Moxy's Playroom

Ruby Braces and other ambiguities

Ok, ruby. You’re confusing me a bit with your syntax. Sometimes I can’t tell when I need a space or brace or carriage return. For example:

puts {:a=>1, :b=>2}

is an error. This got me at first until I realized what was going on. The curly brace does double duty for associative arrays and control blocks. In this case it’s ambiguous since the puts method can take an optional control block. If you want to pass the array directly to puts, you have to be explicit:

puts ( {:a=>1, :b=>2} )

OK. Fine. I understand what’s going on here. This next one I’m not sure about though. The ruby while loop seems to take a block, but it doesn’t act like a normal block. Here’s a normal while loop:

i=0; while i<3
    puts i=i+1
end

If you want to write it on one line you probably just use the inverted “perl” syntax:

i=0; puts i=i+1 while i<3

or you can just write it the same way as the normal syntax with semi-colons:

i=0; while i<3; puts i=i+1; end

or you can use a do-end instead of a semi-colon:

i=0; while i<3 do puts i=i+1; end

What’s going on here? I honestly didn’t know at first. It looks like a normal do-end control block. But I was pretty sure that while was not a normal method – it was a language construct. So this do-end is not actually a do-end block, any more than an if-then-end block uses a literal do-end block. For example you cannot substitute this with:

i=0; while i<3 { puts i=i+1; }

even though braces and do-end are supposed to be interchangeable. That’s because this is a special do, I think. The error message seems to indicate its specialness:

unexpected '{', expecting kDO_COND or ':' or '\n' or ';'

apparently this is a kDO_COND not a kDO. Yet another double-duty piece of syntax. Oh but look! According to the error message a colon also works!

i=0; while i<4: puts i=i+1 end

Of course this is also not the same colon as in a ternary operater although the follow 2 statements look kind of similar:

puts (if true: 1 else 0 end)
puts (true ? 1 : 0)

Probably best to avoid the if/while colon syntax. Does anyone use that?

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Published

11 March 2012

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