Re: inits

Chris Kuklewicz wrote:

Aaron Denney wrote:

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On 2006-04-08, Nils Anders Danielsson wrote:

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On Fri, 07 Apr 2006, "Spencer Janssen" wrote:

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inits xs = [] : (zipWith take [1..] $ map (const xs) xs)

As this version performs much better and will work as a drop in replacement, I suggest that it be included in the hierarchical libraries.

It is not a drop in replacement. The original inits is strict, this one isn't.

The specification of inits (from the Haskell 98 report):

inits :: [a] -> [[a]] inits [] = [[]] inits (x:xs) = [[]] ++ map (x:) (inits xs)

Is that a property many programs depend on? I'd actually call that a bug of the original.

It may break some things:

head (Data.List.inits undefined) is an error

head (New.Spiffy.inits undefined) is []

Is the head of the inits of undefined really an error? Since the head of inits [] is also [] ... But if you really want that undefined to produce an error.. you could just : inits' xn@(_:_) = zipWith take [0..] $ map (const xn) $ undefined:xn inits' _ = undefined