Multiple Patterns in 1 case

Question

In SML, is it possible for you to have multiple patterns in one case statement?

For example, I have 4 arithmetic operators express in string, \"+\", \"-\", \"*

Answer 1

Given that you've tagged your question with the smlnj tag, then yes, SML/NJ supports this kind of patterns. They call it or-patterns and it looks like this:

case str of
  ("+" | "-") => print "PLUS MINUS"
| ("*" | "/") => print "MULT DIV"

Notice the parentheses.

The master branch of MLton supports it too, as part of their Successor ML effort, but you'll have to compile MLton yourself.

val str = "+"

val _ =
  case str of
    "+" | "-" => print "PLUS MINUS"
  | "*" | "/" => print "MULT DIV"

Note that MLton does not require parantheses. Now compile it using this command (unlike SML/NJ, you have to enable this feature explicitly in MLton):

mlton -default-ann 'allowOrPats true' or-patterns.sml