Question on type synonym definition and language extensions
Hi all, I'm trying to define a type synonym for 2D MArray instances. The goal is to keep the function signature simple and readable using type `Matrix` instead of something like `(Ix i, MArray a e m) => m (a i e)`. After some "read, guess, try, error" cycles I came up with this: type Matrix = forall m. forall a. forall i. forall n. (Ix i, MArray a n m, Num i, Num n) => m (a (i,i) n) it requires option -XRank2Types to work. But then I can write my function as: test :: Matrix test = do { a <- newArray ((0,0),(5,8)) 0; writeArray a (0,0) 1; return a } Then I wanted to be able to give the Index and Value types in the type synonym but keep it flexible in terms of which MArray instance is used. I changed it to: type Matrix i n = forall m. forall a. (MArray a n m) => m (a (i,i) n) and the type signature of the test function becomes: test :: Matrix Int Double For this one I had to add an extra -XFlexibleContexts option to build it. It works and I'm quite happy with the look of my new function type signatures, but I'm just wondering if I'm doing something bad or if there is a cleaner/simpler way to define the same type synonym without requiring language extensions. What are the risks associated with using these two language extensions, non compatibility with other compilers or more? Thanks, Olivier.
On Thu, 29 May 2008, Olivier Boudry wrote:
Hi all,
I'm trying to define a type synonym for 2D MArray instances. The goal is to keep the function signature simple and readable using type `Matrix` instead of something like `(Ix i, MArray a e m) => m (a i e)`.
After some "read, guess, try, error" cycles I came up with this:
type Matrix = forall m. forall a. forall i. forall n. (Ix i, MArray a n m, Num i, Num n) => m (a (i,i) n)
Is type Matrix monad array num = monad (array (Int,Int) num) ok for you?
On Thu, May 29, 2008 at 4:38 PM, Henning Thielemann < lemming@henning-thielemann.de> wrote:
Is
type Matrix monad array num = monad (array (Int,Int) num)
ok for you?
Not really what I was looking for but I may end up using it. What I wanted is to hide the Monad and Array and have it inferred from the context of the array STArray or STUArray when run in the ST monad with runST(U)Array, IOArray or IOUArray when used in the IO monad, etc... I'm not sure it's the right thing to do. I may create more troubles than I solve. I was just trying to keep things as generic as possible. Thanks for your reply, Olivier.
2008/5/29 Olivier Boudry
After some "read, guess, try, error" cycles I came up with this:
type Matrix = forall m. forall a. forall i. forall n. (Ix i, MArray a n m, Num i, Num n) => m (a (i,i) n)
I've tried similar things before. You may run into subtle problems later. Such as:
transpose :: Matrix -> Matrix
won't expand into the type signature you want it to, I think. You probably want that to be equivalent to: transpose :: forall m. forall a. forall i. forall n. (Ix i, MArray a n m, Num i, Num n) => m (a (i,i) n) -> m (a (i,i) n) But you'll get: transpose :: forall m. forall a. forall i. forall n. (Ix i, MArray a n m, Num i, Num n) => Matrix -> m (a (i,i) n) which means that the first argument must be a polymorphic value, which isn't very useful. -Antoine -Antoine
On Thu, May 29, 2008 at 7:36 PM, Antoine Latter
I've tried similar things before. You may run into subtle problems later.
Such as:
transpose :: Matrix -> Matrix
won't expand into the type signature you want it to, I think.
You probably want that to be equivalent to:
transpose :: forall m. forall a. forall i. forall n. (Ix i, MArray a n m, Num i, Num n) => m (a (i,i) n) -> m (a (i,i) n)
But you'll get:
transpose :: forall m. forall a. forall i. forall n. (Ix i, MArray a n m, Num i, Num n) => Matrix -> m (a (i,i) n)
which means that the first argument must be a polymorphic value, which isn't very useful.
Right, this is exactly what I'm getting when creating a transpose function: *Main> :t transpose transpose :: (MArray a Double m) => Matrix Int Double -> m (a (Int, Int) Double) Thanks for sharing your experience. I'll try to stay out of the subtle bugs and keep it verbose but simple. Regards, Olivier.
participants (3)
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Antoine Latter -
Henning Thielemann -
Olivier Boudry