[Haskell-cafe] Re: Implementing fixed-sized vectors (using datatype
monnier at iro.umontreal.ca
Fri Feb 8 11:14:55 EST 2008
>> > You seem to write 12 as 1 :+ 2 instead of () :+ 1 :+ 2. But I think, the
>> > latter representation should probably be prefered. With it, :+ always
>> > has a number as its left argument and a digit as its right. Without the
>> > () :+ we get ugly exceptional cases.
>> > You can see this, for example, in the instance
>> > declarations for Compare. With the second representation, we could
>> > reduce the number of instances dramatically. We would define a
>> > comparison of digits (verbose) and than a comparison of numbers based on
>> > the digit comparison (not verbose).
>> Even if () would be preferred from the programmers point of view (I'm
>> not sure how much we could reduce the number of instances though), it
>> makes the representation less attractive on the user-side. Anyone
>> using the library would find it annoying and would wonder why is it
> I wouldn’t wonder. Leaving out the () :* part just works because our
> type-level “values” are not typed, i.e., there aren’t different kinds Digit
> and Number but only kind *. If :+ would be a data constructor (on the value
> level), it would take a number and a digit argument which would forbid using
> a digit as its left argument. So I consider using a digit on the left
> as “unclean”. It’s similar to using a number as the second part of a cons
> cell in LISP.
How 'bout treating :+ as similar to `append' rather than similar to `cons'?
Basically treat :+ as taking 2 numbers (rather than a number and
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