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

Question

Background

I recently attended a beginner Scala meetup and we were talking about the difference between methods and functions (also discussed in-depth here).

For example:

scala> val one = 1
one: Int = 1

scala> val addOne = (x: Int) => x + 1
addOne: Int => Int = <function1>

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:

scala> :type addOne
Int => Int

We can also define methods in objects and classes:

scala> object Foo {
 |   def timesTwo(op: Int) = op * 2
 | }
defined module Foo

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:

scala> :type Foo.timesTwo
<console>: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> :type Foo.timesTwo _
Int => Int

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:

scala> Foo.timesTwo _ apply(4)
res0: Int = 8

scala> addOne.apply(3)
res1: Int = 4

To me, this is quite helpful in learning the language because I can internalize what the syntax is actually implying.

Problematic Example

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:

scala> List(1,2,3).map(_*4)
res2: List[Int] = List(4, 8, 12)

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:

scala> :type List(1,2,3).map _
<console>: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 _
                      ^

From the API definition, I know the signature is:

def map[B](f: (A) ⇒ B): List[B]

But there is also a full signature:

def map[B, That](f: (A) ⇒ B)(implicit bf: CanBuildFrom[List[A], B, That]): That

Question

So there are two things I don't quite understand:

• 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?
• If the reason that the method can't be lifted is due to the full signature "implicit", what exactly is going on there?

Finally, is there a robust way to inspect both types and signatures from the REPL?

Answer 1

The problem you've encountered has to do with the fact that, in Scala, functions are monomorphic, while methods can be polymorphic. As a result, the type parameters B and That must be known in order to create a function value for List.map.

The compiler attempts to infer the parameters but can't come up with anything sensible. If you supply parameters, you'll get a valid function type:

scala> List(1,2,3).map[Char, List[Char]] _
res0: (Int => Char) => List[Char] = <function1>

scala> :type res0
(Int => Char) => List[Char]