[Haskell-cafe] Typing problems with basic arithmetic - help!

[robert dockins]

For the version with type signatures, you are trying to divide integers 
using the (/) function.  This function expects values in the class 
Fractional, and Integer isn't a member.  Replace with div, which does 
integer division.

pnorep days n = (numeratorex days n) `div` (denominatorex days n)


Marcin Tustin wrote:
>     I'm trying to do some basic arithmetic code (in fact it calculates the probability of a birthday clash given year length and group size), and GHC and Hugs are both not liking the code:
> 
> [marcin at ah-m2b2 ~]$ ghci burthday.hs
>    ___         ___ _
>   / _ \ /\  /\/ __(_)
>  / /_\// /_/ / /  | |      GHC Interactive, version 6.4, for Haskell 98.
> / /_\\/ __  / /___| |      http://www.haskell.org/ghc/
> \____/\/ /_/\____/|_|      Type :? for help.
> 
> Loading package base-1.0 ... linking ... done.
> Compiling Main             ( burthday.hs, interpreted )
> 
> burthday.hs:14:37:
>     No instance for (Fractional Integer)
>       arising from use of `/' at burthday.hs:14:37
>     Probable fix: add an instance declaration for (Fractional Integer)
>     In the definition of `pnorep':
>         pnorep days n = (numeratorex days n) / (denominatorex days n)
> Failed, modules loaded: none.
> Prelude> :reload
> Compiling Main             ( burthday.hs, interpreted )
> Ok, modules loaded: Main.
> *Main> pnorep 365 30
> 
> <interactive>:1:0:
>     Ambiguous type variable `a' in the constraints:
>       `Fractional a' arising from use of `pnorep' at <interactive>:1:0-5
>       `Integral a' arising from use of `pnorep' at <interactive>:1:0-5
>     Probable fix: add a type signature that fixes these type variable(s)
> *Main> 
> 
> The first time is with the type annotations, the second without:
> 
> import Ratio
> import Numeric
> 
> numeratorex days n = (numdays days n) - (subtractex days n) -- :: Rational 
> numdays days n = days * n
> --numdays :: Integer -> Integer -> Integer
> subtractex days n = (days * (days - 1)) / 2 --:: Rational
> denominatorex days n = (days^n) 
> -- denominatorex :: Integer -> Integer -> Integer
> pnorep days n = (numeratorex days n) / (denominatorex days n)
> 
> Help!
> 
> Thanks!
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