On 12/22/05, Daniel Carrera <[EMAIL PROTECTED]> wrote: > Paul Moore wrote: > > As I say, I'm not trying to criticize anyone here, but it seems to be > > quite hard to get across to people who have understood and assimilated > > this sort of stuff, just how hard it feels to newcomers. We understand > > the explanations (we do! really! :-)) but even understanding them, we > > are still left with a lack of confidence. It's like being shown a full > > set of carpentry tools, having every one explained, but still reaching > > for the hammer every time and banging something no matter what we're > > trying to do :-) > > I had never heard of mapM, or other -M functions. I can't imagine why > those would be needed. It seems like pointless duplication. >
mapM is like map except you map an IO Action over a list instead of a function of a list. For instance sizes <- mapM getFileSize myListOfFileNames If you used "map" here you'd end up with a list of IO actions, and not a list of file sizes. You'd then have to go through this list of IO actions and using (<-) on each element to get the file sizes. This can, incidentally, be done using the function 'sequence'. liftM lifts a function so that you can use a regular function on an IO Action instead of first having to extract the value of the IO action using (<-). It's just shorthand, so you could do: x <- liftM length (readFile "afile") Instead of having to do f <- readFile "afile" let x = length f The M functions really are useful, get to know them! /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
