What would be the most succinct way to go from
(Rank * Suit) option list -> (Rank * Suit) list option
The reason is, I'm reading from the command line two card hands "4S 4H 4D 4C AS; JS KD QC 10H 2C", so I parse each part of a hand, then return the full hand which obviously could fail.
So if any of the first set of (Rank * Suit) are None then that would shortcircuit and return None (of (Rank * Suit) list)
If they are all Some (of (Rank * Suit) option) then it would return Some (of (Rank * Suit) list)
So I got as far as
type Rank = One | Two | Three
type Suit = Spades | Hearts
type Card = Rank * Suit
[ Some (One, Spades); Some (Two, Hearts); None ]
|> List.map (fun card -> Option.map (fun actualCard -> ) )
And my brain kinda melted... I kinda feel like I need some kind of Option.collect method... or Option.reduce something...
Edit
That last statement got me thinking:
cards
|> Seq.map parseCard
|> Seq.fold (fun agg elem ->
match agg with
| Some l ->
match elem with
| Some card -> Some ( l # [ card ] )
| None -> None
| None -> None ) (Some [])
There must be a more succinct way?
Edit 2
is this it?
cards
|> Seq.map parseCard
|> Seq.fold (fun agg elem ->
agg |> Option.bind ( fun l ->
elem |> Option.map ( fun card ->
l # [ card ]
)
)) (Some [])
The function you're looking for is normally called sequence and it's part of a more generic abstraction called Traversable.
For lists and options you can code it like this:
let sequence t = List.foldBack (Option.map2 (fun x y -> x::y)) t (Some [])
usage
sequence [ Some (One, Spades); Some (Two, Hearts) ]
// val it : (Rank * Suit) list option = Some [(One, Spades); (Two, Hearts)]
sequence [ Some (One, Spades); Some (Two, Hearts); None ]
// val it : (Rank * Suit) list option = None
What about something super simple and easy to understand:
let parseHand cards =
let maybeCards = cards |> List.map parseCard
if maybeCards |> List.contains None
then None
else Some (maybeCards |> List.choose id)
Related
Here is an example:
type Events =
| A of AData
| B of BData
| C of CData
and I have a list of those:
let events : Events list = ...
I need to build a list by event type. Right now I do this:
let listA =
events
|> List.map (fun x ->
match x with
| A a -> Some a
| _ -> None
)
|> List.choose id
and, repeat for each type...
I also thought I could do something like:
let rec split events a b c =
match events with
| [] -> (a |> List.rev, b |> List.rev, c |> List.rev)
| h :: t ->
let a, b, c =
match h with
| A x -> x::a, b, c
| B x -> a, x::b, c
| C x -> a, b, x::c
split t a b c
Is there a more elegant manner to solve this?
This processes a lot of data, so speed is important here.
You can fold back the list of events to avoid writing a recursive function and reversing results. With an anonymous record you will need to define it first and then pipe both arguments ||> to List.foldBack:
let eventsByType =
(events, {| listA = []; listB = []; listC = [] |})
||> List.foldBack (fun event state ->
match event with
| A a -> {| state with listA = a :: state.listA |}
| B b -> {| state with listB = b :: state.listB |}
| C c -> {| state with listC = c :: state.listC |})
With a named record it is more elegant:
{ listA = []; listB = []; listC = [] } |> List.foldBack addEvent events
addEvent is the same as the lambda above except usage of a named record {} instead of {||}.
I think your solution is pretty good, although you do pay a price for reversing the lists. The only other semi-elegant approach I can think of is to unzip a list of tuples:
let split events =
let a, b, c =
events
|> List.map (function
| A n -> Some n, None, None
| B s -> None, Some s, None
| C b -> None, None, Some b)
|> List.unzip3
let choose list = List.choose id list
choose a, choose b, choose c
This creates several intermediate lists, so careful internal use of Seq or Array instead might perform better. You would have to benchmark to be sure.
Test case:
split [
A 1
A 2
B "one"
B "two"
C true
C false
] |> printfn "%A" // [1; 2],[one; two],[true; false]
By the way, your current solution can be simplified to:
let listA =
events
|> List.choose (function A a -> Some a | _ -> None)
If you keep the union cases, you can group the list items like this.
let name = function
| A _ -> "A"
| B _ -> "B"
| C _ -> "C"
let lists =
events
|> List.groupBy name
|> dict
And then you can extract the data you want.
let listA = lists["A"] |> List.map (fun (A data) -> data)
(The compiler doesn't realize the list only consists of "A" cases, so it gives an incomplete pattern match warning😀)
I'm looking for a standard F# function that takes a sequence of 2-choices and returns a pair of sequences:
let separate (choices : seq<Choice<'T1, 'T2>>) : seq<'T1> * seq<'T2> = ...
A naive implementation is pretty simple:
let separate choices =
let ones =
choices
|> Seq.choose (function
| Choice1Of2 one -> Some one
| _ -> None)
let twos =
choices
|> Seq.choose (function
| Choice2Of2 two -> Some two
| _ -> None)
ones, twos
This works fine, but iterates the sequence twice, which is less than ideal. Is this function defined in one of the semi-standard libraries? I looked around, but couldn't find it. (If it exists, I'm sure it goes by some other name.)
For bonus points, versions that work with 3-choices, 4-choices, and so on, would also be nice, as would versions for List, Array, etc. Thanks.
I can't find builtin implementation but can write my own.
It uses IEnumerator<> based approach, so it will work with any collection type but it's not optimal (e.g. arrays will work slower than could be). Order is reversed (easy to fix with ResizeArray but more code). Also this version is not lazy, but can be easily adapted to work with Choice<'a, 'b, 'c> and others
let splitChoices2 (choices: Choice<'a, 'b> seq) =
let rec inner (it: IEnumerator<_>) acc1 acc2 =
if it.MoveNext() then
match it.Current with
| Choice1Of2 c1 -> inner it (c1 :: acc1) acc2
| Choice2Of2 c2 -> inner it acc1 (c2 :: acc2)
else
acc1, acc2
inner (choices.GetEnumerator()) [] []
let choices = [
Choice1Of2 11
Choice2Of2 "12"
Choice1Of2 21
Choice2Of2 "22"
]
choices |> splitChoices2 |> printfn "%A"
Update: ResizeArray based approach without reversed order and potentially less expensive enumeration
let splitChoices2 (choices: Choice<'a, 'b> seq) =
let acc1 = ResizeArray()
let acc2 = ResizeArray()
for el in choices do
match el with
| Choice1Of2 c1 -> acc1.Add c1
| Choice2Of2 c2 -> acc2.Add c2
acc1, acc2
This is sort of inspired by TraverseA but has come out quite different. Here is a single pass solution (UPDATE: however while the core algorithm might be single pass from List to List, but getting it to match your type signature, and ordering the result the same way makes it 3*O(n), it depends how important the ordering and type signature are to you)
let choices = seq {Choice1Of2(1) ; Choice2Of2(2) ; Choice2Of2(3) ; Choice1Of2(4)}
let seperate' choices =
let rec traverse2ChoicesA tupleSeq choices =
match choices with
| [] -> fst tupleSeq |> List.rev |>Seq.ofList , snd tupleSeq |> List.rev |> Seq.ofList
| (Choice1Of2 f)::tl -> traverse2ChoicesA (f::fst tupleSeq, snd tupleSeq) tl
| (Choice2Of2 s)::tl -> traverse2ChoicesA (fst tupleSeq, s::snd tupleSeq) tl
traverse2ChoicesA ([],[]) <| List.ofSeq choices
seperate' choices;;
val seperate' : choices:seq<Choice<'a,'b>> -> seq<'a> * seq<'b>
val it : seq<int> * seq<int> = ([1; 4], [2; 3])
Update: To be clear, if ordering and List instead of Seq are ok then this is a single pass:
let choices = [Choice1Of2(1) ; Choice2Of2(2) ; Choice2Of2(3) ; Choice1Of2(4)]
let seperate' choices =
let rec traverse2ChoicesA (tupleSeq) choices =
match choices with
| [] -> tupleSeq
| (Choice1Of2 f)::tl -> traverse2ChoicesA (f :: fst tupleSeq, snd tupleSeq) tl
| (Choice2Of2 s)::tl -> traverse2ChoicesA (fst tupleSeq, s:: snd tupleSeq) tl
traverse2ChoicesA ([],[]) choices
seperate' choices;;
val choices : Choice<int,int> list =
[Choice1Of2 1; Choice2Of2 2; Choice2Of2 3; Choice1Of2 4]
val seperate' : choices:Choice<'a,'b> list -> 'a list * 'b list
val it : int list * int list = ([4; 1], [3; 2])
You might find something more general, performant and with appropriate type signature in the FSharpPlus "semi-standard" library using TraverseA?
A quick question on how to effectively group/filter list/seq.
Filter for only records where the optional field is not None
Remove the "option" parameter to make future processes easier (as None has been filtered out)
Group (this is of no problem I believe)
Am I using the best approach?
Thanks!
type tmp = {
A : string
B : int option }
type tmp2 = {
A : string
B : int }
let inline getOrElse (dft: 'a) (x: 'a option) =
match x with
| Some v -> v
| _ -> dft
let getGrouped (l: tmp list) =
l |> List.filter (fun a -> a.B.IsSome)
|> List.map (fun a -> {A = a.A ; B = (getOrElse 0 (a.B)) })
|> List.groupBy (fun a -> a.A)
The most natural approach for map+filter when option is involved is to use choose, which combines those two operations and drops the option wrapper from the filtered output.
Your example would look something like this:
let getGrouped (l: tmp list) =
l
|> List.choose (fun a ->
a.B
|> Option.map (fun b -> {A = a.A; B = b})
|> List.groupBy (fun a -> a.A)
The simple solution is just use the property that an option can be transformed to list with one or zero elements then you can define a function like:
let t1 ({A=a; B=b} : tmp) =
match b with
| (Some i) -> [{ A = a; B= i}]
| _ -> []
let getGrouped (l: tmp list) =
l |> List.collect t1
|> List.groupBy (fun a -> a.A)
I'm trying to write a function in F# to get the powersets of a set. So far I have written :
let rec powerset = function
|[] -> [[]]
| [x] -> [[x]; []]
|x::xs -> [x] :: (List.map (fun n -> [x; n]) xs) # powerset xs;;
but this isn't returning the cases that have 3 or more elements, only the pairs, the single elements, and the empty set.
You are on the right track, here is a working solution:
let rec powerset =
function
| [] -> [[]]
| (x::xs) ->
let xss = powerset xs
List.map (fun xs' -> x::xs') xss # xss
See you only have to use this trick:
for each element x you there half of the elements of the powerset will include x and half will not
so you recursively generate the powerset of the remaining elements xss and concat the two parts (List.map (fun xs' -> x::xs') xss will prepend the x to each of those)
But please note that this is not tail recursive and will blow the stack for bigger lists - you can take this idea and try to implement it with seq or make a tail-recursive version if you like
Using seq
Here is a version that uses seq and the bijection between the binary representation of natural numbers (a subset of those) and the subsets of a set (you map the elements to digits and set 1 if the corresponding element is in the subset and 0 if not):
let powerset (xs : 'a seq) : 'a seq seq =
let digits (n : bigint) : bool seq =
Seq.unfold (fun n ->
if n <= 0I
then None
else Some (n &&& 1I = 1I, n >>> 1))
n
let subsetBy (i : bigint) : 'a seq =
Seq.zip xs (digits i)
|> Seq.choose (fun (x,b) -> if b then Some x else None)
seq { 0I .. 2I**(Seq.length xs)-1I }
|> Seq.map subsetBy
this will work for things like powerset [1..100] but it might take a long time to enumerate them all ;) (but it should not take to much memory...)
I've found this question on hubFS, but that handles a splitting criteria based on individual elements. I'd like to split based on a comparison of adjacent elements, so the type would look like this:
val split = ('T -> 'T -> bool) -> 'T list -> 'T list list
Currently, I am trying to start from Don's imperative solution, but I can't work out how to initialize and use a 'prev' value for comparison. Is fold a better way to go?
//Don's solution for single criteria, copied from hubFS
let SequencesStartingWith n (s:seq<_>) =
seq { use ie = s.GetEnumerator()
let acc = new ResizeArray<_>()
while ie.MoveNext() do
let x = ie.Current
if x = n && acc.Count > 0 then
yield ResizeArray.to_list acc
acc.Clear()
acc.Add x
if acc.Count > 0 then
yield ResizeArray.to_list acc }
This is an interesting problem! I needed to implement exactly this in C# just recently for my article about grouping (because the type signature of the function is pretty similar to groupBy, so it can be used in LINQ query as the group by clause). The C# implementation was quite ugly though.
Anyway, there must be a way to express this function using some simple primitives. It just seems that the F# library doesn't provide any functions that fit for this purpose. I was able to come up with two functions that seem to be generally useful and can be combined together to solve this problem, so here they are:
// Splits a list into two lists using the specified function
// The list is split between two elements for which 'f' returns 'true'
let splitAt f list =
let rec splitAtAux acc list =
match list with
| x::y::ys when f x y -> List.rev (x::acc), y::ys
| x::xs -> splitAtAux (x::acc) xs
| [] -> (List.rev acc), []
splitAtAux [] list
val splitAt : ('a -> 'a -> bool) -> 'a list -> 'a list * 'a list
This is similar to what we want to achieve, but it splits the list only in two pieces (which is a simpler case than splitting the list multiple times). Then we'll need to repeat this operation, which can be done using this function:
// Repeatedly uses 'f' to take several elements of the input list and
// aggregate them into value of type 'b until the remaining list
// (second value returned by 'f') is empty
let foldUntilEmpty f list =
let rec foldUntilEmptyAux acc list =
match f list with
| l, [] -> l::acc |> List.rev
| l, rest -> foldUntilEmptyAux (l::acc) rest
foldUntilEmptyAux [] list
val foldUntilEmpty : ('a list -> 'b * 'a list) -> 'a list -> 'b list
Now we can repeatedly apply splitAt (with some predicate specified as the first argument) on the input list using foldUntilEmpty, which gives us the function we wanted:
let splitAtEvery f list = foldUntilEmpty (splitAt f) list
splitAtEvery (<>) [ 1; 1; 1; 2; 2; 3; 3; 3; 3 ];;
val it : int list list = [[1; 1; 1]; [2; 2]; [3; 3; 3; 3]]
I think that the last step is really nice :-). The first two functions are quite straightforward and may be useful for other things, although they are not as general as functions from the F# core library.
How about:
let splitOn test lst =
List.foldBack (fun el lst ->
match lst with
| [] -> [[el]]
| (x::xs)::ys when not (test el x) -> (el::(x::xs))::ys
| _ -> [el]::lst
) lst []
the foldBack removes the need to reverse the list.
Having thought about this a bit further, I've come up with this solution. I'm not sure that it's very readable (except for me who wrote it).
UPDATE Building on the better matching example in Tomas's answer, here's an improved version which removes the 'code smell' (see edits for previous version), and is slightly more readable (says me).
It still breaks on this (splitOn (<>) []), because of the dreaded value restriction error, but I think that might be inevitable.
(EDIT: Corrected bug spotted by Johan Kullbom, now works correctly for [1;1;2;3]. The problem was eating two elements directly in the first match, this meant I missed a comparison/check.)
//Function for splitting list into list of lists based on comparison of adjacent elements
let splitOn test lst =
let rec loop lst inner outer = //inner=current sublist, outer=list of sublists
match lst with
| x::y::ys when test x y -> loop (y::ys) [] (List.rev (x::inner) :: outer)
| x::xs -> loop xs (x::inner) outer
| _ -> List.rev ((List.rev inner) :: outer)
loop lst [] []
splitOn (fun a b -> b - a > 1) [1]
> val it : [[1]]
splitOn (fun a b -> b - a > 1) [1;3]
> val it : [[1]; [3]]
splitOn (fun a b -> b - a > 1) [1;2;3;4;6;7;8;9;11;12;13;14;15;16;18;19;21]
> val it : [[1; 2; 3; 4]; [6; 7; 8; 9]; [11; 12; 13; 14; 15; 16]; [18; 19]; [21]]
Any thoughts on this, or the partial solution in my question?
"adjacent" immediately makes me think of Seq.pairwise.
let splitAt pred xs =
if Seq.isEmpty xs then
[]
else
xs
|> Seq.pairwise
|> Seq.fold (fun (curr :: rest as lists) (i, j) -> if pred i j then [j] :: lists else (j :: curr) :: rest) [[Seq.head xs]]
|> List.rev
|> List.map List.rev
Example:
[1;1;2;3;3;3;2;1;2;2]
|> splitAt (>)
Gives:
[[1; 1; 2; 3; 3; 3]; [2]; [1; 2; 2]]
I would prefer using List.fold over explicit recursion.
let splitOn pred = function
| [] -> []
| hd :: tl ->
let (outer, inner, _) =
List.fold (fun (outer, inner, prev) curr ->
if pred prev curr
then (List.rev inner) :: outer, [curr], curr
else outer, curr :: inner, curr)
([], [hd], hd)
tl
List.rev ((List.rev inner) :: outer)
I like answers provided by #Joh and #Johan as these solutions seem to be most idiomatic and straightforward. I also like an idea suggested by #Shooton. However, each solution had their own drawbacks.
I was trying to avoid:
Reversing lists
Unsplitting and joining back the temporary results
Complex match instructions
Even Seq.pairwise appeared to be redundant
Checking list for emptiness can be removed in cost of using Unchecked.defaultof<_> below
Here's my version:
let splitWhen f src =
if List.isEmpty src then [] else
src
|> List.foldBack
(fun el (prev, current, rest) ->
if f el prev
then el , [el] , current :: rest
else el , el :: current , rest
)
<| (List.head src, [], []) // Initial value does not matter, dislike using Unchecked.defaultof<_>
|> fun (_, current, rest) -> current :: rest // Merge temporary lists
|> List.filter (not << List.isEmpty) // Drop tail element