[bcc haskell, cc haskell-cafe]
On 9/5/07, Tomi Owens
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.
Loading package base ... linking ... done. 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. [1] http://haskell.org/hoogle/?q=RealFrac 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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