Find the sum of the digits in factorial of 100 in Erlang
Don't need rewrite functions which already exist in standard library. Almost always it's will give worse result and just take away your time for writing and testing.  


For converting from integer to list (string) - integer_to_list/1
For getting some single result from set data in a list - use lists:foldl/3 
For extracting the numerical value from digit symbol - just subtract zero-value($0, number representation in Erlang). 


So, if accumulate all of this points, get:

task_(N)-&gt;
lists:foldl(fun(X,Sum)-&gt;Sum+X-$0 end,0,integer_to_list(fact(N))).


If saying generally, I don't see any problem in rule - "one function for one task". It will make your code more readable, flexible and good for testing.