Re: [Haskell-cafe] Need some help with an infinite list

Thomas Davie wrote:

letterCombos = map (:[]) ['a'..'z'] ++ concatMap (\c -> map ((c++) . (: [])) ['a'..'z']) letterCombos

Not hugely efficient, if you generate the strings in reverse then you can use (c:) rather than ((c++) . (:[])), but that may not be useful to you.

Bob

I think the following version increases the sharing between the generated strings, and so might be more space-efficient for consumers which hold on to a significant number of them: number :: [a] -> [[a]] number digits = expand [[]] where expand xss = expanded ++ expand expanded where expanded = concatMap (\d -> map (d:) xss) digits binary = number ['0'..'1'] decimal = number ['0'..'9'] alpha = number ['a'..'z'] Regards, Matthew