On 13 Apr 2006 03:27:03 -0000, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote: > > Creighton Hogg wrote: > > > No instance for (MatrixProduct a (Vec b) c) > > arising from use of `<*>' at <interactive>:1:3-5 > > Probable fix: add an instance declaration for (MatrixProduct a > > (Vec b) c) > > In the definition of `it': it = 10 <*> (vector 10 ([0 .. 9])) > > Let us look at the instance > > class MatrixProduct a b c | a b -> c where > (<*>) :: a -> b -> c > instance (Num a) => MatrixProduct a (Vec a) (Vec a) where > > it defines what happens when multiplying a vector of some numeric type > 'a' by a value _of the same_ type. Let us now look at the error > message: > > (MatrixProduct a (Vec b) c) > > That is, when trying to compile your expression > 10 <*> (vector 10 ([0 .. 9])) > the typechecker went looking for (MatrixProduct a (Vec b) c) > where the value and the vector have different numeric types. There is > no instance for such a general case, hence the error. It is important > to remember that the typechecker first infers the most general type > for an expression, and then tries to resolve the constraints. In your > expression, > 10 <*> (vector 10 ([0 .. 9])) > we see constants 10, 10, 0, 9. Each constant has the type Num a => a. > Within the expression 0 .. 9, both 0 and 9 must be of the same type > (because [n .. m] is an abbreviation for enumFromThen n m, and > according > to the type of the latter > enumFromThen :: a -> a -> [a] > both arguments must be of the same type). > > But there is nothing that says that the first occurrence of 10 must be > of the same numeric type as the occurrence of 9. So, the most general > type assignment will be (Num a => a) for 10, and (Num b => b) for 9.
Thank you very much for the explanation: it makes alot of sense. So, if one does not want to for alot of type declarations into the code, which would be fairly awkward, is there a way to do this with fundeps or other type extensions that will be alot prettier or is any way of defining type classes going to run into the same problems? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
