Inline conditions in Lua (a == b ? "yes" : "no")?

Sure:

print("blah: " .. (a and "blah" or "nahblah"))

If the a and t or f doesn't work for you, you can always just create a function:

function ternary ( cond , T , F )
    if cond then return T else return F end
end

print("blah: " .. ternary(a == true ,"blah" ,"nahblah"))

of course, then you have the draw back that T and F are always evaluated.... to get around that you need to provide functions to your ternary function, and that can get unwieldy:

function ternary ( cond , T , F , ...)
    if cond then return T(...) else return F(...) end
end

print("blah: " .. ternary(a == true ,function() return "blah" end ,function() return "nahblah" end))

Although this question is fairly very old, I thought it would be fair to suggest another alternative that syntactically appears very similar to that of the ternary operator.

Add this:

function register(...)
    local args = {...}
    for i = 1, select('#', ...) do
        debug.setmetatable(args[i], {
            __call = function(condition, valueOnTrue, valueOnFalse)
                if condition then
                    return valueOnTrue
                else
                    return valueOnFalse
                end
            end
        })
    end
end

-- Register the required types (nil, boolean, number, string)
register(nil, true, 0, '')

And then use it like this:

print((true)  (false, true)) -- Prints 'false'
print((false) (false, true)) -- Prints 'true'
print((nil)   (true, false)) -- Prints 'false'
print((0)     (true, false)) -- Prints 'true'
print(('')    (true, false)) -- Prints 'true'

Note: For tables, however, you cannot use them directly with the above method. This is because each and every table has it's own independent metatable and Lua does not allow you to modify all tables at once.

In our case, an easy solution would be to convert the table into a boolean using the not not trick:

print((not not {}) (true, false)) -- Prints 'true'