[Haskell-beginners] Haskell typing question

[Cui Liqiang]

I got it!  

The msg “ No instance for (Fractional Int) arising from a use of `/‘ “ actually has no business with the operands order at all !  

--  
Cui Liqiang


On Tuesday, October 28, 2014 at 8:00 PM, Cui Liqiang wrote:

> Thanks for your help, your suggestion works.
>  
> But I still don’t quite understand. In the following line:
> caluDecimal l = (foldr1 (\x acc -> acc / 10.0 + x) (map digitToInt l)),
> After applying digitToInt, the type of ‘x’ in the expression above is Int indeed, but Haskell consider the ’10.0’ to be a Int, is it?
>  
>  
>  
>  
> ----------------------------------------------------------------------------------------------------------------------------
> Hi,
> I am doing an exercise in Haskell, which is converting a string like ?$123.312? to double value. Below is my code:
>  
> module Main where
> import Data.Char
>  
> caluInt l = foldl1 (\acc x -> acc * 10 + x) (map digitToInt l)
> caluDecimal l = (foldr1 (\x acc -> acc / 10.0 + x) (map digitToInt l))
>  
> convert(x:xs) =
> let num = [e | e <- xs, e /= ',']
> intPart = takeWhile (/='.') num
> decimalPart = tail(dropWhile (/='.') num)
> in (caluInt intPart) + (caluDecimal decimalPart)
>  
>  
> And I got an error in this line: caluDecimal l = (foldr1 (\x acc -> acc / 10.0 + x) (map digitToInt l)),
> which says:
> No instance for (Fractional Int) arising from a use of `/'
> Possible fix: add an instance declaration for (Fractional Int)
> In the first argument of `(+)', namely `acc / 10.0'
> In the expression: acc / 10.0 + x
> In the first argument of `foldr1', namely
> `(\ x acc -> acc / 10.0 + x)'
>  
>  
> Why Haskell insists that 10.0 is a Int? How can I explicitly tell Haskell I want a Fractional?
> --
> Cui Liqiang
>  
>  
>  
> > Why Haskell insists that 10.0 is a Int? How can I explicitly tell Haskell
> > I want a Fractional?
> >  
>  
>  
> Because digitToInt means exactly what it says. If you want it to become
> something other than Int, apply fromIntegral to its result.
>  
>  


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/beginners/attachments/20141028/8b3c4146/attachment.html>