Re: [Haskell-beginners] Matrix and types

Hi, Thanks for the pointers. So I’ve got data M (n :: Nat) a = M [a] deriving Show t2 :: M 2 Int t2 = M [1,2] t3 :: M 3 Int t3 = M [1,2,3] fx :: Num a => M n a -> M n a -> M n a fx (M xs) (M ys) = M (zipWith (+) xs ys) and having g = fx t2 t3 won’t compile. Which is what I want. However… t2 :: M 2 Int t2 = M [1,2] is ‘hardwired’ to 2 and clearly I could make t2 return a list of any length. So what I then tried to look at was a general function that would take a list of Int and create the M type using the length of the supplied list. In other words if I supply a list, xs, of length n then I wan’t M n xs Like this createIntM xs = (M xs) :: M (length xs) Int which compile and has type λ-> :t createIntM createIntM :: [Int] -> M (length xs) Int and all Ms created using createIntM have the same type irrespective of the length of the supplied list. What’s the type jiggery I need or is this not the right way to go? Thanks Mike

On 14 Mar 2019, at 13:12, Frederic Cogny wrote:

The (experimental) Static module of hmatrix seems (I've used the packaged but not that module) to do exactly that: http://hackage.haskell.org/package/hmatrix-0.19.0.0/docs/Numeric-LinearAlgeb... http://hackage.haskell.org/package/hmatrix-0.19.0.0/docs/Numeric-LinearAlgeb...

On Thu, Mar 14, 2019, 12:37 PM Francesco Ariis mailto:fa-ml@ariis.it> wrote: Hello Mike,

On Thu, Mar 14, 2019 at 11:10:06AM +0000, mike h wrote:

...

Multiplication of two matrices is only defined when the the number of columns in the first matrix equals the number of rows in the second matrix. i.e. c1 == r2

So when writing the multiplication function I can check that c1 == r2 and do something. However what I really want to do, if possible, is to have the compiler catch the error.

Type-level literals [1] or any kind of similar trickery should help you with having matrices checked at compile-time.

[1] https://downloads.haskell.org/~ghc/7.10.1/docs/html/users_guide/type-level-l... https://downloads.haskell.org/~ghc/7.10.1/docs/html/users_guide/type-level-l... _______________________________________________ Beginners mailing list Beginners@haskell.org mailto:Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners -- Frederic Cogny +33 7 83 12 61 69 _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners