[Haskell-cafe] Define variable types

[Roel van Dijk]

This is not really an answer to your question, but I think you could
write a slightly more efficient function to calculate the binomial
coefficient:

fac :: Integer -> Integer
fac n = product [1..n]

-- |Product of all positive integers less than or equal to n but
-- larger than s
facFrom :: Integer -> Integer -> Integer
facFrom s n | s > n     = if s == 1 then 1 else 0
            | otherwise = product [max (s + 1) 1 .. n]

boven :: Integer -> Integer -> Integer
boven n k = facFrom (n - k) n `div` fac k

This exploits the fact that "fac n" contains the computation for "fac (n - k)".

2009/2/5 Tsunkiet Man <temp.tsun at gmail.com>:
> 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
> Compilation failed with 3 errors
>
> Some details that might be usefull:
>
> Line 11 is |(n-k)<0 && k>0 = (boven n k) * (p^k) * (1/((1-p)^(-(n-k))))
> Line 12 is |(n-k)<0 && k<0 = (boven n k) * (1/(p^(-k))) *
> (1/((1-p)^(-(n-k))))
>
> So my question is: how can I fix these errors?
>
> (I used these lines of codes because it is not possible to use a negative
> exponent in the Helium interpreter.)
>
> Thank you for answering my question!
>
> Greetings JTKM
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