Jeremy Yallop wrote:
>   module Graph :
>   sig
>     module Mutable :
>     sig
>       type node =         { mutable outgoing : edge list }
>       and  edge =         { from : node ; to_ : node }
>     end
> 
>     module Immutable :
>     sig
>       type node = private { mutable outgoing : edge list }
>       and  edge =         { from : node ; to_ : node }
>     end
> 
>     module Freeze :
>     sig
>       val node : Mutable.node -> Immutable.node
>       val edge : Mutable.edge -> Immutable.edge
>     end
>   end =

I should also mention a drawback of this approach, which is that 
"private" may be stronger than you need.  Values of private record type 
can be neither updated *nor created*; outside the Graph module you 
cannot construct fresh values of Immutable.node, or even use "functional 
update" (i.e. "with" syntax) to construct new values from existing 
Immutable.node values.

Jeremy.

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