[bcc haskell, cc haskell-cafe] On 9/5/07, Tomi Owens <[EMAIL PROTECTED]> 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. > > 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 > -------------------------------------------------------------------------------- > > > Department for Education, Sport and Culture E Mail > This message is for the named person's use only. It may contain > confidential, proprietary or legally privileged information. No > confidentiality or privilege is waived or lost by any mistransmission. > If you receive this message in error, please immediately delete it and all > copies of it from your system, destroy any hard copies of it and notify the > sender. You must not, directly or indirectly, use, disclose, distribute, > print, or copy any part of this message if you are not the intended > recipient. The Department for Education, Sport and Culture and any of its > establishments each reserve the right to monitor all e-mail communications > through its networks. > > Any views expressed in this message are those of the individual sender, > except where the message states otherwise and the sender is authorised to > state them to be the views of any such entity. > > The Department for Education, Sport and Culture shall not be liable to > the recipient or any third party for any loss or damage, however it appears, > from this e-mail or its content. This includes loss or damage caused by > viruses. It is the responsibility of the recipient to ensure that the > opening of this message and its attachments shall not adversely affect > systems or data. > > > -------------------------------------------------------------------------------- > > _______________________________________________ > Haskell mailing list > [EMAIL PROTECTED] > http://www.haskell.org/mailman/listinfo/haskell > > -- Scott Williams
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