Type mismatch with Async in F# - f#

I've just started messing around with F# and am trying to do some basic parallel computation to get familiar with the language. I'm having trouble with type mismatches. Here's an example:
let allVariances list =
seq {
for combination in allCombinations list do
yield (combination, abs(targetSum - List.sum combination))
}
let compareVariance tup1 tup2 =
if snd tup1 < snd tup2 then
tup1
else
tup2
let aCompareVariance tup1 tup2 =
async { return compareVariance tup1 tup2 }
let matchSum elements targetSum =
allVariances elements
|> Seq.reduce aCompareVariance
|> Async.Parallel
|> Async.RunSynchronously
So, "allVariances elements" produces a seq<float list * float>. CompareVariance takes two of those <float list * float> tuples and returns the one with the smaller second item (variance). My goal is to use Reduce to end up with the tuple with the smallest variance. However, I get a type mismatch on the aCompareVariance argument:
Error 1 Type mismatch. Expecting a float list * float -> float list * float -> float list * float but given a float list * float -> float list * float -> Async<float list * float> The type 'float list * float' does not match the type 'Async<float list * float>'
It seems like the Async return type isn't accepted by Reduce?

Seq.reduce takes a function and a sequence and reduces the list using the function. That is, the outcome of reducing a sequence {a1;a2;a3;...;an} with function f will be f(f(...f(f(a1,a2),a3),...),an))...). However, the function you're passing can't be applied this way, because the return type (Async<float list * float>) doesn't match the argument types (float list * float). What exactly are you trying to achieve?
Also, keep in mind that async computations are great for asynchronous work, but not ideal for parallel work. See F#: Asynch and Tasks and PLINQ, oh my! and Task Parallel Library vs Async Workflows.
EDIT
Here's one way to write a function which will reduce items more like you expected, operating sort of like this:
[|a1; a2; a3; a4|]
[|f a1 a2; f a3 a4|]
f (f a1 a2) (f a3 a4)
At each stage, all applications of f will take place in parallel. This uses Async.Parallel, which as mentioned above is probably less appropriate than using the Task Parallel Library (and may even be slower than just doing a normal synchronous Array.reduce). As such, just consider this to be demonstration code showing how to piece together the async functions.
let rec reduce f (arr:_[]) =
match arr.Length with
| 0 -> failwith "Can't reduce an empty array"
| 1 -> arr.[0]
| n ->
// Create an array with n/2 tasks, each of which combines neighboring entries using f
Array.init ((n+1)/2)
(fun i ->
async {
// if n is odd, leave last item alone
if n = 2*i + 1 then
return arr.[2*i]
else
return f arr.[2*i] arr.[2*i+1]})
|> Async.Parallel |> Async.RunSynchronously
|> reduce f
Note that converting items to and from Async values all happens internally to this function, so it has the same type as Array.reduce and would be used with your normal compareVariance function rather than with aCompareVariance.

Related

List.map with two function parameters, in F#

I came across this syntax:
[10.; 11.; 12.]
|> List.map (fun a b -> a * b)
in what case does List.map have two parameters (fun a b)?
This is the output in Jupyter Lab:
That's a fun example, because it looks confusing and usually doesn't come up in practice, so it caught me off-guard initially.
In short, what happens is you're mapping a list of floats into a list of float -> float functions. It's easier to see if you rewrite it as something like this:
let results =
[10.; 11.; 12.]
|> List.map (fun a -> (fun b -> a * b))
You pass a float -> float -> float function as the mapper, and it gets partially applied to the element from the input list. The second argument b is not being applied though, so the output of the mapping is a function that takes a float and multiplies it by the partially applied element of the list, and the overall result is a list of float -> float functions.
You can then apply those functions to some value like this:
results
|> List.map (fun f -> f 2.)

How to avoid changing parameter order

I am currently doing some of the excercises from exercism.io. One of the excercises is summing up all numbers in a sequence that are a multiple of one or more numbers from a different sequence. Splitting the problem into smaller functions seemed like a good idea and i came up with this:
let multipleOf m n =
n % m = 0
let anyMultipleOf (m: int list) n =
m
|> Seq.exists (multipleOf n)
let sumOfMultiples m n =
[1..n-1]
|> Seq.filter (anyMultipleOf m)
|> Seq.sum
The idea being, that i can use partial application to "bake in" the m parameter into my (any)multipleOf functions. But this code doesn't work the way i want it to, because Seq.exists (multipleOf n) actually applies n as my m parameter.
How can i refactor this code without having to reverse the parameter order of my multipleOf function?
Note: I want a solution that uses my multipleOf function inside my anyMultipleOf function. This solution works, but doesn't reuse my first function:
let anyMultipleOf (m: int list) n =
m
|> Seq.exists (fun x -> n % x = 0)
I did type in a suggestion to use flip, but the obvious thing to do is this:
let anyMultipleOf (m: int list) n =
m
|> Seq.exists (fun x -> multipleOf x n)
flip is a nice tool to have, but pipelines of flipped functions are painful to read.
While it's unclear to me why you don't redefine anyMultipleOf to take the the list as the last argument, you can always use flip:
let flip f x y = f y x
This function exists in Haskell, but not in FSharp.Core, which is the reason you'd have to define it yourself.
As an example, flip anyMultipleOf returns a function with the type int -> int list -> bool, which, if I understand the question correctly, is what you want.
You can define yourself a function which just do that :
Takes a function and 2 arguments in reversed order and return the result of applying the arguments in the right order to the function
let flip f y x = f x y

"summing" functions in F#

I have a list of functions in F# which are all of type (float -> float -> float -> float). I want to do some kind of fold on the sequence to get a single function which returns the sum of all of the functions.
For instance, I could pass the values 1.0, 2.0, and 3.0 to every function in the list, and get a return value from each one. Then I could compute the sum of all of these values. However, I want to generalize this.
I know how to do this recursively, but I feel like it should be doable in one line. Is there a concise way to accomplish this task?
The solution by #Lee is a one liner you're looking for. If you wanted to save a few characters, you can use List.sumBy which first applies a given function to an element of the list (similar to List.map) and then sums the result (just like List.sum):
let sumAll (fs:(_ -> _ -> _ -> float) list) a b c =
List.sumBy (fun f -> f a b c) fs
Both this and Lee's version uses type annotations to specify that the functions in the list return float. This is needed, because otherwise the compiler does not know what kind of numbers you want to sum using List.sum (floats, integers, etc.). This ambiguity needs to be resolved to compile the function.
Alternatively, you could mark the function as inline and then it would be inlined when you call it (and it would work for multiple different numeric types). You can also pass the fs parameter as the last one and use partial function application:
let inline sumAll a b c = List.sumBy (fun f -> f a b c)
Now you can call it using pipelining as follows: fs |> sumAll 1 2 3.
let sumAll (fs: (float -> float -> float -> float) list) a b c = List.map (fun f -> f a b c) fs |> Seq.sum
The answers by #Lee and #Tomas are great, but there is a shorter way.
If you can afford passing (a, b, c) as a triple upon invocation:
let inline sumAll() = (|||>) >> List.sumBy
// usage
let predicates =
[
fun a b c -> a
fun a b c -> b * 42.0 - c
]
let ret1 = predicates |> sumAll()(1.0, 2.0, 3.0)
It will be also generic:
let predicates2 =
[
fun a b c -> c - 10
fun a b c -> a + c * 42
]
let ret2 = predicates2 |> sumAll()(1, 2, 3)
A more readable way which supports curried arguments:
let sumAllCurried a b c = (a,b,c) |> (|||>) |> List.sumBy<_, float>
// usage
let ret3 = predicates |> sumAllCurried 1.0 2.0 3.0
Note, I'm using a type parameter on List.sumBy since it looks shorter than typing an entire type specification for f.

Converting a loop to pure functions

I have this code written for a Project Euler problem in c++:
int sum = 0;
for(int i =0; i < 1000; i++)
{
//Check if multiple of 3 but not multiple of 5 to prevent duplicate
sum += i % 3 == 0 && i % 5 != 0 ? i: 0;
//check for all multiple of 5, including those of 3
sum += i % 5 == 0 ? i: 0;
}
cout << sum;
I'm trying to learn f# and rewriting this in f#. This is what I have so far:
open System
//function to calculate the multiples
let multiple3v5 num =
num
//function to calculate sum of list items
let rec SumList xs =
match xs with
| [] -> 0
| y::ys -> y + SumList ys
let sum = Array.map multiple3v5 [|1 .. 1000|]
What I have may be complete nonsense...so help please?
Your sumList function is a good start. It already iterates (recursively) over the entire list, so you don't need to wrap it in an additional Array.map. You just need to extend your sumList so that it adds the number only when it matches the specified condition.
Here is a solution to a simplified problem - add all numbers that are divisible by 3:
open System
let rec sumList xs =
match xs with
| [] -> 0 // If the list is empty, the sum is zero
| y::ys when y % 3 = 0 ->
// If the list starts with y that is divisible by 3, then we add 'y' to the
// sum that we get by recursively processing the rest of the list
y + sumList ys
| y::ys ->
// This will only execute when y is not divisible by 3, so we just
// recursively process the rest of the list and return
/// that (without adding current value)
sumList ys
// For testing, let's sum all numbers divisble by 3 between 1 and 10.
let sum = sumList [ 1 .. 10 ]
This is the basic way of writing the function using explicit recursion. In practice, the solution by jpalmer is how I'd solve it too, but it is useful to write a few recursive functions yourself if you're learning F#.
The accumulator parameter mentioned by sashang is a more advanced way to write this. You'll need to do that if you want to run the function on large inputs (which is likely the case in Euler problem). When using accumulator parameter, the function can be written using tail recursion, so it avoids stack overflow even when processing long lists.
The idea of a accumulator-based version is that the function takes additional parameter, which represents the sum calculated so far.
let rec sumList xs sumSoFar = ...
When you call it initially, you write sumList [ ... ] 0. The recursive calls will not call y + sumList xs, but will instead add y to the accumulator and then make the recursive call sumList xs (y + sumSoFar). This way, the F# compiler can do tail-call optimization and it will translate code to a loop (similar to the C++ version).
I'm not sure if translating from an imperative language solution is a good approach to developing a functional mindset as instrument (C++ in your case) had already defined an (imperative) approach to solution, so it's better sticking to original problem outlay.
Overall tasks from Project Euler are excellent for mastering many F# facilities. For example, you may use list comprehensions like in the snippet below
// multipleOf3Or5 function definition is left for your exercise
let sumOfMultiples n =
[ for x in 1 .. n do if multipleOf3Or5 x then yield x] |> List.sum
sumOfMultiples 999
or you can a bit generalize the solution suggested by #jpalmer by exploiting laziness:
Seq.initInfinite id
|> Seq.filter multipleOf3Or5
|> Seq.takeWhile ((>) 1000)
|> Seq.sum
or you may even use this opportunity to master active patterns:
let (|DivisibleBy|_) divisior num = if num % divisor = 0 the Some(num) else None
{1..999}
|> Seq.map (fun i ->
match i with | DivisibleBy 3 i -> i | DivisibleBy 5 i -> i | _ -> 0)
|> Seq.sum
All three variations above implement a common pattern of making a sequence of members with sought property and then folding it by calculating sum.
F# has many more functions than just map - this problem suggests using filter and sum, my approach would be something like
let valid n = Left as an exercise
let r =
[1..1000]
|> List.filter valid
|> List.sum
printfn "%i" r
I didn't want to do the whole problem, but filling in the missing function shouldn't be too hard
This is how you turn a loop with a counter into a recursive function. You do this by passing an accumulator parameter to the loop function that holds the current loop count.
For example:
let rec loop acc =
if acc = 10 then
printfn "endloop"
else
printfn "%d" acc
loop (acc + 1)
loop 0
This will stop when acc is 10.

Return value in F# - incomplete construct

I've trying to learn F#. I'm a complete beginner, so this might be a walkover for you guys :)
I have the following function:
let removeEven l =
let n = List.length l;
let list_ = [];
let seq_ = seq { for x in 1..n do if x % 2 <> 0 then yield List.nth l (x-1)}
for x in seq_ do
let list_ = list_ # [x];
list_;
It takes a list, and return a new list containing all the numbers, which is placed at an odd index in the original list, so removeEven [x1;x2;x3] = [x1;x3]
However, I get my already favourite error-message: Incomplete construct at or before this point in expression...
If I add a print to the end of the line, instead of list_:
...
print_any list_;
the problem is fixed. But I do not want to print the list, I want to return it!
What causes this? Why can't I return my list?
To answer your question first, the compiler complains because there is a problem inside the for loop. In F#, let serves to declare values (that are immutable and cannot be changed later in the program). It isn't a statement as in C# - let can be only used as part of another expression. For example:
let n = 10
n + n
Actually means that you want the n symbol to refer to the value 10 in the expression n + n. The problem with your code is that you're using let without any expression (probably because you want to use mutable variables):
for x in seq_ do
let list_ = list_ # [x] // This isn't assignment!
list_
The problematic line is an incomplete expression - using let in this way isn't allowed, because it doesn't contain any expression (the list_ value will not be accessed from any code). You can use mutable variable to correct your code:
let mutable list_ = [] // declared as 'mutable'
let seq_ = seq { for x in 1..n do if x % 2 <> 0 then yield List.nth l (x-1)}
for x in seq_ do
list_ <- list_ # [x] // assignment using '<-'
Now, this should work, but it isn't really functional, because you're using imperative mutation. Moreover, appending elements using # is really inefficient thing to do in functional languages. So, if you want to make your code functional, you'll probably need to use different approach. Both of the other answers show a great approach, although I prefer the example by Joel, because indexing into a list (in the solution by Chaos) also isn't very functional (there is no pointer arithmetic, so it will be also slower).
Probably the most classical functional solution would be to use the List.fold function, which aggregates all elements of the list into a single result, walking from the left to the right:
[1;2;3;4;5]
|> List.fold (fun (flag, res) el ->
if flag then (not flag, el::res) else (not flag, res)) (true, [])
|> snd |> List.rev
Here, the state used during the aggregation is a Boolean flag specifying whether to include the next element (during each step, we flip the flag by returning not flag). The second element is the list aggregated so far (we add element by el::res only when the flag is set. After fold returns, we use snd to get the second element of the tuple (the aggregated list) and reverse it using List.rev, because it was collected in the reversed order (this is more efficient than appending to the end using res#[el]).
Edit: If I understand your requirements correctly, here's a version of your function done functional rather than imperative style, that removes elements with odd indexes.
let removeEven list =
list
|> Seq.mapi (fun i x -> (i, x))
|> Seq.filter (fun (i, x) -> i % 2 = 0)
|> Seq.map snd
|> List.ofSeq
> removeEven ['a'; 'b'; 'c'; 'd'];;
val it : char list = ['a'; 'c']
I think this is what you are looking for.
let removeEven list =
let maxIndex = (List.length list) - 1;
seq { for i in 0..2..maxIndex -> list.[i] }
|> Seq.toList
Tests
val removeEven : 'a list -> 'a list
> removeEven [1;2;3;4;5;6];;
val it : int list = [1; 3; 5]
> removeEven [1;2;3;4;5];;
val it : int list = [1; 3; 5]
> removeEven [1;2;3;4];;
val it : int list = [1; 3]
> removeEven [1;2;3];;
val it : int list = [1; 3]
> removeEven [1;2];;
val it : int list = [1]
> removeEven [1];;
val it : int list = [1]
You can try a pattern-matching approach. I haven't used F# in a while and I can't test things right now, but it would be something like this:
let rec curse sofar ls =
match ls with
| even :: odd :: tl -> curse (even :: sofar) tl
| even :: [] -> curse (even :: sofar) []
| [] -> List.rev sofar
curse [] [ 1; 2; 3; 4; 5 ]
This recursively picks off the even elements. I think. I would probably use Joel Mueller's approach though. I don't remember if there is an index-based filter function, but that would probably be the ideal to use, or to make if it doesn't exist in the libraries.
But in general lists aren't really meant as index-type things. That's what arrays are for. If you consider what kind of algorithm would require a list having its even elements removed, maybe it's possible that in the steps prior to this requirement, the elements can be paired up in tuples, like this:
[ (1,2); (3,4) ]
That would make it trivial to get the even-"indexed" elements out:
thelist |> List.map fst // take first element from each tuple
There's a variety of options if the input list isn't guaranteed to have an even number of elements.
Yet another alternative, which (by my reckoning) is slightly slower than Joel's, but it's shorter :)
let removeEven list =
list
|> Seq.mapi (fun i x -> (i, x))
|> Seq.choose (fun (i,x) -> if i % 2 = 0 then Some(x) else None)
|> List.ofSeq

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