algorithm - F# syntax with sets and updating -

in f# im trying remove occurence in set if condition met, it's not working way i'd to.

the trick removing elements set function set.filter, takes function argument - filter feed in every value of set function, , add new set if function returns true. example implementation might be:

let filter f (original : set<'t>) =     set [ value in original if f value yield value ] 

which has type filter : ('t -> bool) -> set<'t> -> set<'t>. example of using be

filter (fun x -> x % 2 = 0) (set [ 1; 2; 3; 4; 5 ]) 

this filters set numbers, return value set [ 2; 4 ].

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i'm not entirely sure problem you're having exactly, here solution game mastermind using knuth's algorithm, albeit random starting guess, rather choice of "1122".

i thought quite nice exercise, though writing checkguess function hardest part of me!

you can run test opening in f# interactive running function playmastermind (), show guesses.

/// colours pegs allowed be. type peg = blue | red | green | yellow | purple | brown  /// shared instance of system.random () class random number /// generators. let private rnd = new system.random ()  /// make random set of 4 peg colours. let randomguess () =     let randompeg () =         match rnd.next(1, 6)         | 1 -> blue         | 2 -> red         | 3 -> green         | 4 -> yellow         | 5 -> purple         | 6 -> brown         | _ -> failwith "random number generation failed."     [ randompeg (); randompeg (); randompeg (); randompeg () ]  /// iterate on colours make of possible combinations. let allpossibles =     let colours = [ blue; red; green; yellow; purple; brown]     set [ in colours b in colours c in colours d in colours -> [ a; b; c; d ] ]   /// number of white , black pegs when comparing solution guess. let checkguess solution guess =     /// create map of (colour -> count).     let tomap = list.countby id >> map.oflist     /// compute how many pegs' colours shared in guesses.     let mapintersect map1 map2 =         let overlap peg count =             match map.tryfind peg map2             | none -> 0             | num -> min num count         map.fold (fun acc peg count -> acc + overlap peg count) 0 map1     /// compare see if each peg in correct place.     let blacks = list.map2 (fun x y -> if x = y 1 else 0) solution guess |> list.sum     // number of pegs of right colour wrong location     // same total number of pegs of right colour subtract ones     // in right place.     let whites = mapintersect (tomap solution) (tomap guess) - blacks     whites, blacks  /// random element of set. let randomsetelement set =     let arr = set.toarray set     arr.[rnd.next (array.length arr)]  let playmastermind () =     // creates closure can check our guess against solution,     // without storing actual value of solution.     let checkanswer = checkguess (randomguess ())     let rec loop turncount remaining =         if set.count remaining = 1             let answer = set.maxelement remaining             printfn "the answer %a, calculated in %d turns." answer (turncount - 1)         else             let guess = randomsetelement remaining             let (whites, blacks) = checkanswer guess             printfn "on turn %d guessed %a, gave %d white pins , %d black pins." turncount guess whites blacks             /// remove possibilities solution wouldn't give             /// same numbers of white , black pins , continue.             loop (turncount + 1) (set.filter (fun possible -> (whites, blacks) = checkguess possible guess) remaining)     // play game!     loop 1 allpossibles