Add float (or finite) type

[davetapley]

Discussed in #1959

^{Originally posted by davetapley September 15, 2022}
Since we already have an integer type:

mobx-state-tree/packages/mobx-state-tree/src/types/primitives.ts

Lines 113 to 117 in 6a50d79

	export const integer: ISimpleType<number> = new CoreType<number, number, number>(
	"integer",
	TypeFlags.Integer,
	(v) => isInteger(v)
	)

Do we want one or both of these?

1. A float type, which would restrict against NaN.
2. A finite type, which would restrict against NaN and infinities (taking inspiration from isFinite).

I'll concede that both could be implemented with refinement, but so could integer and we already have that.

This would have saved me from a bug where I am using parseFloat and a NaN was sneaking in to my model.
It would be great if I could just specify float in my model and have MST throw, instead of having to check for NaN manually.

---

Happy to make a PR if it will be accepted 🤞🏻

[coolsoftwaretyler]

Hey @davetapley - I know it's been a while. I can't answer whether or not a PR would be accepted, but I agree this makes sense to do. I can ask around the maintainers and try to get a sense of what they're thinking in terms of adding functionality like this. Are you still interested in writing a PR for it? If not, no worries, I know it's been quite a while since you brought this up. I just wanted you to at least get an acknowledgment here, albeit belated.

[coolsoftwaretyler]

For folks who aren't following the discussion, we are open for PRs to add new types! Here's the thread: #1959

I'll leave this open for Dave to attempt a PR if they're still interested. But if not, I'll take this on myself and close out the issue once it's all set.