I did a little searching and more than once I've found that "will" is used as opposed to "going to" if there were no previous plan and the speaker just decided it. But if that's the case, how do you explain "I'm going to go to the bathroom." That's certainly not planned but you wouldn't suddenly say to someone "I'll go to the bathroom."
Another example. Say you and your friend have been reading in a room on a hot summer day. Being the hot day that it is you wonder if it's cooler outside so you say to your friend "I'm going to read outside." You wouldn't say "I'll read outside." right?
So what's going on here?
Another example. Say you and your friend have been reading in a room on a hot summer day. Being the hot day that it is you wonder if it's cooler outside so you say to your friend "I'm going to read outside." You wouldn't say "I'll read outside." right?
So what's going on here?