[Haskell-cafe] RealFloat constraint on Complex type

[Bayley, Alistair]

> From: haskell-cafe-bounces at haskell.org 
> [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of 
> Richard A. O'Keefe
> 
> On 21 May 2008, at 9:25 am, Conal Elliott wrote:
> > I think the practice of constraint in type definitions is 
> generally  
> > discouraged,
> 
> Is this true?  If so, why?
> If I have a data type that simply doesn't make sense unless 
> some of the
> type variables belong to certain classes, _shouldn't_ that be stated
> clearly in the declaration rather than hidden elsewhere?


I recall this from Graham Klyne, but I think his use case might be a bit
different:

http://www.ninebynine.org/Software/Learning-Haskell-Notes.html#type-clas
ses-and-data

I don't know all the pros and cons (are there pros, other than the
documentation argument you gave?). I think:
 1. adding the constraint has some costs, and very few benefits
 2. nobody does it much, if at all. Probably for the first reason.

Alistair
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