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