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smlsmlnj

Multiple Patterns in 1 case

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, "+", "-", "*", "/" and I want to print "PLUS MINUS" of it is "+" or "-" and "MULT DIV" if it is "*" or "/".

TL;DR: Is there somewhere I can simplify the following to use less cases?

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

  • Given that you've tagged your question with the 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