This rewrite changes the order of execution. The old version did all of the putDirs and then all of the renameFile calls. The new one interleaves putDirs & renameFile calls. In general, ">>" is not commutative, though in this case you may be as happy with either order. - Conal
On 12/12/06, Bryan Burgers <[EMAIL PROTECTED]> wrote:
On 12/12/06, Louis J Scoras <[EMAIL PROTECTED]> wrote: > I have some IO actions that I want to map over a list of pairs -- > these are just directory names and their down-cased versions. It > wasn't difficult to actually get the right behavior by just doing mapM > twice. > > -- putDirs just outputs something like "mv fst snd" > mapM_ putDirs pairs > mapM_ (uncurry renameFile) pairs > > This bothered me though, because I suspected that this could be done > in one pass. Naively I proceeded to this. > > mapM_ (putDirs >> (uncurry renameFile)) pairs > > Which didn't work. I thought about it a little more before realizing > that putDirs wouldn't get any parameters this way: I needed some way > to distribute the pair over both operations. Here's the higher-order > function I needed: > > foo h f g i = h (f i) (g i) > > which could then be curried and we get: > > mapM_ (foo (>>) putDirs $ uncurry renameFile) pairs > > Works great. So my question: is there a established name for foo? > What about foo partially applied to (>>)? This was a fun exercise, > but I'd like to use the standard implementations if they exist. Before we get too far down the obfuscation road, I'd like to offer what I think is more readable than a liftM2 solution: mapM_ (\p -> putDirs p >> uncurry renameFile p) pairs I haven't tested it, but I hope that does the same thing. To me, this explicitely shows what each is doing, moreso than with a point-free 'foo' combinator. The way my mind worked to get to this solution: mapM_ putDirs pairs mapM_ (uncurry renameFile) pairs ==> mapM_ (\p -> putDirs p) pairs mapM_ (\p -> uncurry renameFile p) pairs ==> mapM_ (\p -> putDirs p >> uncurry renameFile p) pairs Is that a reasonable solution? Bryan Burgers _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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