[Haskell-cafe] partially applied data constructor and corresponding type

TP paratribulations at free.fr
Sat Apr 27 23:35:44 CEST 2013

Thanks Yury,

The problem with this solution is that if I have written a method for the 
Tensor type (for example a method of a typeclass of which Tensor is an 
instance) that uses the order of the tensor (your "ndims") in a general way, I 
cannot reuse it easily for a vector defined like that.

In fact, my problem is to be able to define:
* from my type "Tensor", a type "Vector", by specifying that the dimension 
must be one to have a "Vector" type.
* from my constructor "TensorVar", a constructor "VectorVar", which 
corresponds to TensorVar with the integer equal to 1.

The idea is to avoid duplicating code, by reusing the tensor type and data 
constructor. At some place in my code, in some definition (say, of a vector 
product), I want vectors and not more general tensors.


On Saturday, April 27, 2013 16:16:49 Yury Sulsky wrote:

Hi TP,

Are you looking to use a phantom types here? Here's an example:

data One
data Two

data Tensor ndims a = Tensor { dims :: [Int], items :: [a] }
type Vector = Tensor One
type Matrix = Tensor Two


On Sat, Apr 27, 2013 at 3:33 PM, TP <paratribulations at free.fr> wrote:


I ask myself if there is a way to do the following.
Imagine that I have a dummy type:

data Tensor = TensorVar Int String

where the integer is the order, and the string is the name of the tensor.
I would like to make with the constructor TensorVar a type "Vector", such
that, in "pseudo-language":

data Vector = TensorVar 1 String

Because a vector is a tensor of order 1.

Is this possible? I have tried type synonyms and newtypes without any success.

Thanks a lot,

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