Instant is monotonically nondecreasing

[sfackler]

We don't want to guarantee that Instant::now() != Instant::now() is
always true since that depends on the speed of the processor and the
resolution of the clock.

[rust-highfive]

r? @brson

(rust_highfive has picked a reviewer for you, use r? to override)

[kennytm]

Mathematically speaking, "monotonically increasing" already means x ≥ y ⇒ f(x) ≥ f(y), what you are thinking is "strictly increasing".

[scalexm]

@kennytm actually "strictly increasing" is:
∀x∀y (x ≥ y ⇔ f(x) ≥ f(y))

"Monotonically increasing" (or "nondecreasing") however is:
∀x∀y (x ≥ y ⇒ f(x) ≥ f(y))
(no reciprocal, take for a counterexample a constant function, which is nondecreasing)

[kennytm]

@scalexm Thanks, updated the arrow.

[sfackler]

Ah right - I think only the second fix is needed then.

[sfackler]

Er, actually, I think the first change is still correct - Wolfram defines "monotonic function" as "A monotonic function is a function which is either entirely nonincreasing or nondecreasing." (http://mathworld.wolfram.com/MonotonicFunction.html), so Instant is monotonic, but not monotonically increasing, which is defined as "Always increasing; never remaining constant or decreasing." (http://mathworld.wolfram.com/MonotoneIncreasing.html)

[scalexm]

@sfackler From my understanding of english terminology, "monotonically increasing" or simply "increasing" usually means "strictly increasing", but sometimes it can just mean "nondecreasing", depending on authors and contexts. So I think you should indeed stick with "monotonically nondecreasing" which is unambiguous.

But anyway the second fix is the most important since "greater" indeed means "strictly greater" with standard terminology.

[scalexm]

"be no less" ?

[sfackler]

Fixed, thanks

[carols10cents]

r? @alexcrichton

[alexcrichton]

@bors: r+ rollup

[bors]

📌 Commit 97d046a has been approved by alexcrichton