Haskell/Higherorder functions
At the heart of functional programming is the idea that functions are just like any other value. The power of functional style comes from handling functions themselves as regular values, i.e. by passing functions to other functions and returning them from functions. A function that takes another function (or several functions) as an argument is called a higherorder function. They can be found pretty much anywhere in a Haskell program, and indeed we have already met some of them, such as map
and the various folds. We saw commonplace examples of higherorder functions when discussing map
in Lists II. Now, we are going to explore some common ways of writing code that manipulates functions.
A sorting algorithm edit
For a concrete example, we will consider the task of sorting a list. Quicksort is a wellknown recursive sorting algorithm. To apply its sorting strategy to a list, we first choose one element and then divide the rest of the list into (A) those elements that should go before the chosen element, (B) those elements equal to the chosen one, and (C) those that should go after. Then, we apply the same algorithm to the unsorted (A) and (C) lists. After enough recursive sorting, we concatenate everything back together and have a final sorted list. That strategy can be translated into a Haskell implementation in a very simple way.
 Type signature: any list with elements in the Ord class can be sorted.
quickSort :: (Ord a) => [a] > [a]
 Base case:
 If the list is empty, there is nothing to do.
quickSort [] = []
 The recursive case:
 We pick the first element as our "pivot", the rest is to be sorted.
 Note how the pivot itself ends up included 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
It should be pointed out that our quickSort
is rather naïve. A more efficient implementation would avoid the three passes through filter
at each recursive step and not use (++)
to build the sorted list. Furthermore, unlike our implementation, the original quicksort algorithm does the sorting inplace using mutability.^{[1]} We will ignore such concerns for now, as we are more interested in the usage patterns of sorting functions, rather than in exact implementation.
The Ord
class
edit
Almost all the basic data types in Haskell are members of the Ord
class, which is for ordering tests what Eq
is for equality tests. The Ord
class defines which ordering is the "natural" one for a given type. It provides a function called compare
, with type:
compare :: (Ord a) => a > a > Ordering
compare
takes two values and compares them, returning an Ordering
value, which is LT
if the first value is less than the second, EQ
if it is equal and GT
if it is greater than. For an Ord
type, (<)
, (==)
from Eq
and (>)
can be seen as shortcuts to compare
that check for one of the three possibilities and return a Bool
to indicate whether the specified ordering is true according to the Ord
specification for that type. Note that each of the tests we use with filter
in the definition of quickSort
corresponds to one of the possible results of compare
, and so we might have written, for instance, less
as less = filter (\y > y `compare` x == LT) xs
.
Choosing how to compare edit
With quickSort
, sorting any list with elements in the Ord
class is easy. Suppose we have a list of String
and we want to sort them; we just apply quickSort
to the list. For the rest of this chapter, we will use a pseudodictionary of just a few words (but dictionaries with thousands of words would work just as well):
dictionary = ["I", "have", "a", "thing", "for", "Linux"]
quickSort dictionary
returns:
["I", "Linux", "a", "for", "have", "thing"]
As you can see, capitalization is considered for sorting by default. Haskell String
s are lists of Unicode characters. Unicode (and almost all other encodings of characters) specifies that the character code for capital letters are less than the lower case letters. So "Z" is less than "a".
To get a proper dictionarylike sorting, we need a case insensitive quickSort
. To achieve that, we can take a hint from the discussion of compare
just above. The recursive case of quickSort
can be rewritten as:
quickSort compare (x : xs) = (quickSort compare less) ++ (x : equal) ++ (quickSort compare more)
where
less = filter (\y > y `compare` x == LT) xs
equal = filter (\y > y `compare` x == EQ) xs
more = filter (\y > y `compare` x == GT) xs
While this version is less tidy than the original one, it makes it obvious that the ordering of the elements hinges entirely on the compare
function. That means we only need to replace compare
with an (Ord a) => a > a > Ordering
function of our choice. Therefore, our updated quickSort'
is a higherorder function which takes a comparison function along with the list to sort.
quickSort' :: (Ord a) => (a > a > Ordering) > [a] > [a]
 No matter how we compare two things the base case doesn't change,
 so we use the _ "wildcard" to ignore the comparison function.
quickSort' _ [] = []
 c is our 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
We can reuse our quickSort'
function to serve many different purposes.
If we wanted a descending order, we could just reverse our original sorted list with reverse (quickSort dictionary)
. Yet to actually do the initial sort descending, we could supply quickSort'
with a comparison function that returns the opposite of the usual Ordering
.
 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 caseinsensitive version is left as an exercise!
insensitive = ...
 How can we do caseinsensitive comparisons without making a big list of all possible cases?
Note
Data.List
offers a sort
function for sorting lists. It does not use quicksort; rather, it uses an efficient implementation of an algorithm called mergesort. Data.List
also includes sortBy
, which takes a custom comparison function just like our quickSort'
Exercises 

Write insensitive , such that quickSort' insensitive dictionary gives ["a", "for", "have", "I", "Linux", "thing"] . 
HigherOrder Functions and Types edit
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The concept of currying (the generating of intermediate functions on the way toward a final result) was first introduced in the earlier chapter "Lists II". This is a good place to revisit how currying works.
Our quickSort'
has type (a > a > Ordering) > [a] > [a]
.
Most of the time, the type of a higherorder function provides a guideline about how to use it. A straightforward way of reading the type signature would be "quickSort'
takes, as its first argument, a function that gives an ordering of two a
s. Its second argument is a list of a
s. Finally, it returns a new list of a
s". This is enough to correctly guess that it uses the given ordering function to sort the list.
Note that the parentheses surrounding a > a > Ordering
are mandatory. They specify that a > a > Ordering
forms a single argument that happens to be a function.
Without the parentheses, we would get a > a > Ordering > [a] > [a]
which accepts four arguments (none of which are themselves functions) instead of the desired two, and that wouldn't work as desired.
Remember that the >
operator is rightassociative. Thus, our erroneous type signature a > a > Ordering > [a] > [a]
means the same thing as a > (a > (Ordering > ([a] > [a])))
.
Given that >
is rightassociative, the explicitly grouped version of the correct quickSort'
signature is actually (a > a > Ordering) > ([a] > [a])
. This makes perfect sense. Our original quickSort
lacking the adjustable comparison function argument was of type [a] > [a]
. It took a list and sorted it. Our new quickSort'
is simply a function that generates quickSort
style functions! If we plug in compare
for the (a > a > Ordering)
part, then we just return our original simple quickSort
function. If we use a different comparison function for the argument, we generate a different variety of a quickSort
function.
Of course, if we not only give a comparison function as an argument but also feed in an actual list to sort, then the final result is not the new quickSort
style function; instead, it continues on and passes the list to the new function and returns the sorted list as our final result.
Exercises 

(Challenging) The following exercise combines what you have learned about higher order functions, recursion and I/O. We are going to recreate what is known in imperative languages as 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. The desired behaviour of
Some more challenging exercises you could try

Function manipulation edit
We will close the chapter by discussing a few examples of common and useful generalpurpose higherorder functions. Familiarity with these will greatly enhance your skill at both writing and reading Haskell code.
Flipping arguments edit
flip
is a handy little Prelude function. It takes a function of two arguments and returns a version of the same function with the arguments swapped.
flip :: (a > b > c) > b > a > c
flip
in use:
Prelude> (flip (/)) 3 1 0.3333333333333333 Prelude> (flip map) [1,2,3] (*2) [2,4,6]
We could have used flip to write a pointfree version of the descending
comparing function from the quickSort example:
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 higherorder function.
Composition edit
The (.)
composition operator is another higherorder function. It has the signature:
(.) :: (b > c) > (a > b) > a > c
(.)
takes two functions as arguments and returns a new function which applies the second function to the argument and then the first. (Writing the type of (.)
as (b > c) > (a > b) > (a > c)
can make that easier to see.)
Composition and higherorder functions provide a range of powerful tricks. For a tiny sample, 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],[1,2],[1,2,3]]
We can provide a oneline implementation for inits
(written pointfree for extra dramatic effect) using only the following higherorder functions from Prelude: flip
, scanl
, (.)
and map
:
myInits :: [a] > [[a]]
myInits = map reverse . scanl (flip (:)) []
Swallowing a definition so condensed may look daunting at first, so analyze it slowly, bit by bit, recalling what each function does and using the type signatures as a guide.
The definition of myInits
is super concise and clean with use of parentheses kept to a bare minimum. Naturally, if one goes overboard with composition by writing milelong (.)
chains, things will get confusing; but, when deployed reasonably, these pointfree styles shine. Furthermore, the implementation is quite "high level": we do not deal explicitly with details like pattern matching or recursion; the functions we deployed — both the higherorder ones and their functional arguments — take care of such plumbing.
Application edit
($)
is a curious higherorder operator. Its type is:
($) :: (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")
.
You might think that ($)
is completely useless! However, there are two interesting points about it. First, ($)
has very low precedence,^{[2]} unlike regular function application which has the highest precedence. In effect, that means we can avoid confusing nesting of parentheses by breaking precedence with $
. We write a nonpointfree version of myInits
without adding new parentheses:
myInits :: [a] > [[a]]
myInits xs = map reverse . scanl (flip (:)) [] $ xs
Furthermore, as ($)
is just a function which happens to apply functions, and functions are just values, we can write intriguing expressions such as:
map ($ 2) [(2*), (4*), (8*)]
(Yes, that is a list of functions, and it is perfectly legal.)
uncurry
and curry
edit
As the name suggests, uncurry
is a function that undoes currying; that is, it converts a function of two arguments into a function that takes a pair as its only argument.
uncurry :: (a > b > c) > (a, b) > c
Prelude> let addPair = uncurry (+) Prelude> addPair (2, 3) 5
One interesting use of uncurry
occasionally seen in the wild is in combination with ($)
, so that the first element of a pair is applied to the second.
Prelude> uncurry ($) (reverse, "stressed") "desserts"
There is also curry
, which is the opposite of uncurry
.
curry :: ((a, b) > c) > a > b > c
Prelude> curry addPair 2 3  addPair as in the earlier example. 5
Because most Haskell functions are already curried, curry
is nowhere near as common as uncurry
.
id
and const
edit
Finally, we should mention two functions which, while not higherorder functions themselves, are most often used as arguments to higherorder functions. id
, the identity function, is a function with type a > a
that returns its argument unchanged.
Prelude> id "Hello" "Hello"
Similar in spirit to id
, const
is an a > b > a
function that works like this:
Prelude> const "Hello" "world" "Hello"
const
takes two arguments, discards the second and returns the first. Seen as a function of one argument, a > (b > a)
, it returns a constant function, which always returns the same value no matter what argument it is given.
id
and const
might appear worthless at first. However, when dealing with higherorder functions it is sometimes necessary to pass a dummy function, be it one that does nothing with its argument or one that always returns the same value. id
and const
give us convenient dummy functions for such cases.
Exercises 

