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copied!<p>Quoting from Scala syntax rules:</p> <pre><code>varid ::= lower idrest idrest ::= {letter | digit} [‘_’ op] op ::= opchar {opchar} opchar ::= “all other characters in \u0020-007F and Unicode categories Sm, So except parentheses ([]) and periods” // 'a' and 'a_-' is valid, 'a(' is not. </code></pre> <p>Rule for Typed Pattern match says:</p> <pre><code>Pattern1 ::= varid ‘:’ TypePat | ‘_’ ‘:’ TypePat </code></pre> <p>So</p> <pre><code>def pr(list: Any) = list match { case a :String => // works case a_- :String => // works a_- is valid instance of 'varid' case a() :String => // does not work. case a(b) :List[Int] // does not work either! } </code></pre> <p>Hence:</p> <pre><code>case S(a): S[ta] is not valid syntax for pattern match. </code></pre> <p>However following is valid</p> <pre><code>case (s1 :S[a], s2: S[b]) </code></pre> <p>according to Tuple Pattern matching rule:</p> <pre><code>SimplePattern ::= ‘(’ [Patterns] ‘)’ Patterns ::= Pattern {‘,’ Patterns} </code></pre> <p>Apart from the two possible options that you have listed, you may also use:</p> <pre><code> case tup: (S[_], S[_]) => </code></pre> <p>Following seems to work:</p> <pre><code>trait T case class S[A](a: A, w: A => Int) extends T def makeTwo(t1: T, t2: T) = (t1, t2) match { case (S(a1, w1), S(a2, w2)) => val wNew = { tup:(Any, Any) => w1(tup._1) + w2(tup._2) } S((a1, a2), wNew) } def someString (s: String) = { s.length } val twoS = makeTwo(S("Hello", someString), S("World!", someString)) println(twoS.w(twoS.a)) // gives 11 </code></pre>

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