function - What am I doing wrong with Set.Fold F# -


colouring problem :

hello, i'm trying implement bool function returns true when color can extended country , false otherwise i'm having trouble working sets cannot pattern match them...

my code :

type country = string;; type chart = set<country*country>;; type colour = set<country>;; type colouring = set<colour>;;  (* how tell 2 countries neghbours.  requires chart.*) let areneighbours ct1 ct2 chart =   set.contains (ct1,ct2) chart || set.contains (ct2,ct1) chart;; (* val areneighbours :   ct1:'a -> ct2:'a -> chart:set<'a * 'a> -> bool when 'a : comparison   *)  (* colour col can extended country ct when no neighbours according chart.*)  let canbeextby (col:colouring) (ct:country) (chart:chart) = col |> set.fold (fun x -> (if (areneighbours x ct chart) true else false))

what expected : need check if ct neighbour of country in col (assuming there countries in col), according neighbourhood defined in chart. if 

chart = set     [("andorra", "benin"); ("andorra", "canada"); ("andorra", "denmark");      ("benin", "canada"); ("benin", "denmark"); ("canada", "denmark");      ("estonia", "canada"); ("estonia", "denmark"); ("estonia", "finland");      ...]

and 

col = set      ["andorra"]

then canbeext should return false when when ct = "benin" or "canada" or "denmark" etc... andorra share border countries , cannot coloured in same colour andora.

obviously have type error in canbeextby i'm trying return bool , it's expecting 'a:colouring. don't know how implement it..

thanks !

what this?

type country      = string type neighbours   = set<country*country> type sharescolour = set<country>  let areneighbours (ns : neighbours) (ct1 : country) (ct2 : country) : bool =   set.contains (ct1,ct2) ns || set.contains (ct2,ct1) ns  let cansharecolour (ns : neighbours) (ct : country) (s : sharescolour) : bool =   s |> seq.exists (areneighbours ns ct) |> not  let neighbours : neighbours =    set [|      ("andorra", "benin")  ; ("andorra", "canada") ; ("andorra", "denmark");     ("benin"  , "canada") ; ("benin"  , "denmark"); ("canada" , "denmark");     ("estonia", "canada") ; ("estonia", "denmark"); ("estonia", "finland");   |]  let sharescolour : sharescolour =   set [|     "andorra"   |]  [<entrypoint>] let main argv =   printfn "%a" <| cansharecolour neighbours "estonia" sharescolour   printfn "%a" <| cansharecolour neighbours "benin"   sharescolour   0

changed names made more sense me. might not agree.


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