[Haskell-cafe] Re: wanted: haskell one-liners (in the perl sense
of one-liners)
Lutz Donnerhacke
lutz at iks-jena.de
Tue Mar 20 08:50:01 EDT 2007
* Thomas Hartman wrote:
> You contributed
>
> #12:14 < Thunder> @pl s -> drop (length s - n) s
> #12:14 < lambdabot> drop =<< subtract n . length
>
> But, on second thought, 1) I can't use this as a drop-in replacement
> for the non points free (right term?) version, and 2) I don't really
> understand it.
I did not contribute, but polluted the channel with my own test.
In order to understand this construct you have to switch to the (-> a) Monad:
drop =<< subtract n . length
== do x <- subtract n . length
drop x
This construct consists of partially applied functions, i.e. functions
waiting for an argument.
If you apply an argument to the whole construct, it is applied to each line
seperatly.
So "drop =<< subtract n . length $ s" becomes:
let x = subtract n . length $ s
in drop x s
==
let x = length s - n
in drop x s
==
drop (length s - n) s
I did not expect this monadic approach from lambdabot and was somewhat
surprised. I assumed an application of "liftM2 drop" instead.
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