Re: [Haskell-cafe] generalized list comprehensions

On Sun, 2008-11-09 at 10:15 +0000, Max Bolingbroke wrote:

2008/11/9 Johannes Waldmann :

...

NB: Wasn't there a time (before "do") when "list" notation (brackets) would work in any monad? And "map" was a method in "Functor", and we had "class Functor m => Monad m", etc. Well well well times have changed.

Sure, I believe the feature was called "monad comprehensions". AFAIK it was removed because it gave confusing error messages to new users of the language (what is this Monad thing? I just want a list of stuff!).

List comprehensions really have diverged from being a special "do" notation at the list monad, since you are able to write comprehensions like [(x, y) | x <- xs | y <- ys], and it's not clear how to define "zip" for a monad - but perhaps there is some extension of a monad where it makes sense?

As far as I can tell, no one actually uses parallel list comprehensions. With any luck, the same will be true for generalized list comprehensions.