Didn't recognize the sameness. Aside from there being many ways to do the same thing, partial application makes the mixup even merrier. Thanks, Michael --- On Tue, 1/26/10, Edward Z. Yang wrote: From: Edward Z. Yang Subject: Re: [Haskell-cafe] Maybe, maybe not. To: "michael rice" Cc: "haskell-cafe" Date: Tuesday, January 26, 2010, 10:52 PM Excerpts from michael rice's message of Tue Jan 26 21:34:42 -0500 2010:

fromMaybe d x = case x of {Nothing -> d;Just v  -> v} fromMaybe z = maybe z id

They're equivalent.  Here the definition of maybe:     maybe :: b -> (a -> b) -> Maybe a -> b     maybe n _ Nothing  = n     maybe _ f (Just x) = f x Cheers, Edward

Excerpts from Daniel Peebles's message of Tue Jan 26 23:25:28 -0500 2010:

There are actually only two (extensionally) possible total functions with that type, as far as I can see :)

Is the other one... const? Cheers, Edward

It might be more obvious by giving: fromMaybe :: a -> (a -> x, x) -> x Ivan Miljenovic wrote:

2010/1/27 Edward Z. Yang :

...

Excerpts from Daniel Peebles's message of Tue Jan 26 23:25:28 -0500 2010:

...

There are actually only two (extensionally) possible total functions with that type, as far as I can see :)

Is the other one... const?

As far as I can tell, yes.

-- Tony Morris http://tmorris.net/

Ivan Miljenovic wrote:

2010/1/27 Tony Morris :

...

It might be more obvious by giving:

fromMaybe :: a -> (a -> x, x) -> x

I actually found this more confusing, and am not sure of its validity: should that be "Maybe a" there at the beginning?

Sorry a mistake. Correction: fromMaybe :: a -> ((a -> x, x) -> x) -> x {-# LANGUAGE RankNTypes #-} data Maybe' a = M (forall x. (a -> x, x) -> x) to :: Maybe' t -> Maybe t to (M f) = f (Just, Nothing) from :: Maybe a -> Maybe' a from (Just a) = M (flip fst a) from Nothing = M snd -- Tony Morris http://tmorris.net/