[Haskell-cafe] how can I select all the 3-element-combination out of a list efficiently

[Mirko Rahn]

Jules Bean wrote:

> In the spirit of multiple implementations; another approach is to note 
> that you're really asking for all 3-element sublists:
> 
> power [] = [[]]
> power (x:xs) = power xs ++ map (x:) (power xs)
> 
> triples1' l = [ t | t <- power l, length t == 3]
> 
> (this implementation also preserves sorting)

...but is exponentially slower than necessary, and fails on infinite 
lists. Try this one:

sublistsN 0 _      = [[]]
sublistsN n (x:xs) = map (x:) (sublistsN (n-1) xs) ++ sublistsN n xs
sublistsN _ _      = []

triples = sublistsN 3

BR,

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