Question

I was faced today with the following DUs declarations:

type Grammar = Definition list

and  Definition = Def of string * Expression

and  Range =
     | Char  of char
     | Range of char * char

Why would one use the keyword and instead of type, here?

Answer 1

The and is needed for the definitions of Grammar and Definition to compile correctly. The Grammar type is listed first but depends on the type Definition which is defined later. In order to compile properly it must be linked with and which tells the F# compiler the type definitions are dependent / related.

There is no reason for Range to be declared in such a way and should be declared with type