[Haskell-cafe] Extending the idea of a general Num to other types?

Henning Thielemann lemming at henning-thielemann.de
Tue Sep 4 05:46:24 EDT 2007

On Tue, 4 Sep 2007, Peter Verswyvelen wrote:

> Henning Thielemann wrote:
> > Why not just
> >  FancySequence.fromList [1,2,3,4]
> >   or
> >  FancySequence.fromList $ 1:2:3:4:[]
> > ?
> Yes of course, but the same could be said for numbers, e.g when you need
> an Int, you have to type (Int.fromNumber 1), (Float.fromNumber 1),
> (Double.fromNumber 1), etc... I don't like that because it makes writing
> numeric type independent generic code impossibe, so luckily the Haskell
> compiler automatically inserts fromInteger or fromRational calls to lift
> a generic number into a specific representation. But it does not provide
> the same for lists, e.g. there is no fromList function which is a member
> of some List typeclass that the compiler automatically uses just like it
> does for numbers. I think this is a bit of a discrepancy.

If you are happy with writing "do {1;2;3;4}" you are certainly also happy
with "cv [1,2,3,4]", where cv means 'convert' and is a method of a class
for converting between lists and another sequence type.

class ListCompatible lc where
   cv :: [a] -> lc a
   rt :: lc a -> [a]   {- restore :-) -}

Better don't adapt the names, but the idea would work, wouldn't it?

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