lisp: consolidate a list-of-lists into a single list?

Still working on lisp recipes and idioms.

I have a list like this:

((a b c) (d e f) nil (g h))

I'd like to consolidate that to one list,

(a b c d e f g h)

Seems like there oughta be a one-liner for th开发者_开发技巧at.

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(apply #'append '((a b c) (d e f) (g h i)))

or

(loop for outer in '((a b c) (d e f) (g h i))
      nconcing (loop for inner in outer collecting inner))

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That's a typical homework question. Generally this operation is called FLATTEN (which flattens lists on all levels).

(mapcan #'copy-list '((a b c) (d e f) nil (g h)))

The APPLY variant has the problem that it may run into the CALL-ARGUMENTS-LIMIT when there are more sublists than CALL-ARGUMENTS-LIMIT.

See for example also http://rosettacode.org/wiki/Flatten_a_list#Common_Lisp

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In the vein of the answer involving reduce and append, you can also use the :from-end argument to reduce, which is much more efficient as it avoids the unnecessary copying of the result in progress because append doesn't copy its second argument:

(reduce #'append '((a b c) (d e f) nil (g h)) :from-end t)

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You can also use reduce and append:

(reduce #'append '((a b c) (d e f) nil (g h)))

Unfortunately this is much less time and space efficient than the other solutions because it calls append a lot and copies the result-in-progress unnecessarily.