[Haskell-cafe] Re: what are the points in pointsfree?

Steve Downey sdowney at gmail.com
Fri Dec 15 18:38:04 EST 2006

No fair. Although I've a B.S. in Mathematics, I spent most of my time
in complex analytic dynamical systems, rather than hanging with the
algebraists.  Except for a bit where I did some graph theory.

Rather ironic.

On 12/15/06, Scott Brickner <scottb at brickner.net> wrote:
> Donald Bruce Stewart wrote:
> >sdowney:
> >
> >
> >>i'm not naive enough to think they are the composition function, and
> >>i've gathered it has something to do with free terms, but beyond that
> >>i'm not sure. unless it also has something to do with fix points?
> >>
> >>
> >
> >The wiki knows all! :)
> >
> >    http://haskell.org/haskellwiki/Pointfree
> >
> >    1 But pointfree has more points!
> >
> >    A common misconception is that the 'points' of pointfree style are the
> (.)
> >    operator (function composition, as an ASCII symbol), which uses the
> same
> >    identifier as the decimal point. This is wrong. The term originated in
> >    topology, a branch of mathematics which works with spaces composed of
> points,
> >    and functions between those spaces. So a 'points-free' definition of a
> function
> >    is one which does not explicitly mention the points (values) of the
> space on
> >    which the function acts. In Haskell, our 'space' is some type, and
> 'points' are
> >    values.
> >
> >
> Hm. I've been lurking for a while, and this might be a bit of
> nit-picking as my first post, especially given I'm still a bit of a n00b
> in Haskell. I've been programming a long time, though - coming up on
> three decades now and virtually all of it really programming, no management.
> Anyway, as I understood it, the "points" were the terminal objects of
> the category in which you're working - in this case, pointed continuous
> partial orders (CPO), and the points are effectively values in the
> domain. The usage of "point" for terminal objects comes from the
> category of topological spaces, as you say,. and algebraic topology is
> where category theory found it's first big home - but that's not really
> what we're talking about here, is it?
> Category theory got the term from topology, which got it from geometry.
> So you could say "point" is "position without dimension" - but that's
> just not the "point" we're talking about anymore.
> So, the usage of "point" here refers a terminal object in the CPO
> category, which means a value of some datatype - in this particular
> case, a value in the domain of the function being defined. So when you
> give a definition that uses patterns for the parameters, the variables
> in the patterns get bound to the values in the domain of the function.
> If you write the function in a higher-order style, where you don't bind
> the values, your definition doesn't refer to the "point" at which it's
> being evaluated, hence "point-free".
> --
> -----
> What part of "ph'nglui bglw'nafh Cthulhu R'lyeh wagn'nagl fhtagn" don't you
> understand?

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