Fall-Through by Default for Switch Statements?

The switch statement has a long history, and most languages support it or something similar. In my experience, I found it to be very useful — both for conciseness, and also improving performance. With the recent release of Java 7, you can finally switch over strings.

In Whiley, the syntax for switch statements currently looks like this:

for token in modifiers: modifier = null switch token.id: case "PUBLIC": case "public": modifier = ACC_PUBLIC break case "FINAL": case "final": modifier = ACC_FINAL break ... default: throw SyntaxError("invalid modifier",token)

(This excerpt is based on my Java Assembler/Disassembler benchmark)

Now, I’m having something of a dilemma over this syntax: should I support fall-through case statements or not?

As you can see from above, my initial reaction was: yes. But, I’m starting to think this is a really bad idea and there are a few reasons:

Obviously, fall-through by default is dangerous as its easy to forget the break . I’ve already done this a few times! Generally, I think fall-through is the exception, not the rule. That is, most cases do require the break and, hence, fall-through by default causes additional verbosity. The overloaded break statement can be annoying when you actually want to break out of a loop.

Now, of course, I’m not the first to think along these lines (see e.g. [1][2][3]) and, in fact, I’m probably being a bit slow on the uptake here!

Several languages (e.g. Go, CoffeeScript, Pascal) do not support fall-through by default. For example, Go supports an explicit fallthrough statement, which might look something like this in Whiley:

for token in modifiers: modifier = null switch token.id: case "PUBLIC": fallthrough case "public": modifier = ACC_PUBLIC case "FINAL": fallthrough case "final": modifier = ACC_FINAL ... default: throw SyntaxError("invalid modifier",token)

Go is quite interesting as it provides multi-match case statements as well, which might look like something like this in Whiley:

for token in modifiers: modifier = null switch token.id: case "PUBLIC","public": modifier = ACC_PUBLIC case "FINAL","final": modifier = ACC_FINAL ... default: throw SyntaxError("invalid modifier",token)

I really like this syntax (which I think is originally from Pascal). In Pascal, you can provide ranges as well which certainly makes sense.

So, all things considered, I’m convinced it is a mistake to support fall-through by default in Whiley. Fortunately, it’s not too late for me to correct this!

The big question then is: do I support explicit fallthrough, multi-match cases or both?