On 13/11/2007, Henning Thielemann wrote:

On Tue, 13 Nov 2007, Dougal Stanton wrote:

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-- int a = 3; -- int *pa = &a; ampersand :: t -> Pointer t ampersand a = Just a

What's bad about using 'ampersand' function as replacement for the constructor 'Just'?

I also wanted to use it in pattern matching but have the advantage of all the stuff already written for Maybe. (Note this was an example I made up on the spur of the moment anyway.) I'd often thought it would be nice when adapting standard data structures for specific programs. Like, data Tree a = Leaf a | Branch a [Tree a] data FS a = File a | Folder a [FS a] Once you've got one it would be nice to repurpose it with more appropriate names. Just an idea! :-) D. -- Dougal Stanton dougal@dougalstanton.net // http://www.dougalstanton.net

Henning Thielemann wrote:

On Tue, 13 Nov 2007, Dougal Stanton wrote:

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On 13/11/2007, Henning Thielemann wrote:

...

On Tue, 13 Nov 2007, Dougal Stanton wrote:

...

-- int a = 3; -- int *pa = &a; ampersand :: t -> Pointer t ampersand a = Just a What's bad about using 'ampersand' function as replacement for the constructor 'Just'? I also wanted to use it in pattern matching but have the advantage of all the stuff already written for Maybe.

No problem, write a function like 'maybe' to inspect the data.

Instead of 'f m' with f :: Maybe T -> S f (Just x) = g x f Nothing = h

Yes. It is a problem. Do you write all your code using higher-order functions, never matching explicitly on constructors? I don't. Matching explicitly on constructors is an elegant and easy-to-read way to write programs. It's annoying to have to choose between (a) nicely named constructors and (b) being able to re-use library functions defined for Maybe. Jules

On Tue, Nov 13, 2007 at 11:44:30PM +0100, Henning Thielemann wrote:

On Tue, 13 Nov 2007, Dougal Stanton wrote:

...

-- int a = 3; -- int *pa = &a; ampersand :: t -> Pointer t ampersand a = Just a

What's bad about using 'ampersand' function as replacement for the constructor 'Just'?

It wouldn't work in pattern matching. -- David Roundy Department of Physics Oregon State University