[Haskell-cafe] Re: Implementing fixed-sized vectors
(using datatype algebra?)
westondan at imageworks.com
Fri Feb 8 15:52:39 EST 2008
Dan Weston wrote:
> Brandon S. Allbery KF8NH wrote:
>> On Feb 8, 2008, at 11:14 , Stefan Monnier wrote:
>>>>>> You seem to write 12 as 1 :+ 2 instead of () :+ 1 :+ 2. But I
>>>>>> think, the
>>>>>> latter representation should probably be prefered.
>>> How 'bout treating :+ as similar to `append' rather than similar to
>>> Basically treat :+ as taking 2 numbers (rather than a number and
>>> a digit).
>> Dumb questions department: why not define e.g. D'0 .. D'9 as () :* 0
>> .. () :* 9? Programmers then get D'1 :* 2, but the library sees () :*
>> 1 :* 2.
> No, D'0 should be (), not () :* D0. If you allow () :* D0, then you
> introduce redundant types for the same number:
> In the first case, D'0 :* D'3 == D'3, and D'0 :* D'0 has no instance. In
> your example, D'3 and D'0 :* D'3 are equivalent, but no longer unify.
On second thought, how would you write D'3 :* D0 ? I think maybe using
the () makes it fundamentally difficult to restrict multiple types for
the same number.
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