error FS0001: This expression was expected to have type 'cexp' but here has type 'Label'

Question

Sorry for the long code. I've been trying to make a function that takes in a string representation of a GCL program and produces a string representation of a program graph. I can't seem to get the compilerto read the e.label in this line as cexp (since the type Label is composed of cexp or bexp )

let edge2dot(e: Edge) : string =
    e.source + " -> " + e.target + "[label=" + prettyPrint(e.label) + "]" + ";"

and was wondering if there is a way to do that?

module Graph
open Types
open Parse

open Parser
open FSharp.Text.Lexing
open System
open AST





type Input = { 
    determinism: Determinism
    }

type Output = { dot: string }

type Node = string
type Label = C of cexp | B of bexp | S
type Edge = {
    source : Node;
    label : Label;
    target : Node;
    }



let rec edgesC(ast: cexp, qS:Node,qF:Node): List<Edge>  = 
    match ast with
        | Skip -> [{source =qS ; label = S ; target=qF}]
        | Assign(_,_) -> [{source = qS; label = C ast; target =qF}]
      //  | C (c1,c2) -> edgesC(c1, qS, qF) @ edgesC(c2,qS,qF)
        | If gc -> edgesGC(gc,qS,qF) 
        | Do gc ->  edgesGC(gc,qS,qF) 
        

and edgesGC (ast, qS, qF) = 
    match ast with 
        | Then(b,c) ->  [{source =qS ; label = B b ; target=qF}] @ [{source =qS ; label = C c ; target=qF}]
        | GC(gc1,gc2) -> edgesGC(gc1,qS,qF) @ edgesGC(gc2,qS,qF)



let ast2pg(ast): List<Edge>  = 
    edgesC(ast,"q0","qf")



let edge2dot(e: Edge) : string =
    e.source + " -> " + e.target + "[label=" + prettyPrint(e.label) + "]" + ";"


let rec edges2dot (pg : List<Edge>): string = 
    match pg with
        | [] -> ""
        | e::pg -> edge2dot(e) + edges2dot(pg)



let pg2dot (pg : List<Edge>): string = 
   "digraph program_graph { rankdir=LR;" + edges2dot(pg)+ " }"