Haskell platform proposal: split package

Twan van Laarhoven twanvl at gmail.com
Tue Jul 24 15:13:08 CEST 2012

On 20/07/12 22:35, Brent Yorgey wrote:
> Hello everyone,
> This is a proposal for the split package [1] to be included in the
> next major release of the Haskell platform.
> Missing strategies
> ------------------
> The specific way that the generic splitting algorithm is implemented
> does preclude some imaginable splitting strategies.  For example, a
> few years ago I tried adding a strategy that used a predicate on pairs
> of elements, splitting down the middle of any pairs that satisfy the
> predicate, but gave up because it simply did not fit.

I just needed a function for splitting a list into two equal parts. This is 
something you often need for MergeSort-like functions. Is that something that 
belongs in Data.List.Split?

I came up with this code, I am sure that there are other possible implementations.

     -- | Split a list into two equally sized parts.
     --   If the number of elements in the list is odd,
     --    then the first part will be one element longer.
     splitMiddle :: [a] -> ([a],[a])
     splitMiddle xs = let (_,ys,zs) = go 0 xs in (ys,zs)
         go _ [] = (0,[],[]) -- note: should be made strict in 1st argument
         go i xxs@(x:xs)
             | i <= j    = (j+1,x:ys,zs)
             | otherwise = (j+1,ys,xxs)
           where (j,ys,zs) = go (i+1) xs


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