Hi,

The problem is that Haskell lists don't carry their length in their type hence you cannot enforce their length at compile time. But you can define your M this way instead:

{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE KindSignatures #-}

import GHC.TypeNats
import Data.Proxy

data M (n :: Nat) a where
   MNil  :: M 0 a
   MCons :: a -> M (n-1) a -> M n a

infixr 5 `MCons`

toList :: M k a -> [a]
toList MNil         = []
toList (MCons a as) = a:toList as

instance (KnownNat n, Show a) => Show (M n a) where
   show xs = mconcat
      [ "M @"
      , show (natVal (Proxy :: Proxy n))
      , " "
      , show (toList xs)
      ]

--t2 :: M 2 Integer
t2 = 1 `MCons` 2 `MCons` MNil

--t3 :: M 3 Integer
t3 = 1 `MCons` 2 `MCons` 3 `MCons` MNil

zipM :: (a -> b -> c) -> M n a -> M n b -> M n c
zipM _f MNil         MNil         = MNil
zipM  f (MCons a as) (MCons b bs) = MCons (f a b) (zipM f as bs)

fx :: Num a => M n a -> M n a -> M n a
fx = zipM (+)

Test:

> t2
M @2 [1,2]
> fx t2 t2
M @2 [2,4]
> fx t2 t3

<interactive>:38:7: error:
    • Couldn't match type ‘3’ with ‘2’
      Expected type: M 2 Integer
        Actual type: M 3 Integer

Cheers,
Sylvain

Hi Frederic,

Yeh, my explanation is a bit dubious :) 

What I’m trying to say is:

Looking at the type M (n::Nat) 

If I want an M 2  of Ints say,  then I need to write the function with signature

 f :: M 2 Int

 

If I want a M 3 then I need to explicitly write the function with signature

M 3 Int

and so on for every possible instance that I might want.

What I would like to do is have just one function that is somehow parameterised to create the M tagged with the required value of (n::Nat)

In pseudo Haskell

create :: Int -> [Int] -> M n 

create size ns = (M ns) ::  M size Int

where  its trying to coerce (M ns) into the type (M size Int) where size is an Int but needs to be a Nat.

It’s sort of like parameterising the signature of the function.

Thanks

Mike

On 15 Mar 2019, at 11:37, Frederic Cogny <frederic.cogny@gmail.com> wrote:

I'm not sure I understand your question Mike.
Are you saying createIntM behaves as desired but the data constructor M could let you build a data M with the wrong type. for instance M [1,2] :: M 1 Int ?

If that is your question, then one way to handle this is to have a separate module where you define the data type and the proper constructor (here M and createIntM) but where you do not expose the type constructor. so something like

module MyModule

  ( M -- as opposed to M(..) to not expose the type constructor

  , createIntM

  ) where

Then, outside of MyModule, you can not create an incorrect lentgh annotated list since the only way to build it is through createIntM

Does that make sense?

On Thu, Mar 14, 2019 at 4:19 PM mike h <mike_k_houghton@yahoo.co.uk> wrote:

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 <frederic.cogny@gmail.com> 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-LinearAlgebra-Static.html

On Thu, Mar 14, 2019, 12:37 PM Francesco Ariis <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-literals.html
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Frederic Cogny

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Frederic Cogny

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