Why is the recursive value null? - f#

I'm F# beginner, I'm having this problem when I'm learning "recursive values".
type Type =
| N of int * Type
| E
let rec a = N(2, b)
and b = N(3, E)
a |> printfn "%A"
I expect the output is:
N(2, N(3, E))
but the actual output is:
N (2,null)

This is actually a bug in the F# compiler, filed here: https://github.com/fsharp/fsharp/issues/847
There has been no indication about when (or if) it will be fixed.
For your particular example, I would recommend simply removing the rec modifier: it is not needed, since the values aren't actually recursive. That will make the bug go away.
For more general case, you can create a system of mutually recursive functions for initialization, or simply a nested let block. For example, this should work:
let a, b =
let rec a = N(2, b)
and b = N(3, E)
a, b

Related

How to write efficient list/seq functions in F#? (mapFoldWhile)

I was trying to write a generic mapFoldWhile function, which is just mapFold but requires the state to be an option and stops as soon as it encounters a None state.
I don't want to use mapFold because it will transform the entire list, but I want it to stop as soon as an invalid state (i.e. None) is found.
This was myfirst attempt:
let mapFoldWhile (f : 'State option -> 'T -> 'Result * 'State option) (state : 'State option) (list : 'T list) =
let rec mapRec f state list results =
match list with
| [] -> (List.rev results, state)
| item :: tail ->
let (result, newState) = f state item
match newState with
| Some x -> mapRec f newState tail (result :: results)
| None -> ([], None)
mapRec f state list []
The List.rev irked me, since the point of the exercise was to exit early and constructing a new list ought to be even slower.
So I looked up what F#'s very own map does, which was:
let map f list = Microsoft.FSharp.Primitives.Basics.List.map f list
The ominous Microsoft.FSharp.Primitives.Basics.List.map can be found here and looks like this:
let map f x =
match x with
| [] -> []
| [h] -> [f h]
| (h::t) ->
let cons = freshConsNoTail (f h)
mapToFreshConsTail cons f t
cons
The consNoTail stuff is also in this file:
// optimized mutation-based implementation. This code is only valid in fslib, where mutation of private
// tail cons cells is permitted in carefully written library code.
let inline setFreshConsTail cons t = cons.(::).1 <- t
let inline freshConsNoTail h = h :: (# "ldnull" : 'T list #)
So I guess it turns out that F#'s immutable lists are actually mutable because performance? I'm a bit worried about this, having used the prepend-then-reverse list approach as I thought it was the "way to go" in F#.
I'm not very experienced with F# or functional programming in general, so maybe (probably) the whole idea of creating a new mapFoldWhile function is the wrong thing to do, but then what am I to do instead?
I often find myself in situations where I need to "exit early" because a collection item is "invalid" and I know that I don't have to look at the rest. I'm using List.pick or Seq.takeWhile in some cases, but in other instances I need to do more (mapFold).
Is there an efficient solution to this kind of problem (mapFoldWhile in particular and "exit early" in general) with functional programming concepts, or do I have to switch to an imperative solution / use a Collections.Generics.List?
In most cases, using List.rev is a perfectly sufficient solution.
You are right that the F# core library uses mutation and other dirty hacks to squeeze some more performance out of the F# list operations, but I think the micro-optimizations done there are not particularly good example. F# list functions are used almost everywhere so it might be a good trade-off, but I would not follow it in most situations.
Running your function with the following:
let l = [ 1 .. 1000000 ]
#time
mapFoldWhile (fun s v -> 0, s) (Some 1) l
I get ~240ms on the second line when I run the function without changes. When I just drop List.rev (so that it returns the data in the other order), I get around ~190ms. If you are really calling the function frequently enough that this matters, then you'd have to use mutation (actually, your own mutable list type), but I think that is rarely worth it.
For general "exit early" problems, you can often write the code as a composition of Seq.scan and Seq.takeWhile. For example, say you want to sum numbers from a sequence until you reach 1000. You can write:
input
|> Seq.scan (fun sum v -> v + sum) 0
|> Seq.takeWhile (fun sum -> sum < 1000)
Using Seq.scan generates a sequence of sums that is over the whole input, but since this is lazily generated, using Seq.takeWhile stops the computation as soon as the exit condition happens.

Avoid mutation in this example in F#

Coming from an OO background, I am having trouble wrapping my head around how to solve simple issues with FP when trying to avoid mutation.
let mutable run = true
let player1List = ["he"; "ho"; "ha"]
let addValue lst value =
value :: lst
while run do
let input = Console.ReadLine()
addValue player1List input |> printfn "%A"
if player1List.Length > 5 then
run <- false
printfn "all done" // daz never gunna happen
I know it is ok to use mutation in certain cases, but I am trying to train myself to avoid mutation as the default. With that said, can someone please show me an example of the above w/o using mutation in F#?
The final result should be that player1List continues to grow until the length of items are 6, then exit and print 'all done'
The easiest way is to use recursion
open System
let rec makelist l =
match l |> List.length with
|6 -> printfn "all done"; l
| _ -> makelist ((Console.ReadLine())::l)
makelist []
I also removed some the addValue function as it is far more idiomatic to just use :: in typical F# code.
Your original code also has a common problem for new F# coders that you use run = false when you wanted run <- false. In F#, = is always for comparison. The compiler does actually warn about this.
As others already explained, you can rewrite imperative loops using recursion. This is useful because it is an approach that always works and is quite fundamental to functional programming.
Alternatively, F# provides a rich set of library functions for working with collections, which can actually nicely express the logic that you need. So, you could write something like:
let player1List = ["he"; "ho"; "ha"]
let player2List = Seq.initInfinite (fun _ -> Console.ReadLine())
let listOf6 = Seq.append player1List list2 |> Seq.take 6 |> List.ofSeq
The idea here is that you create an infinite lazy sequence that reads inputs from the console, append it at the end of your initial player1List and then take first 6 elements.
Depending on what your actual logic is, you might do this a bit differently, but the nice thing is that this is probably closer to the logic that you want to implement...
In F#, we use recursion to do loop. However, if you know how many times you need to iterate, you could use F# List.fold like this to hide the recursion implementation.
[1..6] |> List.fold (fun acc _ -> Console.ReadLine()::acc) []
I would remove the pipe from match for readability but use it in the last expression to avoid extra brackets:
open System
let rec makelist l =
match List.length l with
| 6 -> printfn "all done"; l
| _ -> Console.ReadLine()::l |> makelist
makelist []

Is there a library function or operator to create a tuple?

Given a string of digits, I would like to have a sequence of tuples mapping the non-zero characters with their position in the string. Example:
IN: "000140201"
OUT: { (3, '1'); (4, '4'); (6, '2'); (8, '1') }
Solution:
let tuples = source
|> Seq.mapi (fun i -> fun c -> (i, c))
|> Seq.filter (snd >> (<>) '0')
It seems like (fun i -> fun c -> (i, c)) is a lot more typing than it should be for such a simple and presumably common operation. It's easy to declare the necessary function:
let makeTuple a b = (a, b)
let tuples2 = source
|> Seq.mapi makeTuple
|> Seq.filter (snd >> (<>) '0')
But it seems to me that if the library provides the snd function, it should also provide the makeTuple function (and probably with a shorter name), or at least it should be relatively easy to compose. I couldn't find it; am I missing something? I tried to build something with the framework's Tuple.Create, but I couldn't figure out how to get anything other than the single-argument overload.
But it seems to me that if the library provides the snd function, it should also provide the makeTuple function.
F# assumes that you decompose tuples (using fst, snd) much more often than composing them. Functional library design often follows minimal principle. Just provide functions for common use cases, other functions should be easy to define.
I couldn't find it; am I missing something?
No, you aren't. It's the same reason that FSharpPlus has defined tuple2, tuple3, etc. Here are utility functions straight from Operators:
/// Creates a pair
let inline tuple2 a b = a,b
/// Creates a 3-tuple
let inline tuple3 a b c = a,b,c
/// Creates a 4-tuple
let inline tuple4 a b c d = a,b,c,d
/// Creates a 5-tuple
let inline tuple5 a b c d e = a,b,c,d,e
/// Creates a 6-tuple
let inline tuple6 a b c d e f = a,b,c,d,e,f
I tried to build something with the framework's Tuple.Create, but I couldn't figure out how to get anything other than the single-argument overload.
F# compiler hides properties of System.Tuple<'T1, 'T2> to enforce pattern matching idiom on tuples. See Extension methods for F# tuples for more details.
That said, point-free style is not always recommended in F#. If you like point-free, you have to do a bit of heavy lifting yourself.
The #pad's answer is great, just to add my 2 cents: I am using similar operator
let inline (-&-) a b = (a, b)
and it looks very convenient to write let x = a -&- b
Maybe you'll find this operator useful too

Return item at position x in a list

I was reading this post While or Tail Recursion in F#, what to use when? were several people say that the 'functional way' of doing things is by using maps/folds and higher order functions instead of recursing and looping.
I have this function that returns the item at position x in a list:
let rec getPos l c = if c = 0 then List.head l else getPos (List.tail l) (c - 1)
how can it be converted to be more functional?
This is a primitive list function (also known as List.nth).
It is okay to use recursion, especially when creating the basic building blocks. Although it would be nicer with pattern matching instead of if-else, like this:
let rec getPos l c =
match l with
| h::_ when c = 0 -> h
| _::t -> getPos t (c-1)
| [] -> failwith "list too short"
It is possible to express this function with List.fold, however the result is less clear than the recursive version.
I'm not sure what you mean by more functional.
Are you rolling this yourself as a learning exercise?
If not, you could just try this:
> let mylist = [1;2;3;4];;
> let n = 2;;
> mylist.[n];;
Your definition is already pretty functional since it uses a tail-recursive function instead of an imperative loop construct. However, it also looks like something a Scheme programmer might have written because you're using head and tail.
I suspect you're really asking how to write it in a more idiomatic ML style. The answer is to use pattern matching:
let rec getPos list n =
match list with
| hd::tl ->
if n = 0 then hd
else getPos tl (n - 1)
| [] -> failWith "Index out of range."
The recursion on the structure of the list is now revealed in the code. You also get a warning if the pattern matching is non-exhaustive so you're forced to deal with the index too big error.
You're right that functional programming also encourages the use of combinators like map or fold (so called points-free style). But too much of it just leads to unreadable code. I don't think it's warranted in this case.
Of course, Benjol is right, in practice you would just write mylist.[n].
If you'd like to use high-order functions for this, you could do:
let nth n = Seq.take (n+1) >> Seq.fold (fun _ x -> Some x) None
let nth n = Seq.take (n+1) >> Seq.reduce (fun _ x -> x)
But the idea is really to have basic constructions and combine them build whatever you want. Getting the nth element of a sequence is clearly a basic block that you should use. If you want the nth item, as Benjol mentioned, do myList.[n].
For building basic constructions, there's nothing wrong to use recursion or mutable loops (and often, you have to do it this way).
Not as a practical solution, but as an exercise, here is one of the ways to express nth via foldr or, in F# terms, List.foldBack:
let myNth n xs =
let step e f = function |0 -> e |n -> f (n-1)
let error _ = failwith "List is too short"
List.foldBack step xs error n

F#: warning FS0020: This expression should have type 'unit', but has type 'bool'

I'm trying to learn F# by going through some of the Euler problems and I found an issue I haven't been able to figure out. This is my naive solution.
let compute =
let mutable f = false
let mutable nr = 0
while f = false do
nr <- nr + 20
f = checkMod nr
nr
When i do this I get the error message warning FS0020: This expression should have type 'unit', but has type 'bool' on the expression "nr <- nr +20". I've tried rewriting and moving the expressions around and I always get that error on the line below the while statement.
I'm writing this using VS2010 Beta.
Since I can imagine this weg page becoming the 'canonical' place to look up information about warning FS0020, here's my quick summary of the three commonest cases in which you get the warning, and how to fix them.
Intentionally discarding the result of a function that is called only for its side-effects:
// you are calling a function for its side-effects, intend to ignore result
let Example1Orig() =
let sb = new System.Text.StringBuilder()
sb.Append("hi") // warning FS0020
sb.Append(" there") // warning FS0020
sb.ToString()
let Example1Fixed() =
let sb = new System.Text.StringBuilder()
sb.Append("hi") |> ignore
sb.Append(" there") |> ignore
sb.ToString()
Warning is useful, pointing out an error (function has no effects):
// the warning is telling you useful info
// (e.g. function does not have an effect, rather returns a value)
let Example2Orig() =
let l = [1;2;3]
List.map (fun x -> x * 2) l // warning FS0020
printfn "doubled list is %A" l
let Example2Fixed() =
let l = [1;2;3]
let result = List.map (fun x -> x * 2) l
printfn "doubled list is %A" result
Confusing assignment operator and equality comparison operator:
// '=' versus '<-'
let Example3Orig() =
let mutable x = 3
x = x + 1 // warning FS0020
printfn "%d" x
let Example3Fixed() =
let mutable x = 3
x <- x + 1
printfn "%d" x
The following line:
f = checkMod nr
is an equality check, not an assignment as I believe you are intending. Change it to:
f <- checkMod nr
and all should work fine. I'm not sure why you've used the correct syntax on the previous line and not that line...
Also, the line while f = false do should really be simplified to while not f do; equality checks on booleans are rather convoluted.
As I side note, I feel a need to point out that you are effectively trying to use F# as an imperative language. Use of mutable variables and while loops are strongly discouraged in functional languages (including F#), especially when a purely functional (and simpler) solution exists, as in this situation. I recommend you read up a bit on programming in the functional style. Of course, just getting to grips with the syntax is a useful thing in itself.
If you're trying to adopt the functional style, try to avoid mutable values.
For example like this:
let nr =
let rec compute nr =
if checkMod nr then nr else compute (nr + 20)
compute 0
while expressions in F# take a little getting used to if you're coming from an imperative language. Each line in a while expression must evaluate to unit (think void from C++/C#). The overall expression then also evaluates to unit.
In the example:
nr <- nr + 20
evaluates to unit whereas
f = checkMod nr
evaluates to a bool as Noldorin noted. This results in a warning message being reported. You can actually turn the warning off if you so desire. Just put the following at the top of your file:
#nowarn "0020"
I've been programming in an imperative style for a long time, so getting used to the functional programming mindset took a while.
In your example, you're trying to find the first multiple of 20 that passes your checkMod test. That's the what part. For the functional how part, I recommend browsing through the methods available to sequences. What you need is the first element of a sequence (multiples of 20) passing your test, like this:
let multi20 = Seq.initInfinite (fun i -> i*20)
let compute = multi20 |> Seq.find checkMod
The first let generates an infinite list of twentyples (I made that one up). The second let finds the first number in said list that passes your test. Your task is to make sure that there actually is a number that will pass the test, but that's of course also true for the imperative code.
If you want to condense the two above lines into one, you can also write
let computeCryptic = Seq.initInfinite ((*) 20) |> Seq.find checkMod
but I find that pulling stunts like that in code can lead to headaches when trying to read it a few weeks later.
In the same spirit as Brian's post, here is another way to get warning FS0020: In a nutshell, I accidentally tupled the function arguments.
Being an F# newbie, I had a difficult time debugging the code below, which for the second line (let gdp...) gave the warning FS0020: This expression should have type 'unit', but has type '(string -> ^a -> unit) * string * float'. It turns out that line was not the problem at all; instead, it was the printfn line that was messed up. Removing the comma separators from the argument list fixed it.
for country in wb.Regions.``Arab World``.Countries do
let gdp = country.Indicators.``GDP per capita (current US$)``.[2010]
let gdpThous = gdp / 1.0e3
printfn "%s, %s (%.2f)" country.Name, country.CapitalCity, gdpThous

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