Re: Data.List.join

Ross Paterson wrote:

On Mon, Oct 23, 2006 at 01:15:46PM -0400, Samuel Bronson wrote:

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On 10/22/06, John Meacham wrote:

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Although, now that we lost the Monoid instance for functions (which is very dismaying), it is less useful, as the monoid functions were very useful to build things up efficiently with (String -> String) as a type.

WHAT!!!!

Who, when, where, why, how????

Funnily enough, that was my reaction when I saw that endofunctions under composition /was/ the library instance of Monoid for functions, thus preventing the powerful 'pointwise lifting' instance...

The function composition instance is now wrapped in a newtype. The instances available now are listed here:

http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Monoid.html#...

I was who; here is some of the rest:

http://www.haskell.org/pipermail/libraries/2005-September/thread.html#4365

...which we now have instead, allowing things like newtype Parser a = Parser (String -> [(a, String]) deriving Monoid and plenty of other goodies. I have code (part of the Epigram layout library) which systematically iterates pointwise lifting to jack up operators on text boxes to operators on prioritised streams of text boxes, monoidal with respect to a lossy merge operation which discards obviously bad layouts and prevents combinatorial explosion. Pointwise lifting is one way to keep the programs simple even when the underlying structures become complex. While we're at it, would anybody object to a few other instances of the same idea? for <blah> in {State s, IO, Id}, may we have suitable Applicative instances where currently absent, then

instance Monoid x => Monoid (<blah> x) where mempty = pure mempty mappend sx sy = pure mappend <*> sx <*> sy

? Basically, I'd suggest that any monad/idiom/whatever should lift monoids if it doesn't induce a monoid structure of its own (as [], Maybe, etc) do. What to do with StateT etc is more complex... Does this make sense? Cheers Conor This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.