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

Jonathan Cast jcast at ou.edu
Tue Sep 4 11:44:15 EDT 2007

On Tue, 2007-09-04 at 16:06 +0200, Peter Verswyvelen wrote:
> Henning Thielemann wrote:
> > 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?
> >
> >   
> Oh but I will not write "do {1;2;3;4}", this was just an idea :-) Yep, 
> your approach certainly works, but I just found it was a bit of a 
> discrepancy in Haskell (numbers getting better lifting support than lists).

I don't think this has been mentioned explicitly yet, but the
discrepancy is purely for pedagogical purposes.

In Gofer, list comprehensions (and list syntax, IIRC) /was/ generalized
(to an arbitrary instance of MonadPlus).  But that means that any
mistake in your syntax likely brings up a type error mentioning
MonadPlus.  This confuses CS freshmen (who are easily confused anyway);
thus, Haskell restricts list syntax to lists so the type errors are

By contrast, most CS freshman have already used languages with multiple
number types, so all you have to do is explain that type errors
involving Num are Haskell's way of dealing with them.  So the syntax can
be generalized to the type class in that case without confusing freshmen
as much.


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