mergeops :: String -> String -> String
mergeops ns  = ns
mergeops  os = os
mergeops (n:ns) (o:os) = n : o : mergeops ns os
For example two digits then plus then two digits then minus then two digits...
*Main Data.Char> mergeops "123456789" "__+__-__"
Ah yes I will want to filter out the "nothing" operation characters, say like this:-
*Main Data.Char> filter (\c -> c /= '_') it
The bit in the brackets is a lambda, the function taking c and returning a boolean true if c is not equal to the underline character.
And it is the Haskell shell's way of giving you the result of the previous expression you just evaluated.