Report errors through an exported function

Exports can return a numeric error through Result. Plenty callers use the ordinary result; C callers receive a status and separate output pointers:

export def positive(value: i32) -> Result[i32, i32] = "calc_positive":
    if value < 0:
        Err(1)
    else:
        Ok(value)

def main() -> Result[(), AllocError | IoError | i32]:
    print(positive(42)?)?
    print(str.repr(positive(-1))?)?
    Ok(())

42
Result[i32, i32].Err(1)

The generated C declaration is uint32_t calc_positive(int32_t p0, int32_t *out_ok, int32_t *out_error). Status 0 writes out_ok; status 1 writes out_error. Its header explains output storage and borrowing requirements.

Name errors with an enum

A numeric code means little to a C caller. Return an enum whose variants have no payloads instead:

enum CalcError:
    Empty
    Negative

export def checked(value: i32) -> Result[i32, CalcError] = "calc_checked":
    if value == 0:
        Err(CalcError.Empty)
    elif value < 0:
        Err(CalcError.Negative)
    else:
        Ok(value)

def main() -> Result[(), AllocError | IoError | CalcError]:
    print(checked(42)?)?
    print(str.repr(checked(-1))?)?
    Ok(())

42
Result[i32, CalcError].Err(CalcError.Negative)

C receives a uint32_t code, the variant’s position in the declaration. The header names each code:

#define calc_CalcError_Empty UINT32_C(0)
#define calc_CalcError_Negative UINT32_C(1)
uint32_t calc_checked(int32_t p0, int32_t *out_ok, uint32_t *out_error);

The generated calc.plentyi declares its own CalcError, so Plenty consumers match calc.CalcError variants. Enums with payloads and sums such as AllocError | CalcError cannot cross the C boundary.