E4074

This error code is generated when a toplevel declaration is missing a necessary type annotation. MoonBit requires toplevel variables and functions to have fully annotated types, because types are important documentation for toplevel definitions.

There are two exceptions to this rule:

• if the value of a toplevel let is just a simple literal value, type annotation can be omitted:

  pub let forty_two = 42
  pub let float = 1.0
  pub let string = "Hello, world!"
  pub let array = [1, 2, 3]
  

• if the value is a private alias of a foreign value. In this case, the type can be easily inferred. However, for public let, MoonBit still requires annotation for documentation sake.

Erroneous example

///|
let a = 1 + 1 // Error: Cannot infer the type of variable a, please add more type annotation.

Suggestion

This error can be fixed by explicitly annotating the type of the variable:

///|
pub let a : Int = 1 + 1