This is probably a haskell-beginners sort of question, but I usually get about 4x as many responses from cafe, about 10x as fast.
I have code like so:
code:
--------
data Xy a = Xy a a
class Coord2 a where
coords2 :: Fractional b => a b -> Xy b
data CircAppr a b = CircAppr a b b -- number of points, rotation angle,
radius
deriving (Show)
instance Integral a => Coord2 (CircAppr a) where
coords2 (CircAppr divns ang rad) =
let dAng = 2 * pi / (fromIntegral divns) in
let angles = map (* dAng) [0..divns] in
undefined -- To be coded...
--------
In the instance definition divns is an integral trying to divide a
fractional. I hoped wrapping it in fromIntegral would coerce, but
apparently not:
quote:
--------
Could not deduce (Fractional a) arising from a use of `/'
from the context (Integral a)
bound by the instance declaration
at /scratch/cmhoward/pulse-spin/pulse-spin.hs:34:10-42
or from (Fractional b)
bound by the type signature for
coords2 :: Fractional b => CircAppr a b -> Xy b
at /scratch/cmhoward/pulse-spin/pulse-spin.hs:(36,3)-(39,15)
Possible fix:
add (Fractional a) to the context of
the type signature for
coords2 :: Fractional b => CircAppr a b -> Xy b
or the instance declaration
In the expression: 2 * pi / (fromIntegral divns)
In an equation for `dAng': dAng = 2 * pi / (fromIntegral divns)
In the expression:
let dAng = 2 * pi / (fromIntegral divns) in
let angles = map (* dAng) [0 .. divns] in undefined
--------
So, I'm wondering how I can do what I'm trying to do here, while still
keeping my types as generic as possible.
--
frigidcode.com
signature.asc
Description: OpenPGP digital signature
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
