To expand slightly, the Elm compiler will help you out here and show you what you need to fix. Given this function body

  randomReturner n =
      if n < 0.5 then
          123
      else
          "abc"

The compiler will give you the following info:

  The 2nd branch of this `if` does not match all the previous branches:

  22|     if n < 0.5 then
  23|         123
  24|     else
  25|         "abc"
            ^^^^^
  The 2nd branch is a string of type:

      String

  But all the previous branches result in:

      number

  Hint: All branches in an `if` must produce the same type of values. This way, no
  matter which branch we take, the result is always a consistent shape. Read
  <https://elm-lang.org/0.19.1/custom-types> to learn how to “mix” types.

  Hint: Try using String.toInt to convert it to an integer?

You can try it out on Ellie here: https://ellie-app.com/9nXxfKSStG4a1