Anything *else* similar to Haskell's $ in F# other than <|? - f#

I know there's the back-pipe (<|) operator, referenced in several other SO answers. But that doesn't work well when combined with forward pipes (|>), which is common in chaining. However I'm looking for related options. Basically is there any built-in version of the below function definition? Or is this a bad/dangerous practice?
let inline (^%) f = f
let stuff =
[1;2;3]
|> Seq.filter ^% (>) 2
|> Seq.map ^% fun x -> x.ToString()
// compare to this, which doesn't compile (and would be hard to follow even if it did)
let stuff =
[1;2;3]
|> Seq.filter <| (>) 2
|> Seq.map <| fun x -> x.ToString()

There are some Haskell features, like optional infixing using backticks, and sections, which aren't available in F#. That makes certain constructs a bit more verbose.
Usually, I'd simply write a pipe of functions as the above like this:
let stuff =
[1;2;3]
|> Seq.filter (fun x -> x < 2)
|> Seq.map string
This is, in my opinion, much more readable. For example, using Seq.filter ^% (>) 2, I'd intuitively read that as meaning 'all values greater than 2', but that's not what it does:
> let inline (^%) f = f;;
val inline ( ^% ) : f:'a -> 'a
> let stuff =
[1;2;3]
|> Seq.filter ^% (>) 2
|> Seq.map ^% fun x -> x.ToString()
|> Seq.toList;;
val stuff : string list = ["1"]
If you leave the reader of the code in doubt of what the code does, you've just made everyone less productive. Using Seq.filter (fun x -> x < 2) may look more verbose, but is unambiguous to the reader.

Related

Better way to sum rows

I'm learning F# (again) and I'm trying to sum some rows in excel. This is my attempt.
let sumRows (source: seq<double[]>) =
source
|> Seq.reduce (fun a b -> (a, b) ||> Seq.map2 (fun x y -> x + y) |> Seq.toArray)
Can it be done better? I already discovered double forward pipe operator, but now, whole part fun a b -> (a, b) ||> seems quite redundant...
You are right it is redundant, because the double pipe operator is convenient only when you need to convert a tuple into two separate parameters. In this case you already have them as 2 separate parameters so you could just pass them like this:
let sumRows (source: seq<double[]>) =
source
|> Seq.reduce (fun a b -> Seq.map2 (fun x y -> x + y) a b |> Seq.toArray)
We can get rid of the |> Seq.toArray by replacing Seq.map2 with Array.map2:
let sumRows (source: seq<double[]>) =
source
|> Seq.reduce (fun a b -> Array.map2 (fun x y -> x + y) a b)
now we can simplify further by removing the fun a b ->:
let sumRows (source: seq<double[]>) =
source
|> Seq.reduce (Array.map2 (fun x y -> x + y) )
Finally, did you know that an operator like + can be treated as a 2 parameters function
by putting in parenthesis (+)?
let sumRows2 (source: seq<double[]>) =
source
|> Seq.reduce (Array.map2 (+) )
All of this versions have the same signature and they are all correct. You choose whichever suits more your style.
BTW, you maybe tempted to go one further and do this:
let sumRows2 = Seq.reduce (Array.map2 (+) )
but it causes issues with the famous Value restriction error. There are workarounds like adding the type annotation or actually using it somewhere in the code, but the best workaround is to add the parameter, like we had before.

GroupBy Year then take Pairwise diffs except for the head value then Flatten Using Deedle and F#

I have the following variable:
data:seq<(DateTime*float)>
and I want to do something like the following F# code but using Deedle:
data
|> Seq.groupBy (fun (k,v) -> k.Year)
|> Seq.map (fun (k,v) ->
let vals = v |> Seq.pairwise
let first = seq { yield v |> Seq.head }
let diffs = vals |> Seq.map (fun ((t0,v0),(t1,v1)) -> (t1, v1 - v0))
(k, diffs |> Seq.append first))
|> Seq.collect snd
This works fine using F# sequences but I want to do it using Deedle series. I know I can do something like:
(data:Series<DateTime*float>) |> Series.groupBy (fun k v -> k.Year)...
But then I need to take the within group year diffs except for the head value which should just be the value itself and then flatten the results into on series...I am bit confused with the deedle syntax
Thanks!
I think the following might be doing what you need:
ts
|> Series.groupInto
(fun k _ -> k.Month)
(fun m s ->
let first = series [ fst s.KeyRange => s.[fst s.KeyRange]]
Series.merge first (Series.diff 1 s))
|> Series.values
|> Series.mergeAll
The groupInto function lets you specify a function that should be called on each of the groups
For each group, we create series with the differences using Series.diff and append a series with the first value at the beginning using Series.merge.
At the end, we get all the nested series & flatten them using Series.mergeAll.

How do I do in F# what would be called compression in APL?

In APL one can use a bit vector to select out elements of another vector; this is called compression. For example 1 0 1/3 5 7 would yield 3 7.
Is there a accepted term for this in functional programming in general and F# in particular?
Here is my F# program:
let list1 = [|"Bob"; "Mary"; "Sue"|]
let list2 = [|1; 0; 1|]
[<EntryPoint>]
let main argv =
0 // return an integer exit code
What I would like to do is compute a new string[] which would be [|"Bob"; Sue"|]
How would one do this in F#?
Array.zip list1 list2 // [|("Bob",1); ("Mary",0); ("Sue",1)|]
|> Array.filter (fun (_,x) -> x = 1) // [|("Bob", 1); ("Sue", 1)|]
|> Array.map fst // [|"Bob"; "Sue"|]
The pipe operator |> does function application syntactically reversed, i.e., x |> f is equivalent to f x. As mentioned in another answer, replace Array with Seq to avoid the construction of intermediate arrays.
I expect you'll find many APL primitives missing from F#. For lists and sequences, many can be constructed by stringing together primitives from the Seq, Array, or List modules, like the above. For reference, here is an overview of the Seq module.
I think the easiest is to use an array sequence expression, something like this:
let compress bits values =
[|
for i = 0 to bits.Length - 1 do
if bits.[i] = 1 then
yield values.[i]
|]
If you only want to use combinators, this is what I would do:
Seq.zip bits values
|> Seq.choose (fun (bit, value) ->
if bit = 1 then Some value else None)
|> Array.ofSeq
I use Seq functions instead of Array in order to avoid building intermediary arrays, but it would be correct too.
One might say this is more idiomatic:
Seq.map2 (fun l1 l2 -> if l2 = 1 then Some(l1) else None) list1 list2
|> Seq.choose id
|> Seq.toArray
EDIT (for the pipe lovers)
(list1, list2)
||> Seq.map2 (fun l1 l2 -> if l2 = 1 then Some(l1) else None)
|> Seq.choose id
|> Seq.toArray
Søren Debois' solution is good but, as he pointed out, but we can do better. Let's define a function, based on Søren's code:
let compressArray vals idx =
Array.zip vals idx
|> Array.filter (fun (_, x) -> x = 1)
|> Array.map fst
compressArray ends up creating a new array in each of the 3 lines. This can take some time, if the input arrays are long (1.4 seconds for 10M values in my quick test).
We can save some time by working on sequences and creating an array at the end only:
let compressSeq vals idx =
Seq.zip vals idx
|> Seq.filter (fun (_, x) -> x = 1)
|> Seq.map fst
This function is generic and will work on arrays, lists, etc. To generate an array as output:
compressSeq sq idx |> Seq.toArray
The latter saves about 40% of computation time (0.8s in my test).
As ildjarn commented, the function argument to filter can be rewritten to snd >> (=) 1, although that causes a slight performance drop (< 10%), probably because of the extra function call that is generated.

F# Pipelines access data from pipeline stages above

I have written a function like this
let GetAllDirectAssignmentsforLists (spWeb : SPWeb) =
spWeb.Lists
|> Seq.cast<SPList>
|> Seq.filter(fun l -> l.HasUniqueRoleAssignments)
|> Seq.collect (fun l -> l.RoleAssignments
|> Seq.cast<SPRoleAssignment>
|> Seq.map(fun ra -> ra.Member)
)
|> Seq.filter (fun p -> p.GetType().Name = "SPUser")
|> Seq.map(fun m -> m.LoginName.ToLower())
I want to return a tuple which contains the list name (taken from l.Title) in the send pipe and the m.LoginName.ToLower().
Is there a cleanway for me to get something from the above pipe elements?
One way ofcourse would be to tuple the return value in the 2nd stage of the pipe and then pass the Title all the way down.... but that would pollute the code all subsequent stages will then have to accept and return tuple values just for the sake of the last stage to get the value.
I wonder if there is a clean and easy way....
Also, in stage 4 of the pipeline (fun p -> p.GetType().Name = "SPUser") could i use if here to compare the types? rather than convert the typename to string and then match strings?
We exploit the fact that Seq.filter and Seq.map can be pushed inside Seq.collect without changing the results. In this case, l is still available to access.
And the last filter function is more idiomatic to use with type test operator :?.
let GetAllDirectAssignmentsforLists(spWeb: SPWeb) =
spWeb.Lists
|> Seq.cast<SPList>
|> Seq.filter (fun l -> l.HasUniqueRoleAssignments)
|> Seq.collect (fun l -> l.RoleAssignments
|> Seq.cast<SPRoleAssignment>
|> Seq.map (fun ra -> ra.Member)
|> Seq.filter (fun p -> match box p with
| :? SPUser -> true
| _ -> false)
|> Seq.map (fun m -> l.Title, m.LoginName.ToLower()))
To simplify further, you could change the series of Seq.map and Seq.filter to Seq.choose:
Seq.choose (fun ra -> match box ra.Member with
| :? SPUser -> Some (l.Title, ra.Member.LoginName.ToLower())
| _ -> None)
While you can solve the problem by lifting the rest of the computation inside collect, I think that you could make the code more readable by using sequence expressions instead of pipelining.
I could not run the code to test it, but this should be equivalent:
let GetAllDirectAssignmentsforLists (spWeb : SPWeb) = seq {
// Corresponds to your 'filter' and 'collect'
for l in Seq.cast<SPList> spWeb.Lists do
if l.HasUniqueRoleAssignments then
// Corresponds to nested 'map' and 'filter'
for ra in Seq.cast<SPRoleAssignment> l.RoleAssignments do
let m = ra.Member
if m.GetType().Name = "SPUser" then
// This implements the last 'map' operation
yield l.Title, m.LoginName.ToLower() }
The code above corresponds more closely to the version by #pad than to your original code, because the rest of the computation is nested under for (which corresponds to nesting under collect) and so you can see all variables that are already in scope - like l which you need.
The nice thing about sequence expressions is that you can use F# constructs like if (instead of filter), for (instead of collect) etc. Also, I think it is more suitable for writing nested operations (which you need here to keep variables in scope), because it remains quite readable and keeps familiar code structure.

printfn in pipeline

So I have a function SolveEquasion that returns a pair float*float[]. What is the best way to print the number and the array and continue working with the array? I made the following code but it seems there is a better way
...
|> SolveEquasion
|> (fun (det, solution) -> printfn "Determinant = %f\nSolution = %A" det (Array.toList solution), solution )
|> snd
I don't think your solution can improved if you want to do this in a pipeline. Another approach is to use a let binding, along with splitting up the pipelined operations, to avoid having a function that acts like the love child of map and iter.
let (det, solution) = SolveEquasion
printfn "Determinant = %f\nSolution = %A" det (Array.toList solution)
//do something else with solution
I think the original solution is fine, and we can improve its clarity by giving your anonymous function the name I've seen it given in some other libraries based around pipelining higher-order functions: tap.
let tap f x =
f x
x
(1.0, [| 2.0; 3.0 |])
|> tap (fun (s, a) -> printfn "%A %A" s a)
|> snd
Well, for one thing you can skip the use of snd by returning a single value rather than a tuple from the previous function:
...
|> SolveEquasion
|> (fun (det, solution) ->
printfn "Determinant = %f\nSolution = %A" det (Array.toList solution)
solution )
I'd probably use Daniel's approach and just assign the value you want to print to a symbol using let. Alternatively, you could define a variant of printf that takes some arguments and returns one of them. I'm not sure if there is a general scheme how this should be done - for your example it would take a two-element tuple:
let mprintf fmt (a, b) =
Printf.kprintf (fun s -> printf "%s" s; (a, b)) fmt a b
Then you can write:
...
|> SolveEquasion
|> mprintfn "Determinant = %f\nSolution = %A"
|> snd |> // ... more stuff with solution

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