How to write a generic iteration of a function using Java 8?
functor, generics, java, java-8
Solution
Try:
public static<X> UnaryOperator<X> iterate(UnaryOperator<X> f, int n) { ... }
If Y <: X, then a function from X to Y is also a function from X to X, and you should be able to do what you want.
Problem
Please consider the following two functions: ``` public static <X, Y, U, V extends X> Function<U, Y> composite( Function<X, Y> first, Function<U, V> second) { Objects.requireNonNull(first); Objects.requireNonNull(second); return (U arg) -> first.apply(second.apply(arg)); } public static <X, Y extends X> Function<X, ?> iterate(Function<X, Y> function, int n) { if (n < 0) return null; if (n == 0) return (X arg) -> arg; Objects.requireNonNull(function); Function<X, Y> iteration = function; for (; n > 1; --n) iteration = composite(function, iteration); return iteration; } ``` While `composite(first, second)` computes the composition of `first` and `second`, `iterate(function, n)` computes the `nth` iterate of `function`. While the restriction `Y extends X` suffices for any `n > 0`, we've got some problem with `n == 0`. Mathematically, `iterate` should yield the identity function. However, therefore we would need `X extends Y`, i.e. `X == Y`, as well. Please consider the following example ``` Function<Double, Double> nthSquareRoot = iterate(Math::sqrt, n); ``` This yields the error message: ``` Type mismatch: cannot convert from Function<Double,capture#2-of ?> to Function<Double,Double> ``` What's the best option here? If `n == 1`, we could check if `X` is acceptable as `Y`. I would like to hear other options and some ideas how this check can be performed (as far as I know, there is no simple solution to check two generic parameters for equality).