Way to monitor the time elapsed throughout an arbitrary evaluation?

Question

I'd like to have some way to view the time elapsed since the beginning of a long evaluation, viewable while the computation is still evaluating.

My first thought was to use Monitor, but it seems to require a variable's value to update before updating the display. That is, I can't just provide it an expression I'd like re-evaluated 'continuously' throughout the computation:

With[{start=Now},
    Monitor[ 
        Table[Length[FactorInteger[2^n - 1]], {n, 50, 450, 50}],
        DateDifference[start, Now, "Second"]
    ]
]

(Example computation slightly modified from the first example in the docs for Monitor.) During evaluation, the Monitor cell never updates from its initial value -- see above.

Fine -- looks like I was expecting Monitor to do something it doesn't. One would expect it to be difficult for such a function to determine how often to re-evaluate the expression to be monitored anyway.

Then the question becomes whether there's a way to do what I wanted Monitor to do -- evaluate an expression 'continuously' and display its updated value.

Of course, I don't really want this function updated continuously -- just every second or 10th of a second or so, ideally with a configurable duration. Obviously I don't want this monitoring to slow down my computation in any non-neglible sense.

Note in particular that I do not wish to modify the expression being monitored to provide any sort of report on its progress, or anything like that. I want this method to track the evaluation of an arbitrary expression.

Answer 1

Maybe you can use a Dynamic with an UpdateInterval:

SetAttributes[timed,HoldAll]
timed[expr_] := With[{start = AbsoluteTime[]},
    PrintTemporary @ Dynamic[AbsoluteTime[]-start, UpdateInterval->.1];
    expr
]

Example: