[Haskell-beginners] How best to do this?

[Brent Yorgey]

But then \x y -> [x .. y]  would have to have the type

  (Ord a, Enum a) => [a]

whereas now it just has the Enum constraint.  Either that or else the
notation would work differently for literals vs. expressions but that
would be just awful.

-Brent

On Mon, Apr 29, 2013 at 11:57:38AM -0400, David McBride wrote:
> I think it is because that syntax is desugared directly to enumFromTo
> x y and enumFromTo 3 0 is [].  Random things would probably blow up if
> you make that function work in reverse.  But I would love it if ghc
> just checked whether the first was lower than the second and swapped
> them as a convenience.
> 
> On Mon, Apr 29, 2013 at 11:44 AM, emacstheviking <objitsu at gmail.com> wrote:
> > ROFLCOPTER indeed Batman!
> >
> > I had no idea of that... I just *assumed* (usual rules apply I guess) that
> > [3..0] was the "opposite" of [0..3] but sure enough a wuicj ghci session
> > reveals the bitter truth!
> >
> > Thanks again... i can see that it's not just me that is too lazy at times. I
> > guess writing [3,2..0] will do for now but is that a bug or is there some
> > other reasoning behind it?
> >
> > We live and learn, well, I live anyway...
> >
> > :)
> >
> >
> >
> > On 29 April 2013 16:37, David McBride <toad3k at gmail.com> wrote:
> >>
> >> One other gotcha.  I don't know why it is this way, but [3..0]
> >> evaluates to [].  I have no idea why reverse notation is not allowed.
> >> But you can just manually reverse it or you can go [3,2..0].
> >>
> >> On Mon, Apr 29, 2013 at 11:26 AM, emacstheviking <objitsu at gmail.com>
> >> wrote:
> >> > 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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