Question

I want to write a pattern matching like the follows:

match ... with
...
| Const1 (r, c) | Const2 (m, n) 
  -> expr

It returns an error: Error: Variable c must occur on both sides of this | pattern.

Do I have to write expr twice (one time for Const1, the other time for Const2)? Could anyone help?

Answer 1

As the error message said, Or pattern (| pattern) requires binding to the same set of variables. Therefore:

match ... with
...
| Const1 (m, n) | Const2 (m, n) 
  -> expr

or

match ... with
...
| Const1 (m, n) | Const2 (n, m) 
  -> expr

would work.

Of course, you only can do that if Const1 and Const2 accept the same type. In some cases, you still do so if you have parts of constructs with the same type:

match ... with
...
| Const1 (m, _) | Const2 (_, m) 
  -> expr

The pitfall of Or pattern is that you don’t know which constructor you are in. So if logic of expr depends on Const1 or Const2, you couldn’t use Or pattern anymore.