Jacques Garrigue wrote:
> From: Mike Hamburg <hamburg@fas.harvard.edu>
>># let b = let f x () = [|x|] in f [];;
>>val b : unit -> '_a list array = <fun>
 >
> but your second fails, because array is not [covariant] (you may
> assign to an array, and you would have to look at the code to see that
> each call to b creates a different array)

Of course, in this case the usual trick of some greek letter-expansion (was
it eta?) works:

# let b () = let f x () = [|x|] in f [] ();;
val b : unit -> 'a list array = <fun>

Maybe this should be mentioned in the "A type variable starts with _?" part of
the FAQ, like:

| In this case the type checker errs on the conservative side, as the function
| map_id could be fully polymorphic. This is caused by the fact that the
| definition is given in the so called eta-reduced form, and you can recover
| the full polymorphism by giving it in eta-expanded form, which the type
| checker handles more gracefully:
|
| # let map_id l = List.map (function x -> x) l;;
| val map_id : 'a list -> 'a list = <fun>

Yours, Florian.
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