[Haskell-beginners] Cannot compile "where" in a function

[Trung Quang Nguyen]

It works perfectly when I use Fractional with (/) and Integral with (div).
Thanks a lot Brent :)
/Trung



2012/12/16 Brent Yorgey <byorgey at seas.upenn.edu>

> On Sun, Dec 16, 2012 at 06:08:20PM +0100, Trung Quang Nguyen wrote:
> > Hi there,
> >
> > I'm trying to implement Reverse Polish notation calculator from
> > learnyouhaskell.com. I want to add one more operator "/", but I cannot
> > compile. I comment out the line --func (x:y:ys) "/" = (y / x):ys, thing
> > goes well again. Any body knows the cause of this problem?
> >
> > --rpn.hs
> > import System.Environment
> > import Data.List
> >
> > solveRPN :: (Num a, Read a) => String -> a
> > solveRPN = head . foldl func [] . words
> >     where
> >         func (x:y:ys) "+" = (y + x):ys
> >         func (x:y:ys) "-" = (y - x):ys
> >         func (x:y:ys) "*" = (y * x):ys
> >         --func (x:y:ys) "/" = (y / x):ys
> >         func xs numStr = read numStr:xs
> >
> > main = do
> >     (x:_) <- getArgs
> >     putStrLn $ show $ solveRPN x
> >
> > I got this:
> > ~/w/r/s/haskell> ghc --make rpn
> > [1 of 1] Compiling Main             ( rpn.hs, rpn.o )
> >
> > rpn.hs:5:25:
> >     Could not deduce (Fractional a) arising from a use of `func'
> >     from the context (Num a, Read a)
>
> The problem is that you can only use the (/) operator on types which
> are instances of the Fractional type class, but you have stated that
> your function should work for any type as long as it is an instance of
> Num.  But being an instance of Num does not imply an instance of
> Fractional; your function would not work for any type which is an
> instance of Num but not of Fractional, such as Int.
>
> If you only want to use solveRPN with Fractional types like Double or
> Rational, you can just change the Num constraint to a Fractional
> constraint.  If you want solveRPN to work with non-fractional types
> like Int, then you have to decide what exactly division is supposed to
> mean.  Perhaps you want to use 'div' instead of (/) (in which case you
> have to use an Integral constraint instead of Num)?
>
> -Brent
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>



-- 
*Trung Nguyen*
Mobile: +45 50 11 10 63
LinkedIn: http://www.linkedin.com/pub/trung-nguyen/36/a44/187
View my blog at http://www.onextrabit.com/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/beginners/attachments/20121216/41a55312/attachment.htm>