combos n ls = filter ((>= n) . length)
            . map concat
            . sequence
            . map (([]:) . map return)
            $ ls

##</code><code>##

> combos 1 ["abc", "def"]
["d","e","f","a","ad","ae","af","b","bd","be","bf","c","cd","ce","cf"]

##</code><code>##

> combos 4 ["abc", "de", "fgh", "i", "jk", "l", "m"]
["ijlm","iklm","fjlm","fklm","film","fijm", ...
 "cehijlm","cehik","cehikm","cehikl","cehiklm"]