From: Jon Harrop <jon@ffconsultancy.com>

> I've got a type error:
> 
> This type of expression, <height:int; width:int; _..> -> unit, contains type 
> variables that cannot be generalized
> 
> which appears with the form:
> 
> let f =
>   let t = ref None in
>   fun data -> ...
> 
> but not in the unnested form:
> 
> let t = ref None
> 
> let f =
>   fun data -> ...
> 
> Is that supposed to happen? I thought those two forms were exactly
> equivalent.  

They are not.
The criterion to decide whether to generalize type variables in a
definition is whether it is syntactically a "value" or not.
Since [ref None] is not a value, your first definition ends up not
being a value, and cannot be generalized.
The second one is ok, because probably t is later constrained to have
a specific type (not containing type variables) and f is now a "value"
and can be properly generalized.
Note also that this error message does not occur if you provide an
interface for your module. But you may in place have a message about a
type not being included in another, if you really expected the
function to be polymorphic.

Jacques Garrigue