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POTwo ways of defining functions in Scala. What is the difference?
 

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    <p>Here is a little Scala session that defines and tries out some functions:</p> <pre><code>scala&gt; def test1(str: String) = str + str; test1: (str: String)java.lang.String scala&gt; test1("ab") res0: java.lang.String = abab </code></pre> <p>works nicely.</p> <pre><code>scala&gt; val test2 = test1 &lt;console&gt;:6: error: missing arguments for method test1 in object $iw; follow this method with `_' if you want to treat it as a partially applied function val test2 = test1 ^ </code></pre> <p>oops. </p> <pre><code>scala&gt; val test2 = test1 _ test2: (String) =&gt; java.lang.String = &lt;function1&gt; scala&gt; test2("ab") res1: java.lang.String = abab </code></pre> <p>works well!</p> <p>Now, I've seen the <code>_</code> syntax when folding (<code>_ + _</code>, etc). So as I understand it <code>_</code> basically means "an argument". So <code>test1 _</code> basically means a function with an argument, which is given to <code>test1</code>". But why isn't that <em>exactly</em> the same as just <code>test1</code>? Why is there a difference if I append a <code>_</code>?</p> <p>So I kept exploring...</p> <pre><code>scala&gt; val test3 = (str: String) =&gt; str + str test3: (String) =&gt; java.lang.String = &lt;function1&gt; scala&gt; test3("ab") res2: java.lang.String = abab scala&gt; val test4 = test3 test4: (String) =&gt; java.lang.String = &lt;function1&gt; </code></pre> <p>Here it works without <code>_</code>! What's the difference between a <code>def</code>ed function, and a <code>val</code>ed function?</p>
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    1. CORex, I'm going to save you some time. You already answered this on the mailing list on 1/27/2010. It was a good response so I bookmarked it. http://scala-programming-language.1934581.n4.nabble.com/val-def-function-method-tp1956948p1956955.html
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    2. CO@Bradford - Heh, thanks :)
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    3. COI think there is some unfortunate confusion regarding the usage of the term "function" / "defining a function" in Scala. Even *Programming in Scala* says that `def` defines a function, but that's not quite true depending on your definition of "function". I.e. a method returns a result that is a *function of* its parameters, but the method is not a *function value* or *function object*. It would probably be helpful to differentiate between these two usages of the term "function".
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