Can't concatenate a char with a string ML

Question

Hello i am trying to do a incFirst function in ML. The function does the following: incFirst "bad" = "cad" incFirst "shin" = "thin". This is what i try to do fun incFirst s = chr(ord s + 1) ^ substring(s, 1, size s -1); I get the following error: Can't unify string (In Basis) with char (In Basis) (Different type constructors) Found near chr (ord s + 1) ^ substring (s, 1, ... - ...) Exception- Fail "Static Errors" raised Any idea how i can concatenate a char with a string if the "^" operator is not working?

Answer 1

The operator is working, it's just that you can only concatenate strings, and that ord operates on characters, not on strings.
(A character is not the same as a one-character string.)

You need to extract the first character and then convert the result to a string

fun incFirst s = String.str(chr (ord (String.sub (s,0)) + 1)) ^ substring(s, 1, size s - 1)

or you could take a detour through a list

fun incFirst s = let
    fun inc (c::cs) = (chr(ord c + 1))::cs
in
    implode (inc (explode s))
end