I don’t understand how float would be an implementation detail and not the the thing you are trying to operate on. If a programmer uses a float they are most certainly wanting to use a float?
Because many programmers don't understand floats, even ones that have been doing it for years. Not to mention that higherlevel languages are being used by non programmers to script things out. I mean I recently helped oversee a class and heard someone tell people new to programming that floats are basically decimals. I sounded like a pedantic jerk interrupting to explain the difference, and I'm sure none of them remembered.
But more to the point of your question, we use floating point math because that's what computers are good at, not because we want that for it's own sake. We want to figure out sales tax, or how long until our kid will need to buy new shoes, or what effect changing the speed limit had on the total number of accidents, or all kinds of other things that humans care about. Using a floating point representation may be the most efficient way to get some of those answers using the technology available, but it is just a step along the way, not what we actually want. That's what I mean by implementation detail.
Basically, I think that any literals typed into the interpreter should work the same way as a calculator. If you want something special for your implementation because it will work better or faster, then that should be explicit.
If you want to compute sales tax, you’re going to have to define a rounding behavior, too. A decimal type wouldn’t save you from this. The desired rounding behavior will vary by situation.
How your calculator handles rounding is itself an implementation detail. I have no idea what rules your calculator uses to round and it’s not in any standard. Does my calculator do the same thing? Unknowable.
I've always wondered if you use "round-toward-even" rules, or round-down rules when the fractional part of the sales tax amount is exactly 1/2 a cent. But have never wondered hard enough to actually find out.
The programmer _wants_ to operate on a real number; the mental/abstract model of whatever application they’re building almost certainly involves real numbers instead of floats.
It’s the conversation from an abstract model to a concrete instantiation where floats are used, generally out of necessity or ignorance.
The fewer details needed to do this conversion, the easier it is to develop programs. When I say easier, I mean it’s faster AND less buggy — since the conversation often involves introduces errors, subtleties, and logic not present in the abstract model.
