[m-rev.] time additions (was Re: for review: Add random.init/3)

[Peter Wang]

On Wed, 25 May 2016 16:28:32 +1000, Peter Wang <novalazy at gmail.com> wrote:
> On Wed, 25 May 2016 15:55:50 +1000, Paul Bone <paul at bone.id.au> wrote:
> > For review by anyone
> > 
> > ----
> > 
> > Add random.init/3
> > 
> > random.init/3 makes it easier to seed the random number generator.  It will
> > calculate a seed based on the current time and, if known, the process ID.
> > This is sufficient for simple applications but shouldn't be used for
> > security-sensitive applications.

A common way to seed the PRNG in C is

    srand(time(0));

The time in seconds-since-the-Unix-epoch is useful for many other
purposes, in addition to seeding any PRNG implementation.

I would like to see an easy way to get an integer (simple)
representation of Unix time, as well as a way to convert between an
integer representation and the time_t values.

The main problem is probably that a signed 32-bit int cannot represent
Unix times past some time in 2038.  Apart from 32-bit machines, this
problem also affects C# and Java backends.

difftime/2 returns a float.  You can get the current unix time as a
float with

    time(Now, !IO),
    Secs = difftime(Now, unix_epoch)

where `unix_epoch' returns a time_t.

Using floats is not very satisfying but in most cases they will be
double precision, and capable of representing any reasonable time
value precisely.

Here are some suggested additions to time.m

    :- func unix_epoch = time_t.

    :- pred seconds_since_unix_epoch(float::out, io::di, io::uo) is det.

    :- func seconds_since_unix_epoch(time_t) = float.

    :- func from_seconds_since_unix_epoch(float) = time_t.

Thoughts?

Peter