Fold Assignment

Lexical scope is not unique to ML. Nearly all programming languages use lexical scope, including Java.

First, we will build the utility methods: foldLeft (which can be easily built with tail recursion) and foldRight (which cannot).

Next, we will build some applications which use fold: sum (which does not require lexical scope) and countBetweenMinAndMaxExclusive (which does).

Code To Implement

Fold

class:	FoldHof.java
methods:	foldLeft foldRight
package:	hof.fold.assignment
source folder:	src/main/java

foldLeft

public static <A, B> A foldLeft(BiFunction<A, B, A> f, A acc, ImmutableList<B> list)

foldLeft(f, initial_value, [x1, x2, ..., xn])

returns f(xn,...,f(x2, f(x1, initial_value))...) or initial_value if the list is empty.

foldRight

public static <A, B> A foldRight(BiFunction<A, B, A> f, A acc, ImmutableList<B> list)

foldRight(f, initial_value, [x1, x2, ..., xn])

returns f(x1, f(x2, ..., f(xn, init)...)) or initial_value if the list is empty.

Fold Apps

class:	FoldHofApps.java
methods:	sum countBetweenMinAndMaxExclusive
package:	hof.map.assignment
source folder:	src/main/java

sum

public static int sum(ImmutableList<Integer> xs)

countBetweenMinAndMaxExclusive

public static int countBetweenMinAndMaxExclusive(int min, int maxExclusive, ImmutableList<Integer> xs)

Test

class:	FoldTestSuite.java
package:	hof.fold.assignment
source folder:	src/test/java