too excited, but
quickSort is certainly one of the quickest. Have you heard of it? If so, you can skip the following subsection and go straight to the next one.
The Idea Behind
The idea is very simple. For a big list, we pick an element, and divide the whole list into three parts.
The first part has all elements that should go before that element, the second part consists of all of the elements that are equal to the picked element, the third has the elements that ought to go after that element. And then, of course, we are supposed to concatenate these. What we get is somewhat better, right?
The trick is to note that only the first and the third are yet to be sorted, and for the second, sorting doesn't really make sense (they are all equal!). How to go about sorting the yet-to-be-sorted sub-lists? Why... apply the same algorithm on them again! By the time the whole process is finished, you get a completely sorted list.
So Let's Get Down To It!
-- if the list is empty, we do nothing -- note that this is the base case for the recursion quickSort  =  -- if there's only one element, no need to sort it -- actually, the third case takes care of this one pretty well -- I just wanted you to take it step by step quickSort [x] = [x] -- this is the gist of the process -- we pick the first element as our "pivot", the rest is to be sorted -- don't forget to include the pivot in the middle part! quickSort (x : xs) = (quickSort less) ++ (x : equal) ++ (quickSort more) where less = filter (< x) xs equal = filter (== x) xs more = filter (> x) xs
And we are done! Even if you have met
quickSort before, perhaps you thought recursion was a neat trick but difficult to implement.
Now, How Do We Use It?
quickSort at our disposal, sorting any list is a piece of cake. Suppose we have a list of
String, maybe from a dictionary, and we want to sort them; we just apply
quickSort to the list. For the rest of this chapter, we will use a pseudo-dictionary of words (but a 25,000 word dictionary should do the trick as well):
dictionary = ["I", "have", "a", "thing", "for", "Linux"]
We get, for
["I", "Linux", "a", "for", "have", "thing"]
But, what if we wanted to sort them in the descending order? Easy, just reverse the list,
reverse (quickSort dictionary) gives us what we want.
But wait! We didn't really sort in the descending order, we sorted (in the ascending order) and reversed it. They may have the same effect, but they are not the same thing!
Besides, you might object that the list you got isn't what you wanted. "a" should certainly be placed before "I". "Linux" should be placed between "have" and "thing". What's the problem here?
The problem is, the way
Strings are represented in a typical programming settings is by a list of Unicode characters. Unicode (and almost all other encodings of characters) specifies that the character code for capital letters are less than the small letters. Bummer. So "Z" is less than "a". We should do something about it. Looks like we need a case insensitive
quickSort as well. It might come handy some day.
But, there's no way you can blend that into
quickSort as it stands. We have work to do.
Tweaking What We Already Have
What we need to do is to factor out the comparisons
quickSort makes. We need to provide
quickSort with a function that compares two elements, and gives an
Ordering, and as you can imagine, an
Ordering is any of
LT, EQ, GT.
To sort in the descending order, we supply
quickSort with a function that returns the opposite of the usual
Ordering. For the case-insensitive sort, we may need to define the function ourselves. By all means, we want to make
quickSort applicable to all such functions so that we don't end up writing it over and over again, each time with only minor changes.
quickSort, Take Two
quickSort will take two things this time: first, the comparison function, and second, the list to sort.
A comparison function will be a function that takes two things, say,
y, and compares them. If
x is less than
y (according to the criteria we want to implement by this function), then the value will be
LT. If they are equal (well, equal with respect to the comparison, we want "Linux" and "linux" to be equal when we are dealing with the insensitive case), we will have
EQ. The remaining case gives us
GT (pronounced: greater than, for obvious reasons).
-- no matter how we compare two things -- the first two equations should not change -- they need to accept the comparison function though -- but the result doesn't need it quickSort _  =  quickSort _ [x] = [x] -- we are in a more general setting now -- but the changes are worth it! -- c is the comparison function quickSort c (x : xs) = (quickSort c less) ++ (x : equal) ++ (quickSort c more) where less = filter (\y -> y `c` x == LT) xs equal = filter (\y -> y `c` x == EQ) xs more = filter (\y -> y `c` x == GT) xs
But What Did We Gain?
Reuse. We can reuse
quickSort to serve different purposes.
-- the usual ordering -- uses the compare function from the Ord class usual = compare -- the descending ordering, note we flip the order of the arguments to compare descending x y = compare y x -- the case-insensitive version is left as an exercise! insensitive = ... -- can you think of anything without making a very big list of all possible cases?
And we are done!
quickSort usual dictionary
should, then, give
["I", "Linux", "a", "for", "have", "thing"]
The comparison is just
compare from the
Ord class. This was our
quickSort, before the tweaking.
quickSort descending dictionary
["thing", "have", "for", "a", "Linux", "I"]
quickSort insensitive dictionary
["a", "for", "have", "I", "Linux", "thing"]
Exactly what we wanted!
Higher-Order Functions and Types
quickSort has type
(a -> a -> Ordering) -> [a] -> [a].
Most of the time, the type of a higher-order function provides a good guideline about how to use it. A straightforward way of reading the type signature would be, "
quickSort takes a function that gives an ordering of
as, and a list of
as, to give a list of
as". It is then natural to guess that the function sorts the list respecting the given ordering function.
Note that the parentheses surrounding
a -> a -> Ordering is mandatory. It says that
a -> a -> Ordering altogether form a single argument, an argument that happens to be a function. What happens if we omit the parentheses? We would get a function of type
a -> a -> Ordering -> [a] -> [a], which accepts four arguments instead of the desired two (
a -> a -> Ordering and
[a]). Furthermore none of the four arguments, neither
[a] are functions, so omitting the parentheses would give us something that isn't a higher order function.
Furthermore, it's worth noting that the
-> operator is right-associative, which means that
a -> a -> Ordering -> [a] -> [a] means the same thing as
a -> (a -> (Ordering -> ([a] -> [a]))). We really must insist that the
a -> a -> Ordering be clumped together by writing those parentheses... but wait... if
-> is right-associative, wouldn't that mean that the correct signature
(a -> a -> Ordering) -> [a] -> [a] actually means...
(a -> a -> Ordering) -> ([a] -> [a])?
Is that really what we want?
If you think about it, we're trying to build a function that takes two arguments, a function and a list, returning a list. Instead, what this type signature is telling us is that our function takes one argument (a function) and returns another function. That is profoundly odd... but if you're lucky, it might also strike you as being profoundly beautiful. Functions in multiple arguments are fundamentally the same thing as functions that take one argument and give another function back. It's OK if you're not entirely convinced. We'll go into a little bit more detail below and then show how something like this can be turned to our advantage.
The following exercise combines what you have learned about higher order functions, recursion and IO. We are going to recreate what programmers from more popular languages call a "for loop". Implement a function
for :: a -> (a->Bool) -> (a->a) -> (a-> IO ()) -> IO () for i p f job = -- ???
An example of how this function would be used might be
for 1 (<10) (+1) (print)
which prints the numbers 1 to 9 on the screen.
Starting from an initial value
Some more challenging exercises you could try
I hope you followed the reasoning of the preceding chapter closely enough. If you haven't, you should, so give it another try.
Currying is a technique that lets you partially apply a multi-parameter function. When you do that, it remembers those given values, and waits for the remaining parameters.
quickSort takes two parameters, the comparison function, and the list. We can, by currying, construct variations of
quickSort with a given comparison function. The variation just "remembers" the specific comparison, so you can apply it to the list, and it will sort the list using the that comparison function.
descendingSort = quickSort descending
What is the type of descendingSort?
(a -> a -> Ordering) -> [a] -> [a], and the comparison function
a -> a -> Ordering. Applying
descending (that is, applying it partially, we haven't specified the list in the definition) we get a function (our
descendingSort) for which the first parameter is already given, so you can scratch that type out from the type of
quickSort, and we are left with a simple
[a] -> [a]. So we can apply this one to a list right away!
["thing", "have", "for", "a", "Linux", "I"]
It's rather neat. But is it useful? You bet it is. It is particularly useful as sections, you might notice. Currying often makes Haskell programs very concise. More than that, it often makes your intentions a lot clearer. Remember
less = filter (< x) xs
from the first version of
quickSort? You can read it aloud like "keep those in
xs that are less than
(< x) is just a shorthand for
(\y -> y < x), try reading that aloud!
We will close the chapter discussing a few examples of very common and very useful general-purpose higher-order functions, beginning with
flip is a handy little Prelude function that has the following type:
flip :: (a -> b -> c) -> b -> a -> c
It takes a function of two arguments and returns a version of the same function with the arguments swapped, so that:
Prelude> (flip (/)) 3 1 0.3333333333333333 Prelude> (flip map) [1,2,3] (*2) [2,4,6]
We could have used flip to write the
descending comparing function from the quickSort example point-free, as simply:
descending = flip compare
flip is particularly useful when we want to pass a function with two arguments of different types to another function and the arguments are in the wrong order with respect to the signature of the higher-order function.
(.) composition operator is, as should be clear by now, just another higher-order function. It has the signature:
(.) :: (b -> c) -> (a -> b) -> a -> c
(.) takes two functions as argument and returns a new function, which applies the second argument and then the first.
Composition and higher-order functions provide a range of powerful tricks. For a very small sample of that, first consider the
inits function, defined in the module
Data.List. Quoting the documentation, it "returns all initial segments of the argument, shortest first", so that:
Prelude Data.List> inits [1,2,3] [,,[1,2],[1,2,3]]
With the higher order functions
map we can, using only functions in Prelude, provide the following one line implementation for
inits (written point-free for extra dramatic effect):
myInits :: [a] -> [[a]] myInits = map reverse . scanl (flip (:)) 
Swallowing a definition so condensed may look daunting at first; so slowly analyse it bit by bit, recalling what each function does and using the type signatures as a guide.
Note that the definition of
myInits is not only very concise but also clean, in particular with parentheses usage kept to a bare minimum. Naturally, if one goes overboard with composition by writing mile-long
(.) chains things are bound to get confusing, but when deployed reasonably this is a very attractive style. Furthermore, the implementation is quite "high level": we do not deal explicitly at all with details like pattern matching or recursion; the functions we deployed - both the higher-order ones and their functional arguments - take care of such plumbing.
Finally, we will present a very curious higher-order operator,
($). Its type is:
($) :: (a -> b) -> a -> b
It takes a function as its first argument, and all it does is to apply the function to the second argument, so that, for instance,
head $ "abc" == head "abc".
By now, it would be entirely justified if you thought that
($) was completely useless! However, there are two interesting points about it. First,
($) has very low precedence, unlike regular function application which has very high precedence. In effect, that means we can write a non-point-free version of
myInits without having to add any parentheses in spite of the intricate expression at the left of the argument:
myInits :: [a] -> [[a]] myInits xs = map reverse . scanl (flip (:))  $ xs
($) is just a function which happens to apply functions, and functions are just values, we can write intriguing expressions such as:
map ($ 2) [(2*), (4*), (8*)]
(Yes, that is a list of functions, and it is perfectly legal.)
Function manipulation through higher-order gives us a great deal of power. As we continue throughout this book we will see many examples of such power being harnessed.
- named after the outstanding logician Haskell Brooks Curry, the same guy after whom the language is named.
- Precedence here is meant in the sense that
+has lower precedence than
*, and therefore
2 + 3 * 4equals 14 and not 20.