[Haskell-cafe] Name that function =)

[Conal Elliott]

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 <bryan.burgers at gmail.com> wrote:
>
> On 12/12/06, Louis J Scoras <louis.j.scoras at gmail.com> 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
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