[Haskell-cafe] RE: [Haskell] is $ a no-op?
GK at ninebynine.org
Wed Oct 13 15:26:59 EDT 2004
[Switched to haskell-cafe]
At 13:24 13/10/04 -0400, Jacques Carette wrote:
> > > -- It's kind of like an converse map.
> > I have attempted, unsuccessfully, to write flist above in a point-free
> > manner. Is it possible?
> > Of course it is, but why?
> > flist = flip (map . flip ($))
>Some functions are simpler point-free, others are simpler with points. I
>was curious about this one (I like the pointwise version better).
>Also, the statement "It's kind of like a converse map" becomes quite clear
>from the point-free way to write it, while still not so obvious in the
The point-free form can be deduced thus, I think:
flist fs a = map ($ a) fs
(flip flist) a fs = map ($ a) fs -- swap flist args
(flip flist) a = map ($ a) -- redundant arg
(flip flist) a = map ((flip ($)) a) -- expand section
(flip flist) a = (map . (flip ($))) a -- definition of .
(flip flist) = map . (flip ($)) -- redundant arg
flist = flip (map . (flip ($))) -- move flip to RHS
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