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    <p>There's no difference between a def'ed function and a val'ed function:</p> <pre><code>scala&gt; def test1 = (str: String) =&gt; str + str test1: (String) =&gt; java.lang.String scala&gt; val test2 = test1 test2: (String) =&gt; java.lang.String = &lt;function1&gt; scala&gt; val test3 = (str: String) =&gt; str + str test3: (String) =&gt; java.lang.String = &lt;function1&gt; scala&gt; val test4 = test2 test4: (String) =&gt; java.lang.String = &lt;function1&gt; </code></pre> <p>See? All of these are functions, which is indicated by the <code>X =&gt; Y</code> type they have.</p> <pre><code>scala&gt; def test5(str: String) = str + str test5: (str: String)java.lang.String </code></pre> <p>Do you see an <code>X =&gt; Y</code> type? If you do, go see an ophthalmologist, because there's none. The type here is <code>(X)Y</code>, commonly used to denote a <em>method</em>.</p> <p>Actually, <code>test1</code>, <code>test2</code>, <code>test3</code> and <code>test4</code> are all methods, which return functions. <code>test5</code> is a method which returns a <code>java.lang.String</code>. Also, <code>test1</code> through <code>test4</code> do not take parameters (only <code>test1</code> could, anyway), while <code>test5</code> does.</p> <p>So, the difference is pretty simple. In the first case, you tried to assign a method to a val, but did not fill in the parameters the method take. So it failed, until you added a trailing underscore, which meant <em>turn my method into a function</em>.</p> <p>In the second example you had a function, so you didn't need to do anything else.</p> <p>A method is not a function, and vice versa. A function is an object of one of the <code>FunctionN</code> classes. A method is a handle to some piece of code associated with an object.</p> <p>See various questions about methods vs functions on Stack Overflow.</p>
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    1. USDaniel C. Sobral
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    1. COIs `(X)Y` a type? Oh, and thanks for the new word.
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    2. COI love this answer. Some follow-up questions: **1)** *Do you see an X => Y type?* How about the second line? Is it not `X => Y` because it has `(` and `)` around `String`? (Otherwise I'd have to look for an oftamologist in the yellow-pages.) **2)** From your description it seems to me that functions are strictly more general than methods, and that I could use your `test1`-notation instead of `test5`-notation everywhere? What would the drawback be?
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    3. CO@Synesso Yes, no, kind of. It's REPL's representation of an hypothetical method type.
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