Scala String Concatentation Problem

Question

This was a fairly hard bug to find in my code but once i found it i was surprised the compiler didn't catch it or understand why it was valid.

val my_string =
    "abc" +
    "def"
    "ghi"

The value of my_string ended up being "abcdef" since i missed the + sign after "def". Why didn't the compiler complain and what happened to "ghi"?

will now enclose multi-line strings in triple quotes to avoid future bugs — Mark Spangler
– Mark Spangler, Commented Nov 10, 2010 at 16:47
A common practice is to wrap all tokens of an expression inside parenthesis to avoid such issues, e.g., val my_string = ( ... ) — Alex Boisvert
– Alex Boisvert, Commented Nov 10, 2010 at 20:27

sepp2k · Accepted Answer · 2010-11-10 16:17:57Z

10

The code is valid because "ghi" is a valid expression on its own.

If this is inside a function (and not followed by anything else) then "ghi" is the return value of that function. Otherwise it's just ignored (like if you'd written 42 + 23 on a line on its own).

answered Nov 10, 2010 at 16:17

sepp2k

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3 Comments

Mark Spangler Over a year ago

it's being declared as an instance variable inside my class. I guess you wouldn't be able to reference it, it would just execute and move on?

sepp2k Over a year ago

@Mark: Any expression in the class body is considered part of the constructor. So every time an instance of your class is created the expressions "ghi" is evaluated (which doesn't have any effect at all).

Mark Spangler Over a year ago

that's right! i remember reading that now / i appreciate it, thanks

hbatista · Accepted Answer · 2010-11-10 16:18:06Z

1

"ghi" is just an expression of type String, why should the compiler complain?

answered Nov 10, 2010 at 16:18

hbatista

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2 Comments

yura Over a year ago

because in java it is not statment so it is logical to assume that compiler would fail if you came from java world

hbatista Over a year ago

@yura: yes, but this is not Java, and you should not make the mistake of looking at Scala as Java with some "extensions". I find that one of the most useful mind shifts if you're learning Scala, is to understand that everything is just an expression that evaluates to something.

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