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POWhy Scala doesn't allow List.map _ and type signatures in REPL
 

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    <h1>Background</h1> <p>I recently attended a beginner Scala meetup and we were talking about the difference between methods and functions (also discussed in-depth <a href="https://stackoverflow.com/questions/2529184/difference-between-method-and-function-in-scala">here</a>).</p> <p>For example:</p> <pre><code>scala&gt; val one = 1 one: Int = 1 scala&gt; val addOne = (x: Int) =&gt; x + 1 addOne: Int =&gt; Int = &lt;function1&gt; </code></pre> <p>This demonstrates that vals can not only have an integer type, but can have a function type. We can see the type in the scala repl:</p> <pre><code>scala&gt; :type addOne Int =&gt; Int </code></pre> <p>We can also define methods in objects and classes:</p> <pre><code>scala&gt; object Foo { | def timesTwo(op: Int) = op * 2 | } defined module Foo </code></pre> <p>And while a method doesn't have a type (but rather is has a type signature), we can lift it into a function to see what it is:</p> <pre><code>scala&gt; :type Foo.timesTwo &lt;console&gt;:9: error: missing arguments for method timesTwo in object Foo; follow this method with `_' if you want to treat it as a partially applied function Foo.timesTwo ^ scala&gt; :type Foo.timesTwo _ Int =&gt; Int </code></pre> <p>So far, so good. We even talked about how functions are actually objects with an apply method and how we can de-syntactic sugarify expressions to show this:</p> <pre><code>scala&gt; Foo.timesTwo _ apply(4) res0: Int = 8 scala&gt; addOne.apply(3) res1: Int = 4 </code></pre> <p>To me, this is quite helpful in learning the language because I can internalize what the syntax is actually implying.</p> <h1>Problematic Example</h1> <p>We did, however, run into a situation that we could not identify. Take, for example, a list of strings. We can map functions over the values demonstrating basic Scala collections and functional programming stuff:</p> <pre><code>scala&gt; List(1,2,3).map(_*4) res2: List[Int] = List(4, 8, 12) </code></pre> <p>Ok, so what is the type of List(1,2,3).map()? I would expect we would do the same :type trick in the repl:</p> <pre><code>scala&gt; :type List(1,2,3).map _ &lt;console&gt;:8: error: Cannot construct a collection of type Nothing with elements of type Nothing based on a collection of type List[Int]. List(1,2,3).map _ ^ </code></pre> <p>From the API definition, I know the signature is:</p> <pre><code>def map[B](f: (A) ⇒ B): List[B] </code></pre> <p>But there is also a full signature:</p> <pre><code>def map[B, That](f: (A) ⇒ B)(implicit bf: CanBuildFrom[List[A], B, That]): That </code></pre> <h1>Question</h1> <p>So there are two things I don't quite understand:</p> <ul> <li>Why doesn't the normal function lift trick work with List.map? Is there a way to de-syntactic sugar the erroneous statement to demonstrate what is going on?</li> <li>If the reason that the method can't be lifted is due to the full signature "implicit", what exactly is going on there? </li> </ul> <p>Finally, is there a robust way to inspect both types and signatures from the REPL?</p>
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    1. COQuestion about the implicit type signature is explained here: http://stackoverflow.com/a/18533437/125901
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