Declaring internal types in F#

Question

Recently I came across a code listing like so:

type MyType =
  | MyType of int

and internal MyInternal =
  | One of MyType
  | Two of MyType

I am not familiar with the usage of 'and internal' (on the third line) and I was wondering if there was any difference between using 'type internal' like this:

type MyType =
  | MyType of int

type internal MyInternal =
  | One of MyType
  | Two of MyType

I have experimented with both forms briefly, and I can't see a difference. Is this just two different ways to write the same thing?

Answer 1

In this instance, the and keyword would be used to define mutally recursive types MyType and MyInternal. Since MyType and MyInternal are not mutually recursive, the and is not required.

As the MSDN page states:

The and keyword replaces the type keyword on all except the first definition

so the definitions are equivalent.

Answer 2

No, your suggested code is definitely more idiomatic.

type T = ...
and U = ...

is typically only used if the types are mutually recursive. Using it with an internal type would be even more strange, since the constructors of the first type would also need to be made internal:

type T = internal | T of U
and internal U = | U of T