E4144

This is a constant, not a constructor, it cannot be applied to arguments.

This error occurs when you try to use a constant as a constructor in pattern. One possible reason for this error is that you have a constant with the same name as an constructor from some other packages, and in such case you need to either use qualified name or type annotations to disambiguate.

Erroneous example

pub const Value : Int = 1

pub fn classify(value : Int) -> String {
  match value {
    Value(_) => "Value"
  //^~~~~
  // Error: 'Value' is a constant, not a constructor, it cannot be applied to arguments.
    _ => "Other"
  }
}

Suggestion

If you want to match against the constant, you can remove the payload from the pattern.

pub const Value : Int = 1

pub fn classify(value : Int) -> String {
  match value {
    Value => "Value"
    _ => "Other"
  }
}

Or if the constant has the same name as a constructor from some other packages, you can use qualified name or type annotations to disambiguate.

pub fn classify_qualified(item : @a.Item) -> String {
  match item {
    @a.Value(_) => "Value"
  }
}