5014.md

5014

A constructor had an argument in a pattern match, but it was defined to have no argument in the datatype.

datatype d = A | B of int

fun f x =
  case x of
    A y => y + 1
(** ^^^ unexpected argument for constructor pattern *)
  | B z => z - 1

To fix

Try one of the following:

• Remove the argument from the pattern:

  datatype d = A | B of int
  
  fun f x =
    case x of
      A => 1
    | B z => z - 1

• Define the constructor to have an argument:

  datatype d = A of int | B of int
  
  fun f x =
    case x of
      A y => y + 1
    | B z => z - 1