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copied!<h1>Currying, That's Why</h1> <p>Daniel did a great job at explaining why parameterless methods are necessary. I'll explain why they are regarded distinctly from zero-parameter methods.</p> <p>Many people view the distinction between parameterless and zero-parameter functions as some vague form of syntactic sugar. In truth it is purely an artifact of how Scala supports currying (for completeness, see below for a more thorough explanation of what currying is, and why we all like it so much).</p> <p>Formally, a function may have zero or more parameter lists, with zero or more parameters each.<br> This means the following are valid: <code>def a</code>, <code>def b()</code>, but also the contrived <code>def c()()</code> and <code>def d(x: Int)()()(y: Int)</code> etc...</p> <p>A function <code>def foo = ???</code> has zero parameter lists. A function <code>def bar() = ???</code> has precisely one parameter list, with zero parameters. <strong>Introducing additional rules that conflate the two forms would have undermined currying as a consistent language feature</strong>: <code>def a</code> would be equivalent in form to <code>def b()</code> and <code>def c()()</code> both; <code>def d(x: Int)()()(y: Int)</code> would be equivalent to <code>def e()(x: Int)(y: Int)()()</code>.</p> <p>One case where currying is irrelevant is when dealing with Java interop. Java does not support currying, so there's no problem with introducing syntactic sugar for zero-parameter methods like <code>"test".length()</code> (which directly invokes <code>java.lang.String#length()</code>) to also be invoked as <code>"test".length</code>.</p> <h1>A quick explanation of currying</h1> <p>Scala supports a language feature called 'currying', named after mathematician Haskell Curry.<br> Currying allows you to define functions with several parameter lists, e.g.:</p> <pre><code>def add(a: Int)(b: Int): Int = a + b add(2)(3) // 5 </code></pre> <p>This is useful, because you can now define <code>inc</code> in terms of a partial application of <code>add</code>:</p> <pre><code>def inc: Int => Int = add(1) inc(2) // 3 </code></pre> <p>Currying is most often seen as a way of introducing control structures via libraries, e.g.:</p> <pre><code>def repeat(n: Int)(thunk: => Any): Unit = (1 to n) foreach { _ => thunk } repeat(2) { println("Hello, world") } // Hello, world // Hello, world </code></pre> <p>As a recap, see how <code>repeat</code> opens up another opportunity to use currying:</p> <pre><code>def twice: (=> Any) => Unit = repeat(2) twice { println("Hello, world") } // ... you get the picture :-) </code></pre>

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