E4051

The identifier is declared twice.

Mutually recursive definitions in MoonBit shall not have the same identifier. This include:

• All toplevel definitions, including variables, functions, types, traits, etc.

• Locally named functions. (Reported as E4006)

• Local types.

Note, locally defined variables are not mutually recursive, so they can have the same identifier, and the later definition shadows the previous one.

Erroneous example

• Toplevel variable definition:

  pub let a = 0
  pub let a = 1 // Error: The toplevel identifier a is declared twice: it was previously defined at ...
  

• Toplevel function definition:

  pub fn f() -> Unit {}
  pub fn f() -> Unit {} // Error: The toplevel identifier f is declared twice: it was previously defined at ...
  

• Toplevel type definition:

  pub enum A {}
  pub struct A {} // Error: The type A is declared twice: it was previously defined at ...
  

• Local type definition:

  pub fn g() -> Unit {
    struct A {}
    struct A {} // Error: The local type A is declared twice: it was previously defined at ...
  }
  

Suggestion

Rename the identifier to a different name.

pub let a = 0
pub let b = 1

If you want to shadow the previous definition, you can use a block and put the definition in the block.

pub let a = {
  let a = 0
  1
}