Re: [Caml-list] recursive records

[Andreas Rossberg <rossberg@mpi-sws.org>]

On Nov 8, 2013, at 21:10 , Brigitte Pientka <bpientka@cs.mcgill.ca> wrote:
> I am playing around with using recursive records to define observations about
> streams simulating essentially the ideas described in "Copatterns:Programming infinite structures by observations" presented at POPL'13.
> 
> type 'a susp = Susp of (unit -> 'a)
> 
> type 'a str = {hd: 'a  ; tl : ('a str) susp}
> 
> let rec ones = {hd = 1 ; tl = Susp (fun () -> ones)}
> 
> This works fine and many examples can be elegantly written this way.  However,
> when I define the stream ones via the function delay, OCaml fails.
> 
> let delay f = Susp f
> 
> let rec ones = {hd = 1 ; tl = delay (fun () -> ones)};;
> Characters 15-53:
>  let rec ones = {hd = 1 ; tl = delay (fun () -> ones)};;
>                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Error: This kind of expression is not allowed as right-hand side of `let rec'
> 
> Could someone explain why this fails?

Roughly speaking, let rec only allows syntactic values as right-hand sides (to ease implementation and to avoid issues with potential observable side effects during tying of the recursive knot). An application like the one of delay is not a value. If you replace it by a direct application of the Susp constructor it should work.

/Andreas