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POMissing parameter type for generic scan
 

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  1. POMissing parameter type for generic scan
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    <p>I can't seem to understand why my program needs explicit parameter types for this case. Using the following method:</p> <pre><code>trait DistTraversableLike[+T, +Repr] extends DistTraversable[T] { self: Repr =&gt; ... def scan1lD[T1 &gt;: T, That](assOp: (T1, T1) =&gt; T1)(implicit bf: CanBuildFrom[Repr, T1, That]): That = { val b = bf(this) for (x &lt;- group.prefixSumSeq(part, assOp)) b += x b.result } } </code></pre> <p>I try to use it on a specific implementation like so:</p> <pre><code>val x:DistSeq[Int] = DistSeq((0 until worldSize): _*) val scan = x.scan1lD(_+_) </code></pre> <p>Which yields the following error message:</p> <pre><code>Multiple markers at this line - missing parameter type for expanded function ((x$1: &lt;error&gt;, x$2) =&gt; x$1.$plus{&lt;null&gt;}(x$2{&lt;null&gt;}){&lt;null&gt;}) {&lt;null&gt;} - missing parameter type for expanded function ((x$1, x$2) =&gt; x$1.$plus{&lt;null&gt;}(x$2{&lt;null&gt;}){&lt;null&gt;}){&lt;null&gt;} - missing parameter type for expanded function ((x$1: &lt;error&gt;, x$2) =&gt; x$1.$plus(x$2)) - missing parameter type for expanded function ((x$1, x$2) =&gt; x$1.$plus(x$2)) </code></pre> <p>group is a mixed trait using a method from FooParOps-trait (prototype implementation):</p> <pre><code> def prefixSumSeq[T](part: Seq[T], combiner: (T, T) =&gt; T): Seq[T] = { implicit val srlz = getSerializerFor(part) var result = part.head var msg = part.reduce(combiner) for (i &lt;- 0 until log2i(size)) { val partner = localRank ^ math.pow(2, i).toInt send(partner, msg) val number: T = rcv(partner) msg = combiner(msg, number) if (partner &lt; localRank) result = combiner(number, result) } return (part.tail).scanLeft(result)(combiner) } </code></pre> <p><code>part</code> is from <code>DistTraversable[+T]</code> and defined as <code>def part: Seq[T]</code></p> <p>I don't quite see why explicit parameters are needed here? Please tell me if more information is needed. The program is quite complex right now.</p>
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    1. USJørgen R
    1. USFelix
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    1. COWhat are the variables `group` and `part` in the for loop in `scan1lD`?
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      1. USEmil H
    2. COI added a more thorough explanation
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    3. COI tried simplifying your problem, but I end up with quite a few changes just to get something that compiles... Do you get the same compilation error if you are more explicit e.g. `x.scan1lD((x:Int,y:Int) => x + y)`? Could a problem be that it can't implicitly resolve the right `CanBuildFrom` since it doesn't know the type of `That`?
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      1. USEmil H
 

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