> why 
> 
> let _ = List.map printf (["abc\n"; "def\n"]:('a,'b,'c) format list)
> 
> work properly, but:
> 
>  let _ = List.map printf ["abc\n"; "def\n"]
> 
> doesn't compiled with message
> 
>  This expression has type ('a, out_channel, unit) format -> 'a
>  but is here used with type string -> 'b
> 
> I know, O'Caml format type is a hack, but I can't undertand difference 
> between thees two expressions for the type inference algorighm.

You have the answer: this is a hack, and there are no guarantees
anything will type (or have the expected side-effects) if you do not
follow the documentation.

If you understand how the hack works, then you can write the
following (which does not seem very useful):

# let map_printf l = List.map Printf.printf l;;
val map_printf : ('a, out_channel, unit) format list -> 'a list = <fun>
# map_printf ["abc"; "def"];;
abcdef- : unit list = [(); ()]

        Jacques Garrigue
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