F# how to return have value a tuple or null

    let retVal =
      if reader.Read() then
        (reader.GetString(0), getBytesData reader 1, reader.GetDateTime(2))
      else
        null

F# don't allow null to returned

How can i have value return as a tuple or a null?

It is not that F# does not allow you to return null.

It is because then part and else part have different types.

You can use Option type.

let retVal =
  if reader.Read() then
    Some (reader.GetString(0), getBytesData reader 1, reader.GetDateTime(2))
  else
    None

when you use retVal, you use pattern matching:

match retVal with
| Some v -> ...
| None -> // null case

To add some additional information to the Yin Zhu's answer, the situation with null value in the F# language is following:

F# types such as tuples (e.g. int * int), which is exactly your case don't have null as a valid value, so you cannot use null in this case (other such types are function values e.g. int -> int, lists and most of the F# library types)
Types from the .NET framework can have null value, so you can for example write:
```
let (rnd:Random) = null
```
This isn't idiomatic F# style, but it is allowed.
If you define your own F# type, it won't automatically allow you to use null as a valid value of the type (which follows the goal to minimize the use of null in F#). However, you can explicitly allow this:
```
[<AllowNullLiteral>]
type MyType = ...
```

来源：https://stackoverflow.com/questions/2983242/f-how-to-return-have-value-a-tuple-or-null

标签