[Haskell-beginners] Further constraining types

[David Virebayre]

2011/8/5 Brent Yorgey <byorgey at seas.upenn.edu>:

> Your particular problem could be solved by creating a type Positive
> which consists of an Integer value paired with a *proof* that it is
> positive.  Then the squaring function (for example) could exploit the
> properties of multiplication to return the result along with a proof I
> realize that's a bit vague; I could explain how this would work in
> more detail if you like.

I realiser the offer wasn't for me but yes, please, I'd like to know
how you could do that, also with a subtract function example, and with
a user input example.

Pretty please ?

David