let rec for nonfunction values
 guttman@m...
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Date:   (:) 
From:  guttman@m... 
Subject:  let rec for nonfunction values 
I don't understand when OCaml will allow me to define nonfunctional values via let rec. Here's a puzzling example. It arose in the course of implementing some algorithms manipulating nondeterministic finite automata. type 'a t = Node of bool * (* Accepting state? *) ('a * 'a t) list * (* Next states for different events *) 'a t list (* Next states after tau (silent event) *) Then the following is accepted by ocaml v. 3: let star_sketch (Node(b, i_0, t_0)) = let rec n = let rec traverse_initial (e,n_1) = e, traverse n_1 and traverse = function Node (true,i,t) > Node (true, List.map traverse_initial i, (n :: (List.map traverse t)))  Node (false,i,t) > Node (false, List.map traverse_initial i, List.map traverse t) in Node (b, List.map traverse_initial i_0, List.map traverse t_0) in n # val star_sketch : 'a t > 'a t = <fun> I know it's not a good definition of Kleene star, because it will loop if the argument already has cycles. But it's syntactically OK. By contrast: let rec star_sketch_2 (Node(b, i_0, t_0)) = let rec n = Node (b, List.map traverse_initial i_0, List.map traverse t_0) and traverse_initial (e,n_1) = e, traverse n_1 and traverse = function Node (true,i,t) > Node (true, List.map traverse_initial i, (n :: (List.map traverse t)))  Node (false,i,t) > Node (false, List.map traverse_initial i, List.map traverse t) in n is syntactically not OK: Characters 59133: This kind of expression is not allowed as righthand side of `let rec' And yet star_sketch_2 seems to be derived from star_sketch by a sound transformation for unnesting let recs. The manual is not terribly informative: The current implementation also supports a certain class of recursive definitions of nonfunctional values, such as let rec name_1 = 1 :: name_2 and name_2 = 2 :: name_1 in expr which binds name_1 to the cyclic list 1::2::1::2::..., and name_2 to the cyclic list 2::1::2::1::... Informally, the class of accepted definitions consists of those definitions where the defined names occur only inside function bodies or as argument to a data constructor. I ask this question because using let rec to define immutable cyclic data is pretty attractive, but clearly one can't write a program in that style if such small differences break things. Many variants were either accepted or rejected in patterns that I found inscrutable. Thanks  Joshua  Joshua D. Guttman <guttman@mitre.org> MITRE, Mail Stop A150 202 Burlington Rd. Tel: +1 781 271 2654 Bedford, MA 017301420 USA Fax: +1 781 271 3816