Scala's tuple unwrapping nuance

问题

I've noticed the following behavior in scala when trying to unwrap tuples into vals:

scala> val (A, B, C) = (1, 2, 3)
<console>:5: error: not found: value A
       val (A, B, C) = (1, 2, 3)
            ^
<console>:5: error: not found: value B
       val (A, B, C) = (1, 2, 3)
               ^
<console>:5: error: not found: value C
       val (A, B, C) = (1, 2, 3)
                  ^

scala> val (u, v, w) = (1, 2, 3)
u: Int = 1
v: Int = 2
w: Int = 3

Is that because scala's pattern matching mechanism automatically presumes that all identifiers starting with capitals within patterns are constants, or is that due to some other reason?

Thanks!

回答1:

Yes, and it gets worse:

val (i, j) : (Int, Int) = "Hello" -> "World"

The above will compile and fail at runtime with a ClassCastException. It is easy to forget that the (i, j) declaration is a pattern.

EDIT: for ziggystar, the Scala assignment rules state that in the statement:

val p = expr //or var

p can be either an identifier or a pattern (see section 15.7 of Programming in Scala, pp284). So for example, the following is valid:

val x :: y :: z :: rest = List(1, 2, 3, 4)

Taking this together with the fact that patterns are erased (i.e. parametric type information is unchecked) means that my original example will compile.

回答2:

From [scala] Question about naming conventions you can read

The initial capital letter has an advantage when pattern matching. Identifiers with an initial capital letter are considered to be values to match against instead of a variable to be bound.

回答3:

A workaround exists if you need to initialize a large number of constants and want to avoid writing val = for each or you are hitting the tuple size limit (22).

From section 4.1 of The Scala Language Speciﬁcation:

A value deﬁnition val x: T = e deﬁnes x as a name of the value that results from the evaluation of e. A value deﬁnition val p1, ..., pn = e is a shorthand for the sequence of value deﬁnitions val p1 = e; ...; val pn = e.

Per the specification, one can initialize a sequence of values all with names starting with capital letters by specifying an expression on the right-hand which returns each value in order.

val iter = Iterator(1, 2, 3)
val A, B, C = iter.next()

Another example:

val next = { var n = 0; () => { n = n + 1; n } }
val A, B, C, D, E, F, G, H = next()

In these trivial cases above this approach is not very useful. Below is a more useful example that initializes a constant for each of the 64 squares of the chessboard (see Square.scala#L31 for source):

val squareIter = squares.iterator
val A1, A2, A3, A4, A5, A6, A7, A8,
  B1, B2, B3, B4, B5, B6, B7, B8,
  C1, C2, C3, C4, C5, C6, C7, C8,
  D1, D2, D3, D4, D5, D6, D7, D8,
  E1, E2, E3, E4, E5, E6, E7, E8,
  F1, F2, F3, F4, F5, F6, F7, F8,
  G1, G2, G3, G4, G5, G6, G7, G8,
  H1, H2, H3, H4, H5, H6, H7, H8 = squareIter.next()

来源：https://stackoverflow.com/questions/2727612/scalas-tuple-unwrapping-nuance

标签

scala

design-patterns

tuples