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    <p>I've digged into <a href="http://www.scala-lang.org/docu/files/ScalaReference.pdf" rel="nofollow">The Scala Language Specification</a> for Scala 2.9, and found out that part 6.23 describes how anonymous function have to be defined: </p> <pre><code>Expr ::= (Bindings | [‘implicit’] id | ‘_’) ‘=&gt;’ Expr ResultExpr ::= (Bindings | ([‘implicit’] id | ‘_’) ‘:’ CompoundType) ‘=&gt;’ Block Bindings ::= ‘(’ Binding {‘,’ Binding} ‘)’ Binding ::= (id | ‘_’) [‘:’ Type] </code></pre> <p>As you can see <code>Bindings</code> requires to be placed inside two surrounding parenthesis. So if your anonymous function defines the type of the parameter, it <strong>has</strong> to be defined in this way:</p> <pre><code>(c:Char) =&gt; 1 </code></pre> <p>And the call to map should look like that:</p> <pre><code>s.toList map((c:Char) =&gt; 1) </code></pre> <p>Also in the same part of reference documentation you can find:</p> <blockquote> <p>If an anonymous function (x: T) => e with a single typed parameter appears as the result expression of a block, it can be abbreviated to x: T => e.</p> </blockquote> <p>So it says that if anonymous function is defied as last expression in a code block, and has exacly one parameter, you can use abbreviated syntax to define anonymous function - without parenthesis. So, in your case, you can write <code>c:Char =&gt; 1</code> but only when you place it inside a code block <code>{c:Char =&gt; 1}</code>. And you can call map function this way:</p> <pre><code>s.toList map({c:Char =&gt; 1}) </code></pre> <p>or with abbreviated syntax without parenthesis:</p> <pre><code>s.toList map {c:Char =&gt; 1} </code></pre> <p>On the other hand, when we look back at the anonymous function specification:</p> <pre><code>Expr ::= (Bindings | [‘implicit’] id | ‘_’) ‘=&gt;’ Expr </code></pre> <p>We can see that we can define your anonymous function, if we don't want to specify argument type, without parameter binding in those two ways:</p> <pre><code>s.toList map (c =&gt; 1) // or s.toList map (_ =&gt; 1) </code></pre> <hr> <p>Also, this is summarized in changelog at the end of the documentation (Changes in Version 2.1.7 (19-Jul-2006)):</p> <p>Closure Syntax</p> <p>The syntax of closures has been slightly restricted (§6.23). The form</p> <pre><code>x: T =&gt; E </code></pre> <p>is valid only when enclosed in braces, i.e. <code>{ x: T =&gt; E }</code>. The following is illegal, because it might be read as the value x typed with the type T => E:</p> <pre><code>val f = x: T =&gt; E </code></pre> <p>Legal alternatives are:</p> <pre><code>val f = { x: T =&gt; E } val f = (x: T) =&gt; E </code></pre>
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