[Haskell-cafe] where to put handy functions?
chad.scherrer at gmail.com
Fri Aug 10 11:41:09 EDT 2007
Agreed. I like "select" better too, and the regular vs "Asc" version
is a nice parallel with fromList and fromAscList.
On 8/10/07, Tillmann Rendel <rendel at rbg.informatik.tu-darmstadt.de> wrote:
> Non-negative is obvious for a list of indexes. Ordered makes sense
> implementation-wise, and should be easy to match for many applications.
> But is it a sensible constraint on a standard library function?
> For Data.List, I would prefer a multi-pass select function like this:
> select :: Integral n => [n] -> [a] -> [a]
> select ns xs = select' 0 ns xs where
> select' k  _ = 
> select' k (n:ns)  = select' k ns 
> select' k nns@(n:ns) yys@(y:ys) = case k `compare` n of
> LT -> select' (succ k) nns ys
> EQ -> y : select' k ns yys
> GT -> select nns xs
> *Main> select [0, 2, 2, 1] "abcde"
> There could be selectAsc for the special case of ordered indexes, to
> avoid keeping the whole input list in memory.
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