4025.md

4025

A name bound in a pattern in a matcher (e.g. case) was the same as the name of a fun declaration that contained the pattern.

fun foo x =
  case x of
    0 => 1
  | foo => foo
(** ^^^ name bound in pattern inside a `case` matches name of a `fun` that contains the `case` *)

This is at best a possibly confusing case of shadowing.

This warning occurs in the following somewhat common scenario:

fun foo 0 y = y
  | foo 1 y =
      case y of
        0 => 1
      | 2 => 3
      | _ => 4
  | foo x y = x + y
(** ^^^ name bound in pattern inside a `case` matches name of a `fun` that contains the `case` *)

This looks like a fun with many cases, one of which has an inner case. However, most SML parsers (including Millet) attempt to parse the final foo x y as part of the case instead as part of the fun. This leads to confusing errors, often 3001 and 3002.

To fix

Try one of the following:

• Put parentheses around the inner matcher (i.e. case, fn, or handle).
• Rename the binding to avoid shadowing.

fun foo 0 y = y
  | foo 1 y =
      (case y of
        0 => 1
      | 2 => 3
      | _ => 4)
  | foo x y = x + y