Recursive call in if expression - ocaml - function

Question

module Dfs = struct
let rec dfslsts g paths final =
let l = PrimePath.removeDuplicates (PrimePath.extendPaths g paths)
in
let f elem =
if (List.mem "%d" (List.flatten final) = false) then (dfslsts g ["%d"] (List.flatten l)::final)
else final
in
List.iter f (Graph.nodes g)
end
Error: This expression has type string but an expression was expected of type int list
This error occurred when I called dfslsts function, which is recursive, inside the if condition.
The function dfslsts returns a list of lists.
If I try to replace the complex expression in if statement to
if (List.mem "%d" (List.flatten final) = false) then "%d"
else "%d"
then I get
Error: This expression has type 'a -> string
but an expression was expected of type 'a -> unit
Type string is not compatible with type unit
at List.iter line.
How do I solve this problem and are we allowed to call a recursive function inside the if expression.
This is the definition of my graph type:
module Graph = struct
exception NodeNotFound of int
type graph = {
nodes : int list;
edges : (int * int) list;
}
let makeGraph () =
{
nodes = [];
edges = [];
}
let rec isNodeOf g n = List.mem n g.nodes
let nodes g = g.nodes
let edges g = g.edges
let addNode g n =
let nodes = n::g.nodes and edges = g.edges in
{
nodes;
edges;
}
let addEdge g (n1, n2) =
if ((isNodeOf g n1) = false) then
raise (NodeNotFound n1)
else if ((isNodeOf g n2) = false) then
raise (NodeNotFound n2)
else
let nodes = g.nodes
and edges = (n1, n2) :: g.edges in
{
nodes;
edges;
}
let nextNodes g n =
let rec findSuccessors edges n =
match edges with
[] -> []
| (n1, n2) :: t ->
if n1 = n then n2::findSuccessors t n
else findSuccessors t n
in
findSuccessors g.edges n
let rec lastNode path =
match path with
[] -> raise (NodeNotFound 0)
| n :: [] -> n
| _ :: t -> lastNode t
end
module Paths = struct
let extendPath g path =
let n = (Graph.lastNode path) in
let nextNodes = Graph.nextNodes g n in
let rec loop path nodes =
match nodes with
[] -> []
| h :: t -> (List.append path [h]) :: (loop path t)
in
loop path nextNodes
let rec extendPaths g paths =
match paths with
[] -> []
| h :: t -> List.append (extendPath g h) (extendPaths g t)
(* Given a list lst, return a new list with all duplicate entries removed *)
let rec removeDuplicates lst =
match lst with
[]
| _ :: [] -> lst
| h :: t ->
let trimmed = removeDuplicates t in
if List.mem h trimmed then trimmed
else h :: trimmed
end

Answer 1

Any expression can be a recursive function call. There are no limitations like that. Your problem is that some types don't match.
I don't see any ints in this code, so I'm wondering where the compiler sees the requirement for an int list. It would help to see the type definition for your graphs.
As a side comment, you almost certainly have a precedence problem with this code:
dfslsts g ["%d"] (List.flatten l)::final
The function call to dfslsts has higher precedence that the list cons operator ::, so this is parsed as:
(dfslsts g ["%d"] (List.flatten l)) :: final
You probably need to parenthesize like this:
dfslsts g ["%d"] ((List.flatten l) :: final)