v1: Backported IsSet() from alpha branch.

[jeeswg]

Replaces #185. The commit is now stored in its own branch.

IsSet

IsSet for AHK v1.1.

For reference:
The v2 commit: Added IsSet(Var).
The v2 documentation: IsSet.

Btw, I noticed that in AHK v2, the Exception.What property contains the function name, but not so in AHK v1.1.
E.g. try the Hotstring function.

Test code

;==================================================

;test code: IsSet (AHK v1)

;==================================================

var := 123
MsgBox, % IsSet(notavar) ;0
MsgBox, % IsSet(var) ;1

;==================================================

Future functions

I intend to post (all AHK v1.1):
StrCount.
StrRept. (By repeated doubling, then the remainder.)
StrJoin. (Accepts an array for an alternating join string.)
Range. (Returns an array of int64s/doubles.) (Performance with a for loop was fine for 100,000 items.)
Base64Get/Base64Put and HexGet/HexPut. (The code, as is.)