Firstly, let me say that I'm very new to Haskell, so it may be that
I'm just going about things in a fundamentally wrong way.
I'm writing an application in Haskell which, if I were to write it in
C, would be an event-driven select-loop based program. It needs to
handle timeouts, and know when particular external (network) events
happen.
I was pleased to see that GHC has a library misc/Select which will
allow me to call select(2). This will allow me to build the
event-processing arrangements.
My first observation is that the timeout is represented as a TimeOut,
which is Maybe Int, where the integer is in microseconds. Ints only
have an advertised range of around +/-2^29, which corresponds to
something under 10 minutes. This is too short for my application. I
think that Select.TimeOut should be Maybe Integer - or, alternatively,
that a new type TimeVal or something should be created to represent
the full precision and range of struct timeval, and then TimeOut would
be Maybe TimeVal.
My second problem is that to know what timeout parameter to pass to
select I need to know what the current time is, to the precision of
the timeout for select. In a C program I would use gettimeofday,
which (effectively) returns a struct timeval.
GHC does provide access to gettimeofday in the form of the
getClockTime call in the std/Time library. It returns a ClockTime,
which, as the comments suggest, is advertised in the Library Report
only as an abstract type. In fact, the ClockTime is concrete in GHC
and is pretty much the TimeVal type I suggest above.
However, arithmetic operations on ClockTimes all take place after
conversion to and from civil time (y/m/d/h/m/s&c). I can't take
ClockTimes and simply (eg) add a number of us to calculate the next
event point, or subtract a pair of ClockTimes to get the core of a
TimeOut - without unpicking it myself and converting to an Integer,
anyway.
So, it all seems somewhat irregular. This clearly needs to be fixed,
and I think I can do it, but in order for my patch to be accepted I
think I should ask what the best way would be.
Questions include:
* Is it reasonable to consider the internal construction of a
ClockTime exposed ? (I presume not.)
* Should there be a conversion from ClockTime to Integer ? In what
units should the Integer be - us or ps ?
* How much is the interface in misc/Select.lhs cast in stone ? Would
it be reasonable to simply change hSelect to take an Integer, or a
ClockTime ?
Any and all advice, specific or general, appreciated.
Ian.
