Question

Let’s say we have a function:

generate([],Result) ->
    Result;
generate([H|T],Result) ->
    case H > 0 of
        true -> generate(T,[H*2|Result]);
        false -> generate(T,Result)
    end.

So, basically the function does something with each H and adds this something to Result.

Now, if I call generate() from another function:

do_generate([],Result) ->
    Result;
do_generate([List|Other_lists],Result) ->
    do_generate(Other_lists,[generate(List,[])|Result]).

The problem is that I do not want the do_generate() function to add an empty list [] to its result. What if I call do_generate([[0,0],[1,2]],[])? I can end up with something like this: [[2,4],[]].

If I embed do_generate() into other functions I might end up with even more empty lists inside the result list, which is very inconvenient for later use.

How can I avoid adding [] to Result in other way than checking with case generate(List,[]) == [] of each time?

Is there any ‘best practice’ for calling a sequence of deeply nested functions and getting rid of empty lists [] in the final result with ease?

Answer 1

you can get rid of all empty lists from the result, like

 do_generate([],Result) -> [M|| M <-  Result, length(M) >0];