This question already has an answer here:
Coming from a background in Java and C# I am very pleased with the brevity of Golang's ability to use the shortcut method for variable declaration for private variables within functions, which allows me to write:
x := 1.5
It reminds me of duck typing in a dynamic language such as Python. However, in declaring global variables outside of the scope of a function you still need to use the more verbose syntax of:
var x float64 = 1.5
I'm just wondering why the shortcut method works for private variables and not globals? I know that the designers of the language are quite experienced so I'm assuming that this isn't reflective of a feature overlooked. Is there a technical reason why this kind of type inference (and I realize that the := shortcut is not the same as proper type inference) wouldn't work at the global scope? It just seems somewhat inconsistent in terms of the design, and as an inexperienced Gopher I must admit to being thrown off by this on a couple of occasions. On the whole however I'm really enjoying Go.
</div>
See the Ian's answer in this thread: https://groups.google.com/forum/#!msg/golang-nuts/qTZemuGDV6o/IyCwXPJsUFIJ
At the top level, every declaration begins with a keyword. This simplifies parsing.
Actually you don't need to specify type in many cases.
var x = 1.5
should work fine. It's probably as short as it can get and it's not much longer than local variable shortcut. So there is a shortcut for global.
As to why := can't be used, I would guess that calling out var makes code structure more consistent as other global constructs start with a keyword - func, const, import, type.