Question

I’ve tried the following, which don’t work.

exception MyError<'a> of 'a
exception 'a MyError of 'a

Do I have to use the long form:

type MyError<'a>(value) =
  inherit System.Exception()
  member this.Value : 'a = value

Answer 1

According to the specification, you have to use the long form. I didn’t find any explanation why that’s the case, but the grammar for exception declarations looks like this (and maybe also hints why the behavior is as you described):

exception-defn := attributes_opt exception union-type-case-data

union-type-case-data :=
     ident                                (nullary union case)
     ident of type * … * type   (n-ary union case)
     ident : uncurried-sig        (n-ary union case)

This is quite interesting, because it suggests that exception declarations are more like discriminated union cases than like types. I guess you can think of an exception declaration…

exception MyExn of int

…as a declaration adding new case to the standard System.Exception type (if it was a discriminated union). In this case you wouldn’t expect to be able to use a generic type parameter:

type System.Exception = 
  | ...
  | MyExn of int