[Haskell] Re: Fixed-length vectors in Haskell, Part 2: Using no extensions

[Daan Leijen]

Ashley Yakeley wrote:

>In article 
><r02010400-1040-A0BE089CBF6111D99910000393758032@[10.0.1.2]>,
> David Menendez <zednenem at psualum.com> wrote:
>  
>
>>>data Vec 0   a = Nil
>>>data Vec s v a = Cons a (v a)
>>>      
>>>
>>Some aliases for convenience:
>>    
>>
>>>type Vec 1 = Vec s Vec 0
>>>type Vec 2 = Vec s Vec 1
>>>type Vec 3 = Vec s Vec 2
>>>type Vec 4 = Vec s Vec 3
>>>      
>>>
>
>I don't understand this at all, and neither does GHC.
>  
>
You seem to have lost the underscores :-)
It can be written without underscores as:

 data Vec0 a = Nil
 data VecS v a = Cons a (v a)

 type Vec1 = VecS Vec0
 type Vec2 = VecS Vec1


All the best,
 Daan Leijen

Btw. David: nice example of useful GADT use!


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