concept console in category functional programming

This is an excerpt from Manning's book Functional Programming in Java: How functional techniques improve your Java programs. Login to get full access to this book.

No printing to the console or to any device

This is very neat and safe. No bad things can happen; no exceptions can be thrown. This is the beauty of functional programming: you have a program that will always work, whatever data you use as input. But the question is, how can you use this result? Suppose you want to display the result on the console—how can you do this?

To see your new methods in action, use the following Console class.

Listing 13.11. The Console class

It’s important to note that these two methods are purely functional. They don’t throw any exceptions, nor do they read from or print to the console. They only return programs that do those things.

Listing 13.17. The new Console class using the stack-safe version

public class Console { private static BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); public static IO<String> readLine(Nothing nothing) { return new IO.Suspend<>(() -> { try { return br.readLine(); } catch (IOException e) { throw new IllegalStateException((e)); } }); } /** * A possible implementation of readLine as a function */ public static Function<Nothing, IO<String>> readLine_ = x -> new IO.Suspend<>(() -> { try { return br.readLine(); } catch (IOException e) { throw new IllegalStateException((e)); } }); /** * A simpler implementation of readLine as a function using a method reference */ public static Function<Nothing, IO<String>> readLine = Console::readLine; /** * A convenience helper method allowing calling the readLine method without * providing a Nothing. */ public static IO<String> readLine() { return readLine(Nothing.instance); } public static IO<Nothing> printLine(Object s) { return new IO.Suspend<>(() -> println(s)); } private static Nothing println(Object s) { System.out.println(s); return Nothing.instance; } public static IO<Nothing> printLine_(Object s) { return new IO.Suspend<>(() -> { System.out.println(s); return Nothing.instance; }); } public static Function<String, IO<Nothing>> printLine_ = s -> new IO.Suspend<>(() -> { System.out.println(s); return Nothing.instance; }); public static Function<String, IO<Nothing>> printLine = Console::printLine; }

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13.2.1. Reading data from the console

As an example, you’ll read data from the console in a way that, although imperative, allows testing by making your programs deterministic. The approach you’ll use is similar to what you did with the random generator in chapter 12.