5015.md

5015

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

datatype d = A | B of int

fun f x =
  case x of
    A => 1
  | B => 2
(** ^ missing argument for constructor pattern *)

To fix

Try one of the following:

• Add an argument to the pattern:

  datatype d = A | B of int
  
  fun f x =
    case x of
      A => 1
    | B _ => 2

• Define the constructor to not have an argument:

  datatype d = A | B
  
  fun f x =
    case x of
      A => 1
    | B => 2