Chad Scherrer wrote:
extract :: [Int] -> [a] -> [a]
[...]
This behaves roughly as
extract ns xs == map (xs !!) ns
extract sounds like removing the elements to be extracted from the
original list. I would therefore expect it's type signature to be
extract :: [Int] -> [a] -> ([a], [a])
with
extract [0, 2] "abcde" == ("ac", "bde").
For your extract I would prefer "select" as name, in analogy to
relational algebra, viewing a list as a one-column table.
Oh, and "ns" are required to be ordered and non-negative.
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"
"accb"
There could be selectAsc for the special case of ordered indexes, to
avoid keeping the whole input list in memory.
Tillmann
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