Wed Sep 5 17:28:21 EDT 2007

```[bcc haskell, cc haskell-cafe]

On 9/5/07, Tomi Owens <t.owens at hautlieu.sch.je> wrote:
>
> Hi there. I'm a teacher of Maths and am working my way through the Euler
> Project problems for fun. I have mostly been using Basic, but have read up
> about Haskell and think it looks like a sensible way to solve many of the
> problems.
>
> OK, so I've downloaded GHCi and am trying to teach myself.
>
> So far I have done this:
>
>   ___         ___ _
>  / _ \ /\  /\/ __(_)
> / /_\// /_/ / /  | |      GHC Interactive, version 6.6.1, for Haskell 98.
> / /_\\/ __  / /___| |      http://www.haskell.org/ghc/
> \____/\/ /_/\____/|_|      Type :? for help.
>
> Prelude> let f (a,b) = a * floor (100000/b)
> Prelude> f(2,5)
> 40000

Here you can find out type ghci has inferred for this function.
> :t f
f :: (RealFrac b, Integral b1) => (b1, b) -> b1

This function works just as I want it to.
>
> Now I try creating a list:
>
> Prelude> [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a]
> [(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)]

Let's assign this to an intermediate variable so we can query it's type:

Prelude> let lst = [(a ^ 2 + b ^ 2, a) | a <- [1..4], b <- [1..4], a^2 + b^2
< 20, b <= a]
Prelude> lst
[(2,1),(5,2),(8,2),(10,3),(13,3),(18,3),(17,4)]
Prelude> :t lst
lst :: [(Integer, Integer)]

aha; here's the source of the type mismatch:
Prelude> :t floor
floor :: (RealFrac a, Integral b) => a -> b

Floor has to take a RealFrac. According to hoogle[1], we can use various
floating-point approximations (Float, Double, CFloat, etc) or we can use the
exact Rational type.

You can get your types to match by declaring your list to be of type
[(Rational, Rational)] either by explicitly typing one of the untyped
variables or the entire expression:
Prelude> let lst = [(a ^ 2 + b ^ 2, a) | (a::Rational) <- [1..4], b <-
[1..4], a^2 + b^2 < 20, b <= a]
Prelude> :t lst
lst :: [(Rational, Rational)]
Prelude> let lst :: [(Rational, Rational)] = [(a ^ 2 + b ^ 2, a) | a <-
[1..4], b <- [1..4], a^2 + b^2 < 20, b <= a]
Prelude> :t lst
lst :: [(Rational, Rational)]

and this works
> So now I try to apply the function to the list:
>
> Prelude> map (f) [(a2+b2,a)| a <- [1..4] , b<- [1..4], a2+b2<20, b<=a]
>
> and I get this result:
>
> <interactive>:1:5:
>    Ambiguous type variable `t' in the constraints:
>      `Integral t' arising from use of `f' at <interactive>:1:5
>      `RealFrac t' arising from use of `f' at <interactive>:1:5
>    Probable fix: add a type signature that fixes these type variable(s)

I'm sorry, but I don't quite get how to set the type signature and how it
> will apply to my function...
>
> Thanks,

Hope this helps

Tomi
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