[Haskell-cafe] Define variable types

[Roman Cheplyaka]

* Tsunkiet Man <temp.tsun at gmail.com> [2009-02-05 12:37:22+0100]
> Hello,
> 
> I'm new to Haskell and it seems like a very nice language to learn. However
> I'm not really familiar with the errormessages it produces. I am using a
> Helium interpreter. I've created the following module (however it is just a
> small sketch). I've written the following code:
> 
> fac :: Int -> Int
> fac n = product[1..n]
> 
> boven :: Int -> Int -> Int
> boven n k = (fac n) `div` fac k * fac (n-k)
> 
> bin :: Int -> Int -> Int -> Int
> bin n k p   |(n-k)>0 && k>0 = (boven n k) * (p^k) * (1-p)^(n-k)
>             |(n-k)<0 && k>0 = (boven n k) * (p^k) * (1/((1-p)^(-(n-k))))
>             |(n-k)<0 && k<0 = (boven n k) * (1/(p^(-k))) *
> (1/((1-p)^(-(n-k))))
> 
> When I load this into my interpreter it says:
> 
> Compiling ./Test.hs
> (11,55): Type error in infix application
>  expression       : 1 / ((1 - p) ^ (-(n - k)))
>  operator         : /
>    type           : Float -> Float -> Float
>    does not match : Int   -> Int   -> Int
> (12,47): Type error in infix application
>  expression       : 1 / (p ^ (-k))
>  operator         : /
>    type           : Float -> Float -> Float
>    does not match : Int   -> Int   -> a
> (12,62): Type error in infix application
>  expression       : 1 / ((1 - p) ^ (-(n - k)))
>  operator         : /
>    type           : Float -> Float -> Float
>    does not match : Int   -> Int   -> a

You can't use fractional division (/) with integers. You can convert
integers to fractions using fromIntegral. E.g.:

    1 / fromIntegral ((1 - p) ^ (-(n - k)))

(literals like 1 doesn't need to be converted because they are
polymorphic)

-- 
Roman I. Cheplyaka :: http://ro-che.info/
"Don't let school get in the way of your education." - Mark Twain