Hi all,
I just started working with Haskell and am having some difficulties with its type system. Here is function that is supposed to calculate the distance between two coordinates: distBetween (x1,y1) (x2,y2) = sqrt((x2-x1)^2 + (y2-y1)^2) I am trying to explictly give it a type signature. Here is what I have tried and the errors generated by Hugs: type Point = (Int,Int) distBetween :: Point -> Point -> Float >>ERROR - Type error in explicitly typed binding *** Term : distBetween *** Type : Point -> Point -> Int *** Does not match : Point -> Point -> Float distBetween :: Point -> Point -> Int >>Instance of Floating Int required for definition of distBetween Any help is appreciated... Deech _______________________________________________ Haskell-Cafe mailing list [hidden email] http://www.haskell.org/mailman/listinfo/haskell-cafe |
One thing to try:
type Point = (Float, Float) The problem is that x1, y1, x2, and y2 are Ints, and so (x2-x1)^2 + (y2-y1)^2 is also an Int, but then you try to take the square root, which is an operation only available on floating point values (more specifically, types in the class Floating, which also includes things like Complex Double). Another option is to leave the Point type as-is, but do a conversion just before applying the sqrt, say, by applying the fromIntegral function, which serves to convert from types in the class Integral, like Int and Integer to any numeric type you need. distBetween (x1,y1) (x2,y2) = sqrt . fromIntegral $ (x2-x1)^2 + (y2-y1)^2 hope this helps, - Cale On 27/04/06, Aditya Siram <[hidden email]> wrote: > Hi all, > I just started working with Haskell and am having some difficulties with its > type system. > Here is function that is supposed to calculate the distance between two > coordinates: > distBetween (x1,y1) (x2,y2) = sqrt((x2-x1)^2 + (y2-y1)^2) > > I am trying to explictly give it a type signature. Here is what I have tried > and the errors generated by Hugs: > > type Point = (Int,Int) > distBetween :: Point -> Point -> Float > >>ERROR - Type error in explicitly typed binding > *** Term : distBetween > *** Type : Point -> Point -> Int > *** Does not match : Point -> Point -> Float > > distBetween :: Point -> Point -> Int > >>Instance of Floating Int required for definition of distBetween > > Any help is appreciated... > Deech > > > _______________________________________________ > Haskell-Cafe mailing list > [hidden email] > http://www.haskell.org/mailman/listinfo/haskell-cafe > Haskell-Cafe mailing list [hidden email] http://www.haskell.org/mailman/listinfo/haskell-cafe |
In reply to this post by Aditya Siram
It looks like somewhere else in your program (or a type signature
somewhere) is trying to force the result of sqrt to be an Int which won't work since square roots are irrational (represented by the computer as a Float or Double). You might try (1) making sure the place where distBetween is used isn't trying to use only Ints or Integers and (2) taking off explicit type signatures and seeing if that works (the compiler can usually get things working for you as long as you don't give it misinformation in type signatures). Also, a more useful type of Point if you are taking distances would be type Point = (Float,Float) instead (or you can leave that out because the function type for distBetween will be inferred to be the "right thing", i.e. (Float,Float) is already a valid type without needing to be named). Hope that helps, Jared. -- http://www.updike.org/~jared/ reverse ")-:" On 4/27/06, Aditya Siram <[hidden email]> wrote: > Hi all, > I just started working with Haskell and am having some difficulties with its > type system. > Here is function that is supposed to calculate the distance between two > coordinates: > distBetween (x1,y1) (x2,y2) = sqrt((x2-x1)^2 + (y2-y1)^2) > > I am trying to explictly give it a type signature. Here is what I have tried > and the errors generated by Hugs: > > type Point = (Int,Int) > distBetween :: Point -> Point -> Float > >>ERROR - Type error in explicitly typed binding > *** Term : distBetween > *** Type : Point -> Point -> Int > *** Does not match : Point -> Point -> Float > > distBetween :: Point -> Point -> Int > >>Instance of Floating Int required for definition of distBetween > > Any help is appreciated... > Deech > > > _______________________________________________ > Haskell-Cafe mailing list > [hidden email] > http://www.haskell.org/mailman/listinfo/haskell-cafe > Haskell-Cafe mailing list [hidden email] http://www.haskell.org/mailman/listinfo/haskell-cafe |
In reply to this post by Aditya Siram
On 4/28/06, Aditya Siram <[hidden email]> wrote:
> type Point = (Int,Int) > distBetween :: Point -> Point -> Float > >>ERROR - Type error in explicitly typed binding > *** Term : distBetween > *** Type : Point -> Point -> Int > *** Does not match : Point -> Point -> Float > > distBetween :: Point -> Point -> Int > >>Instance of Floating Int required for definition of distBetween It's saying that you explicitly gave the function the type "Point -> Point -> Int" but that it actually has "Point -> Point -> Int". If you look at the type of sqrt: Prelude> :t sqrt sqrt :: (Floating a) => a -> a You'll see that it returns a floating-point number. (Also, a minor style point: you should probably get in the habit of putting a space between the "sqrt" and its arguments. It'll make more sense as you gain more experience.) _______________________________________________ Haskell-Cafe mailing list [hidden email] http://www.haskell.org/mailman/listinfo/haskell-cafe |
Thanks to all for their assistance....
By changing the output of distBetween to Float the function seemed to work. And using the fromIntegral I was able to get my program, which accepts only Ints, to work. I was a little antsy about using fromIntegral because it isn't covered in my text (Haskell:SOE) until later chapters. But I can find no other way Deech _______________________________________________ Haskell-Cafe mailing list [hidden email] http://www.haskell.org/mailman/listinfo/haskell-cafe |
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