F# won't accept type abbreviations that don't use all of the declared type parameters in the type being abbreviated, whereas OCaml will.

For example, OCaml will accept type ('a, 'b) t = 'a, whereas F# will result in an FS0035 error.

These types come up frequently, since F* allows this style of type definition.

In FStar.Pervasives,

let st_pre_h  (heap:Type)          = heap -> GTot Type0
let st_post_h (heap:Type) (a:Type) = a -> heap -> GTot Type0
let st_wp_h   (heap:Type) (a:Type) = st_post_h heap a -> Tot (st_pre_h heap)

is extracted to

type 'Aheap st_pre_h =
'Aheap  ->  obj
type ('Aheap, 'Aa) st_post_h =
'Aa  ->  'Aheap  ->  obj
type ('Aheap, 'Aa) st_wp_h =
Prims.unit  ->  'Aheap st_pre_h

Since 'Aa appears as a parameter of st_wp_h but not in the type abbreviation that it is defined by, the F# compiler throws FS0035.

A workaround is to use single constructor DUs when defining types like this.