scala sys.process_ illegal start of simple expression error

Question

I got an error "illegal start of simple expression" in scala while trying to do this:

def test() = {
    val h = "ls"!
    if (h != 0)
        println("error")

}

this is the error

[error]         if (h != 0)
[error] one error found
[error] (compile:compileIncremental) Compilation failed

Can anyone tell me what I did wrong?

yǝsʞǝla · Accepted Answer · 2016-04-08T06:23:53.623

1

Scala compiler is getting confused here because you are calling ! as a postfix operator. It can't figure out which version to use and where to place the implied semicolon. You can either add a semicolon like this:

def test() = {
    val h = "ls"!;
    if (h != 0)
        println("error")
}

or call it as a method:

def test() = {
    val h = "ls".!
    if (h != 0)
        println("error")
}

or add a new line:

def test() = {
    val h = "ls"!

    if (h != 0)
        println("error")
}

edited Apr 08 '16 at 06:23

answered Apr 08 '16 at 04:16

yǝsʞǝla

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Or add blank line. Note that it is syntax, independent of the overload. – som-snytt Apr 08 '16 at 04:22
@som-snytt indeed. I didn't want to suggest a new line approach, but I added it for completeness. – yǝsʞǝla Apr 08 '16 at 06:24

score 0 · Answer 2 · edited May 23 '17 at 12:24

Best practice to use a postfix operator is to use dot . operator. You should invoke it like val h = "ls".!

It makes code less ambiguous as semicolon is optional in Scala and compiler might treat it as an infix notation.

From Scala doc:

This style is unsafe, and should not be used. Since semicolons are optional, the compiler will attempt to treat it as an infix method if it can, potentially taking a term from the next line.

For full reference see this link: Suffix Notation. This post may also be helpful Scala's "postfix ops".

scala sys.process_ illegal start of simple expression error

2 Answers2