[Haskell-beginners] How best to do this?

[emacstheviking]

damn that lazy evaluation! LMAO  ...a good point brent and yuo have no
doubt saved me hours of head scratching this evening when I try out the
"new improved software". Oh dear oh dear oh dear...

doOption dev (Forward n)  = do
  putStrLn $ "> STEP FORWARD " ++ (show n)
  stepBits dev ioPORTA [3..0]

doOption dev (Backward n) = do
  putStrLn $ "> STEP BACKWARD " ++ (show n)
  stepBits dev ioPORTA [0..3]

stepBits dev port = mapM_ stepIt
  where stepIt bit = mapM_ (\s -> HW.setPortBit dev port bit s >>
stepDelay) [0,1]

I now have the above as my current "final" implementation... hopefully that
*does* do what I think it does because mapM_ is driving it and will cause
evaluation of the actions?



On 29 April 2013 15:56, Brent Yorgey <byorgey at seas.upenn.edu> wrote:

> On Mon, Apr 29, 2013 at 02:59:29PM +0100, emacstheviking wrote:
> > I have built a library for using the Hexwax expandIO-USB chip and I have
> > now got some code to drive a stepper motor:
> >
> > doOption :: HWHandle -> Flag -> IO ()
> > doOption dev (Backward n) = do
> >   putStrLn $ "> STEP BACKWARD " ++ (show n)
> >   let x = [ stepBit b | b <- [3..0]]
> >   return ()
> >     where
> >       stepBit p b = setBit p b 0 >> setBit p b 1
> >         where setBit p b s = HW.setPortBit dev p b s >> stepDelay
>
> The other posted solutions are good, but I also want to make a very
> important comment about the above code: it does not actually step any
> bits!  All it does is print some stuff.  x is simply a name for a list
> of IO actions; it is never used so it just gets garbage collected and
> the IO actions are never run.
>
> -Brent
>
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