How to get length info from expression.Constant correctly?

🛰️ 应用开发者专区
1

Hello, I am confuse about how to get the length of expression.Constant value correctly (eg: the instance stores a string in Value and want to get the length of the string)?
I notice expression.Constant has Value types.Datum and RetType *types.FieldType fields. They all have certain field related to length info ( length in type.Datum and Flen in types.FieldType ).

type Constant struct {
    Value   types.Datum
    RetType *types.FieldType
    ...
 }

type Datum struct {
    length    uint32      // length can hold uint32 values.
    ...
}

type FieldType struct {
    Flen    int
    ...
}

What’s the different between them and which one is the right one to use? Any docs or suggestion would be appreciated.