Last updated on Monday, October 14, 2024

Higher-order functions

Welcome to week 2 of CS-214 — Software Construction!

In the first part of this exercise set, we’ll revisit week 1’s exercises, and search for ways to leverage the power of higher-order functions to write shorter, simpler code. This first part is important for this week’s lab, and it will work much better if you work in groups.
In the second part of this exercise set, we’ll make ourselves familiar with the notion of “functions as values”. We’ll store functions in vals, combine them, make functions that make functions that make functions, etc. We also encourage you to work in groups on this part

This week, we’re trying something a bit different. At the front of the class you should find a stack of printed index cards (also available here), containing the solutions to all of the list function problems of week 1. Here is the protocol:

Form a small group (2 to 4 people).
Come to the front of the class to pick up a sheet of index cards (two pages).
Cut the sheets into individual cards.
Start working on exercises. The cards will be useful in “Part 1: Observation” and “Part 2: Conjecture”.

As before, ⭐️ indicates the most important exercises and questions and 🔥 indicates the most challenging ones. 🔜 indicates exercises that are useful to prepare for future lectures.

You do not need to complete all exercises to succeed in this class, and you do not need to do all exercises in the order they are written.

Higher-order functions on lists and trees ⭐️

You may have noted that we asked to write very similar code many, many times in last week’s exercises. Let’s explore this observation in more detail.

Part 1: Observation ⭐️

Let’s look at five categories of functions, each illustrated by a pair of recursive functions from last week’s exercises. In your stack of index cards, find the cards for the following functions:

sum and product (we’ll call this pair “associative”)
increment and capAtZero (we’ll call this pair “map”)
collectEven and removeZeroes (we’ll call this pair “filter”)
countEven and multiplyOdd (we’ll call this pair “foldRight”)
min and last (we’ll call this pair “reduceRight”)

Read the implementation of these functions carefully. For each pair, ask yourself: what do these two functions have in common? Which parts differ? Use underlines, colors, or boxes to highlight the differences (you can write on the cards).

Once you’re done, put each of these pairs down, leaving some space around each pair to add more cards.

Part 2: Conjecture ⭐️

Let’s see how well the patterns that you have found generalize.

You should have about 20 cards left. Try to classify each of one the cards into one of the five categories above, based on how well it fits the pattern that you identified earlier. If you feel that you need more categories, that’s OK. Oh, and beware: two of the cards don’t fit!

Hint

In particular, you may decide that allEven/allPositiveOrZero and anyOdd/anyNegative should be their own categories. That’s fine!

Hint

reverseAppend and init are the ugly ducklings. reverseAppend is an instance of a different pattern (a “left fold”) that we will study later.
Find another group of students, and compare your results with them. Do you agree on everything? Did you create the same additional categories? If you disagree, is it because one category is a special case of another one?

Hint

If you look very carefully, you will see that all categories are special cases of “fold” and “reduce”.

Part 3: Experiment ⭐️

Time for higher-order functions! Take the categories above one by one, and for each of them, write one single function that is more general than all the examples belonging to that category. To help with this, consider starting with the following warm-up exercise:

Warm-up

Higher-order functions can be used to “abstract away” part of a function.

Consider the following two functions, which check whether the head of a list has a certain property, and return false if the list is empty:
```
def headIsEven(l: IntList): Boolean =
  !l.isEmpty && l.head % 2 == 0
def headIsPositive(l: IntList): Boolean =
  !l.isEmpty && l.head > 0
```
src/main/scala/hofs/fun.scala
The bodies of these functions are very similar, so we can abstract away the common parts into a separate higher-order function (a function that takes a function):
```
def headHasProperty(p: Int => Boolean, l: IntList): Boolean =
  !l.isEmpty && p(l.head)
```
src/main/scala/hofs/fun.scala
Now, use headHasProperty to refactor (i.e. rewrite in a more succinct way) the functions headIsEven and headIsPositive:
Solution
```
def headIsEven1(l: IntList): Boolean =
  headHasProperty(i => i % 2 == 0, l)
def headIsPositive1(l: IntList): Boolean =
  headHasProperty(i => i > 0, l)
```
src/main/scala/hofs/fun.scala

Now take a look at these three functions:

def DoubleTriple(x: Int) =
  IntCons(x * 2, IntCons(x * 3, IntNil()))

def DivideTrivide(x: Int) =
  IntCons(x / 2, IntCons(x / 3, IntNil()))

def IncrementDeuxcrement(x: Int) =
  IntCons(x + 1, IntCons(x + 2, IntNil()))

src/main/scala/hofs/fun.scala

What parts do they have in common? What parts are different?

Write a single function ConstructTwo that abstracts away the common parts of the three functions above, and rewrite all three functions to use it:

def ConstructTwo(f: Int => Int, g: Int => Int): Int => IntList =
  ???

val DoubleTriple2 = TODO
val DivideTrivide2 = TODO
val IncrementDeuxcrement2 = TODO

src/main/scala/hofs/fun.scala

Refactoring recursive functions

Your task is now to generalize this process of abstraction to the groups of functions that you have identified.

The general functions that you come up with will need additional parameters, just like in the refactoring exercise. They should be such that you can rewrite all the functions that fit into the categories as special cases of the general functions, by passing a few parameters.

Here is a series of steps to follow:

Pick a category.
Highlight the parts that differ between functions of that category.
Rewrite all the functions to isolate the parts that differ, representing the differences as vals.
Transform the new vals into parameters of the function.

The following hint shows you this process step by step for “associative” (the category that contains sum and product), but don’t look just yet! Try it on your own and with other students first.

Hint: Step-by-step example

Let’s pick the “associative” category.
There are two differences:
- The base case (0 vs 1)
- The recursive case (+ vs *)

We isolate the + and * into val functions, and the 0 and 1 into simple vals:

l.head + sum(list.tail) becomes f(l.head, sum(list.tail)), where fis (x, y) => x + y.
l.head * product(list.tail) becomes f(l.head, product(list.tail)) where f is ((x, y) => x * y).

The result is:

def sumRewritten(l: IntList): Int =
  val base = 0
  val f = (x: Int, y: Int) => x + y
  if l.isEmpty then base
  else f(l.head, sumRewritten(l.tail))

def productRewritten(l: IntList): Int =
  val base = 1
  val f = (x: Int, y: Int) => x * y
  if l.isEmpty then base
  else f(l.head, productRewritten(l.tail))

src/main/scala/hofs/listOps.scala

Finally, we extract base and f into parameters:

def sp(l: IntList, base: Int, f: (Int, Int) => Int): Int =
  if l.isEmpty then base
  else f(l.head, sp(l.tail, base, f))

def sumDef(l: IntList): Int =
  sp(l, 0, (x, y) => x + y)

def productDef(l: IntList): Int =
  sp(l, 1, (x, y) => x * y)

src/main/scala/hofs/listOps.scala

Alternatively, we could also curry the last argument of sp to make the definitions of sum and product more succinct:

def spCurried(base: Int, f: (Int, Int) => Int): IntList => Int =
  def sp(l: IntList): Int =
    if l.isEmpty then base
    else f(l.head, sp(l.tail))
  sp // Same as (l: IntList) => sp(l)

val sumVal = spCurried(0, (x, y) => x + y)
val productVal = spCurried(1, (x, y) => x * y)

src/main/scala/hofs/listOps.scala

Your turn! Once you get used to it, you will find that almost all of last week’s functions can be succinctly reimplemented.

You will find that sometimes you cannot fully abstract across a whole category, because of types: for example, you will be able to write countEven and multiplyOdd using a single function, and allPositiveOrZero and allEven using a single function, but unifying the two will not work, due to mismatched types. It’s possible to unify them using a concept called polymorphism, which we will study in week 4.

Once you’re done with this exercise on paper, use the REPL, last week’s tests, or last week’s worksheet to confirm that your rewritten (“refactored”) functions work.

Here is some scaffold code (ZIP). Note that it covers only the first part of the set (this part). For the second part (below), we have only provided you with a simple worksheet, with no tests, so that you can learn to test your functions directly yourself (either directly in the worksheet, or in the REPL).

Part 4: Conclusion

The higher-order functions that we have discovered today are useful beyond Scala, and have names common across programming languages:

“associative” (sum/product), once suitably generalized, is almost identical to “foldRight”, so it typically doesn’t have its own name.
“map” (increment/multiply) and “filter” (collectEven/removeOdd) are special cases of foldRight. They are so useful that they are typically defined (and optimized) separately from foldRight, with their own name. They are present in the vast majority of modern programming languages (including JavaScript, Python, and Java).
“foldRight” (countEven/multiplyOdd) and “reduceRight” (minMax/last) are similar to each other, but reduceRight uses the last element of the list as it’s starting point, and does not work for empty lists.

Variants of these functions are found in many fields: in data science with MapReduce; in databases with SELECT/WHERE (filter); in graphics with GPUs and SIMD programs; and in many other computational sciences, where they help process large data sets. There are other such higher-order functions that capture common patterns, which we will explore throughout the course.

The supporting code for this week’s lab includes definitions of all of these functions, should you want to use them. We found them useful in our own solution.

🔜 map and filter are special cases of foldRight. Can you rewrite them using foldRight instead of direct recursion?
```
def mapAsFoldRight(f: Int => Int): IntList => IntList =
  ???

def filterAsFoldRight(p: Int => Boolean): IntList => IntList =
  ???
```
src/main/scala/hofs/fun.scala
⭐️ allEven and anyNegative can be implemented using foldRight, but is the result as efficient as the original function? Why or why not?

Hint

Try evaluating each implementation with the substitution model and counting evaluation steps.
Another way to implement allEven and anyNegative would be to define two functions:
- one function, forall, to check whether a predicate (a function from Int to Boolean) holds (i.e., evaluates to true) for all values in a list, and
- one function, exists, to check whether a predicate holds for any value in a list:
```
def forall(p: Int => Boolean)(l: IntList): Boolean =
  if l.isEmpty then true
  else p(l.head) && forall(p)(l.tail)

def exists(p: Int => Boolean)(l: IntList): Boolean =
  if l.isEmpty then false
  else p(l.head) || exists(p)(l.tail)
```
src/main/scala/hofs/listOps.scala
1. Rewrite allEven and anyNegative using forall/exists.
```
def allEven(l: IntList): Boolean =
  if l.isEmpty then true
  else (l.head % 2 == 0) && allEven(l.tail)
```
  src/main/scala/hofs/listOps.scala
```
def anyNegative(l: IntList): Boolean =
  if l.isEmpty then false
  else l.head < 0 || anyNegative(l.tail)
```
  src/main/scala/hofs/listOps.scala
2. The two implementations provided above use if with a constant branch (if … then true else … and if … then false else …). Can you simplify them to eliminate the ifs?
```
def forallNoIf(p: Int => Boolean)(l: IntList): Boolean =
  ???

def existsNoIf(p: Int => Boolean)(l: IntList): Boolean =
  ???
```
  src/main/scala/hofs/fun.scala

Part 5: Going further

The functions that we have seen today are applicable beyond lists; we will look at this in more depth next week, but you may be interested to get started right away:

🔜 Look over the tree functions of week 1. Do you see patterns and categories similar to the ones we saw for lists? Could you operate the same generalization by defining map, filter, and reduce on trees?
🔜 Look over the string functions of week 1. Are they different from the list functions? Is there anything preventing you to use your list functions on strings?

Functions as values

Operations on functions ⭐️

Think about the types that you already know in Scala: Boolean, Int, String, … (what else?): each of them has operations that you can use to combine them: * on Ints, || on Booleans, + on Strings, == on most types… (what else?)

Which one of these operations make sense for functions? Do they make sense of all functions, or just for some functions? What could it mean to “or” or “add” two functions together?

Hint

Consider concrete examples: if I have two functions isOdd and isGreaterThan5, how can I combine them? What operators can I apply to their results?

How about two functions $f: x ↦ x + 1$ and $g: x ↦ x^2$? The answer should be a bit different from the Boolean case. Why? Consider the argument and return types.

Hint

Take time to think about what == may mean for functions. If we define equality as f(x) ≣ g(x) for all x, then could you write a program that checks whether two functions are equal? How long would that program run for?
Can you think of operations that make sense for functions, but not for the other types you know?

Hint

Think again about $f: x ↦ x + 1$ and $g: x ↦ x^2$. I can add or multiply their results, of course, but what else can I do with them?

Then think about logical negation (not), isEven, and isOdd. Can I define one in terms of the other two?

Function combinations

Composition ⭐️

⭐️ Write a function that takes two functions f and g, and returns a new function that applies them in sequence (f, then g). This is called composition and usually written g ∘ f in mathematics. In Scala, it is typically written g `compose` f or f `andThen` g (both are built-in):
```
def andThen(f: Int => Double, g: Double => String) =
  ???
```
src/main/scala/hofs/fun.scala
Can you write variants of compose for other types of functions? Bool => Bool, or String => String, for example. Does the implementation look different?
More generally, assume that f has type A => B and g has type C => D. Under what conditions can you compose f and g to form g ∘ f? What about f ∘ g?

Identity

0 is the neutral element for +: $0 + x = x + 0 = x$ for all $x$. 1 is the neutral element for *.

What is the neutral element for compose? That is, which function id is such that id ∘ f ≡ f ∘ id ≡ f for all f? (we use f ≡ g here to mean that f(x) == g(x) for all x.

val id: Int => Int =
  TODO

src/main/scala/hofs/fun.scala

Flip

Define a function flip. It takes a function and returns the same function, but with the arguments flipped.
```
def flip(f: (Int, Int) => Int): (Int, Int) => Int =
  ???
```
src/main/scala/hofs/fun.scala
What happens if you compose flip with itself (in other words, what does flip ∘ flip do?)

Refactoring with composition

Higher-order functions are often very useful to capture repeated patterns. For example, here are some closely related functions:

val squareMinusOne     = (x: Int) => (x - 1) * (x - 1)
val squarePlusOne      = (x: Int) => (x + 1) * (x + 1)
val squareSquare       = (x: Int) => (x * x) * (x * x)
val squareMinusTwo     = (x: Int) => (x - 2) * (x - 2)
val squareSquareSquare = (x: Int) =>
  ((x * x) * (x * x)) * ((x * x) * (x * x))

src/main/scala/hofs/fun.scala

Can you define these functions in terms of the following ones?

val square = (x: Int) => x * x
val plusOne = (x: Int) => x + 1
val minusOne = (x: Int) => x - 1
def composeInt(f: Int => Int, g: Int => Int): Int => Int =
  x => f(g(x))

src/main/scala/hofs/fun.scala

Lifting ⭐️

Write a function that takes two functions Int => Int and returns a new function Int => Int whose results are the sum of the results of the first two functions.
```
def adder(f: Int => Double, g: Int => Double): Int => Double =
  ???
```
src/main/scala/hofs/fun.scala
Hint

The result should be such that adder(f, g)(x) == f(x) + g(x).

This is called a lifted version of +. In mathematics, this is the usual definition of + on functions.
Can you do the same for *, -, /, other operators? Do you notice any similarities?
```
def multiplier(f: Int => Double, g: Int => Double): Int => Double =
  ???
```
src/main/scala/hofs/fun.scala
We saw previously that 0 is the neutral element of +. What is the neutral element of adder? In other words, what function is such adder(f, g) ≡ f?

Heavy lifting

Let’s extract the common code between the functions in the previous exercise.

Write a function that takes a single function op (a binary operator such as +) and returns a lifted version of that operation (like adder above).
```
def lifter(op: (Double, Double) => Double): (Int => Double, Int => Double) => (Int => Double) =
  ???
```
src/main/scala/hofs/fun.scala
lifter((x, y) => x + y) should be the same as adder. Look for the common parts in the implementation, and extract the ones that vary between them.
Rewrite adder and other related functions in terms of this one.
```
val adder2 = TODO
val multiplier2 = TODO
```
src/main/scala/hofs/fun.scala

Multi-lifting

Write a lifted version of the boolean && (and) operator.
```
def meet(f: Int => Boolean, g: Int => Boolean): (Int => Boolean) =
  ???
```
src/main/scala/hofs/fun.scala
Hint

This function should be such that meet(f, g)(x) == f(x) && g(x).

Functions that return booleans are often called “predicates” (in other words, f: Int => Boolean is a predicate, and meet above combines two predicates into one).
🔜 Generalize meet to accept more than one predicate. That is, write a function Meet which, given a list of predicates, returns a single predicate that lifts && across all of the predicates.
```
def Meet(l: IntPredicateList): (Int => Boolean) =
  ???
```
src/main/scala/hofs/fun.scala

Values as functions: `def`s, `val`s and currying

The idea of currying is to change (slightly) the way a function is invoked (called) to make it easier to use. Let us explore this on two examples:

Translating between `def`s and `val`s (named and anonymous functions) ⭐️

Using the template below, rewrite (and show how to call) each of the following functions:

def isGreaterThanBasic(x: Int, y: Int): Boolean =
  x > y
val isGreaterThanAnon: (Int, Int) => Boolean =
  (x, y) => x > y
val isGreaterThanCurried: Int => Int => Boolean =
  x => y => x > y // Same as `x => (y => x > y)`
def isGreaterThanCurriedDef(x: Int)(y: Int): Boolean =
  x > y

// How to call:
//   For all x, y:
//     isGreaterThan(x, y)
//       == isGreaterThanAnon(x, y)
//       == isGreaterThanCurried(x)(y)
//       == isGreaterThanCurriedDef(x)(y)

src/main/scala/hofs/fun.scala

incrHeadByX

def incrHeadByXBasic(x: Int, l: IntList): IntList =
  if l.isEmpty then l
  else IntCons(l.head + x, l.tail)

val incrHeadByXAnon: (Int, IntList) => IntList =
  TODO

val incrHeadByXCurried: Int => IntList => IntList =
  TODO

def incrHeadByXCurriedDef(x: Int)(l: IntList): IntList =
  ???

src/main/scala/hofs/fun.scala

addToFront

def addToFrontBasic(x: Int, y: Int, l: IntList): IntList =
  IntCons(x, IntCons(y, l))

val addToFrontAnon: (Int, Int, IntList) => IntList =
  TODO

val addToFrontPartlyCurried: (Int, Int) => IntList => IntList =
  TODO

val addToFrontCurried: Int => Int => IntList => IntList =
  TODO

def addToFrontCurriedDef(x: Int)(y: Int)(l: IntList): IntList =
  ???

src/main/scala/hofs/fun.scala

contains

def containsBasic(l: IntList, n: Int): Boolean =
  !l.isEmpty && (n == l.head || contains(l.tail, n))

def containsAnon: (IntList, Int) => Boolean =
  TODO

def containsCurried: IntList => Int => Boolean =
  TODO

def containsCurriedDef(l: IntList)(n: Int): Boolean =
  ???

src/main/scala/hofs/fun.scala

It is debatable whether containsAnon is really anonymous, since it must use its own name (the one being defined in the def) for the recursive call. This recursion issue is also why we use def instead of val here.

headHasProperty (from “Part 3”, above)

def headHasPropertyBasic(p: Int => Boolean, l: IntList): Boolean =
  !l.isEmpty && p(l.head)

val headHasPropertyAnon: ((Int => Boolean), IntList) => Boolean =
  TODO

val headHasPropertyCurried: (Int => Boolean) => IntList => Boolean =
  TODO

def headHasPropertyCurriedDef(p: Int => Boolean)(l: IntList): Boolean =
  ???

src/main/scala/hofs/fun.scala

Finally, using this new headHasPropertyCurried, rewrite headIsEven and headIsPositive. Which version is shorter?

def headIsEven(l: IntList): Boolean =
  !l.isEmpty && l.head % 2 == 0
def headIsPositive(l: IntList): Boolean =
  !l.isEmpty && l.head > 0

src/main/scala/hofs/fun.scala

val headIsEven2 =
  TODO
val headIsPositive2 =
  TODO

src/main/scala/hofs/fun.scala

Currying container functions ⭐️

Now let’s see how these different styles can help:

Let’s assume we have a list of EPFL Sciper numbers registered for this course (students and staff), and a separate list of EPFL Scipers for just the staff. Thus, registered students are those that occur in the first list but not the second one:

val cs214All =
  IntCons(123456, IntCons(654321, IntCons(111222, IntCons(333444, IntCons(555666, IntCons(787878, IntNil()))))))

val cs214Staff =
  IntCons(654321, IntCons(333444, IntNil()))

src/main/scala/hofs/fun.scala

Write a function isRegisteredForCS214 that checks whether a given Sciper appears in the cs214All list. Write two versions: one using containsBasic, as a def; and one using containsCurried, as a val. For the val, do not create an anonymous function: the definition should have no mention of a sciper variable.
```
def isRegisteredForCS214Def(sciper: Int): Boolean =
  ???

val isRegisteredForCS214Val =
  TODO
```
src/main/scala/hofs/fun.scala
Write a function isCS214Student that checks whether a Sciper corresponds to a registered student. Write two versions: one using containsBasic, as a def; and one using containsCurried, notLifter, and andLifter, as a val. For the val, as before, do not create an anonymous function: the definition should have no mention of a sciper variable.
```
def isCS214StudentDef(sciper: Int): Boolean =
  ???
```
src/main/scala/hofs/fun.scala
```
def andLifter(f: Int => Boolean, g: Int => Boolean): Int => Boolean =
  n => f(n) && g(n)

def notLifter(f: Int => Boolean): Int => Boolean =
  n => !f(n)

val isCS214StudentVal =
  TODO
```
src/main/scala/hofs/fun.scala
The val style is often called “point-free style”, which means using only function combinators like andLifter and notLifter instead of explicit parameter names.
Notice that the two versions of the function above will always scan both lists. Using the difference function on lists, write a more general, curried function that takes two lists, computes the difference, and then returns a function that takes a sciper and validates it against the resulting list. Make sure that the list difference is computed once. Can you do it in point-free style, with no anonymous functions?
```
def isCourseStudentDefPartlyCurried(all: IntList, staff: IntList): Int => Boolean =
  ???
```
src/main/scala/hofs/fun.scala

Equality 🔥

Mathematicians generally say that two functions are “equal” when they have the same outputs for all inputs.

By this definition, which of the following functions are equal?

val f0 = (x: Long) => x
val f1 = (x: Long) => if x > 0 then x else -x
val f2 = (x: Long) => x + 1 - 1
val f3 = (x: Long) =>
  Math.sqrt(x.toDouble * x.toDouble).round
val f4: Long => Long = x =>
  if x < 0 then f4(x + 1) - 1
  else if x > 0 then f4(x - 1) + 1
  else 0

src/main/scala/hofs/fun.scala

Can you define a function that checks whether two functions of type Boolean => Boolean are equal?
```
def eqBoolBool(
    f: Boolean => Boolean,
    g: Boolean => Boolean
) =
  ???
```
src/main/scala/hofs/fun.scala
What about functions of the following types?
- Int => Boolean
- Boolean => Int
- IntList => Boolean
- Boolean => IntList
- Int => Boolean => IntList
Can you come up with a general result? For which types A and B can one write a function eqAB that checks whether two functions of type A => B return the same outputs for all inputs?
What do you think of this definition of equality? Is f4 really “the same” as f0, or do they differ in any way?

Fixed points

A value x is a fixed point of f if f(x) == x. Do the following functions have fixed points ? If so, which one(s)?

val a = (x: Int) => x
val b = (x: Int) => -x
val c = (x: Int) => x + 1
val d = (x: Int) => (x / 2) + 5
val e = (x: Int) => if x % 10 == 0 then x else (x + 1)
val f = (x: Int) => -(x * x)
val g = (x: Int) => /* 🔥 */ /* assuming x > 0 */
  if x == 1 then 1
  else if x % 2 == 0 then x / 2
  else 3 * x + 1

src/main/scala/hofs/fun.scala

Write a function fixedPoint that takes a function f and an integer x, checks whether x is already a fixpoint of f, and then looks for a fixed point by repeatedly calling f, until it converges.
```
def fixedPoint(f: Int => Int, start: Int): Int =
  ???
```
src/main/scala/hofs/fun.scala
For example, fixedPoint(((x: Int) => x / 2 + 5), 20) will call itself recursively with x = 15 (20 / 2 + 5), then x = 12 (15 / 2 + 5), then x = 11 (12 / 2 + 5), then x = 10 (11 / 2 + 5).
For each of the following expressions, indicate whether it terminates, and if so, what value is returned:

a. fixedPoint(((x: Int) => x / 2), 4) b. fixedPoint(((x: Int) => -x), 3) c. fixedPoint(((x: Int) => x), 123456) d. fixedPoint(((x: Int) => x + 1), 0) e. fixedPoint(((x: Int) => if (x % 10 == 0) then x else x + 1), 35) f. fixedPoint(((x: Int) => x / 2 + 5), 20)

What happens when there is no fixed point? Does fixedPoint work for all functions above that have a fixed point? Does it depend on the starting value of x?
🔥 Finally, a question that is not directly relevant to the class but interesting to think about. For which functions and which inputs does fixedPoint work?