Error with floating point literal in Scala REPL

Question

The definiton for floating point literal in Scala is

floatingPointLiteral  ::=  digit {digit} ‘.’ digit {digit} [exponentPart] [floatType]
                        |  ‘.’ digit {digit} [exponentPart] [floatType]
                        |  digit {digit} exponentPart [floatType]
                        |  digit {digit} [exponentPart] floatType
exponentPart          ::=  (‘E’ | ‘e’) [‘+’ | ‘-’] digit {digit}
floatType             ::=  ‘F’ | ‘f’ | ‘D’ | ‘d’

When I try to input floating point literal that starts with a dot I get an error:

scala> .123
        ^
   error: ';' expected but double literal found.

If is assign such literal to some variable everything is fine

scala> val x = .123
x: Double = 0.123

Why it behaves like that?

Answer 1

Probably the quote you pasted is from the Syntax Summary.

Before we answer your question, let's notice the following:

scala> 1.213
res1: Double = 1.213

So the issue here is not the floating point, but the fact that the expression starts with a dot. It makes the console, just like a regular Scala program, to evaluate the expression on the last object calculated.

Let's do an example:

scala> val d = 1.123
d: Double = 1.123

scala> .equals(1.123)
res10: Boolean = true

scala> .equals(1.123)
res11: Boolean = false

scala> .toString
res12: String = false

As you can see, every new line, is a follow up of the previous. You can go back, for example:

scala> res10
res13: Boolean = true

But the object in cache is the last result.

In your example, probably the last successful run of your console was val x = .123, which put the last res as a Double. When you ran .123 you are starting another BlockStat. The lexer identifies it is a Block, and therefore expects a semi (which is a ;) between the different BlockStats.

When starting a new console, it works as expected:

scala> .123
res0: Double = 0.123