[Haskell-cafe] Typing problems with basic arithmetic - help!
robert dockins
robdockins at fastmail.fm
Fri Sep 23 14:48:12 EDT 2005
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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