E4108

E4108#

The loop is expected to yield a value, please add an else branch.

This error occurs when the compiler infers that a for-loop is expected to yield a non-Unit value based on its context (such as the function’s return type), but the loop lacks an else branch to provide that value.

In MoonBit, when a for-loop is used in a context where a value is expected (for example, when the function returns a non-Unit type), the loop must have an else branch that specifies what value to return when the loop completes normally. This is because:

  1. The loop body itself cannot yield a value (unlike loop expressions)

  2. Without an else branch, there’s no way to determine what value should be returned when the loop finishes without breaking

This commonly occurs in two situations:

  • When the for-loop is the last expression in a function that returns a non-Unit type

  • When the for-loop’s result is assigned to a variable or used in an expression that expects a non-Unit value

错误示例#

pub fn f(x: Int) -> Int {
  for i = 0, acc = 0; i < x; i = i + 1, acc = acc + i {
//^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Error: The for loop is
//                                                      not expected to yield a
//                                                      value, please add an
//                                                      `else` branch.
  }
}

pub fn g(x: Int) -> Int {
  for i in 0..=x {
  }
}

建议#

To fix this error, you can:

  • add an else branch to the for-loop:

pub fn f(x: Int) -> Int {
  for i = 0, acc = 0; i < x; i = i + 1, acc = acc + i {
  } else {
    acc
  }
}

  • change the function’s return type to Unit if you don’t need to return a value from the loop:

pub fn g(x: Int) -> Unit {
  for i in 0..=x {
  }
}
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