F# Sequences - how to return multiple rows - f#

let x = [for p in db.ParamsActXes do
if p.NumGroupPar = grp then
yield p.Num, p.Name]
Here is my sequence but the problem is it returns the list of tuples, I can't access single tuple element like
let str = "hello" + x.[1]
and that is the trouble.
how can I realize this functionary ?

To access the second element of a 2-tuple you can either use snd or pattern matching. So if tup is a 2-tuple, where the second element of tup is a string, you can either do:
let str = "hello" + snd tup
or
let (a,b) = tup
let st = "hello" + b
Note that snd only works with 2-tuples, not tuples with more than two elements.

To give you one more alternative solution, you can just create a filtered sequence containing values of the original type by writing yield p:
let x = [ for p in db.ParamsActXes do
if p.NumGroupPar = grp then
yield p ]
And then just access the property you need:
let str = "hello" + x.[1].Name
This is probably a good idea if you're returning only several properties of the p value. The only reason for yielding tuples would be to hide something from the rest of the code or to create sequence that matches some function you use later.
(Also, I would avoid indexing into lists using x.[i], because this is inefficient - but maybe this is just for ilustration in the sample you posted. Use an array if you need index based access by wrapping sequence expression into [| ... |])

Just to throw one more possibility out there, if you are running the .NET 4.0 build of F# 2.0 you can perform a runtime cast from the F# tuple to the .NET 4.0 System.Tuple and then use the ItemX properties of the .NET 4.0 tuples to access the tuple element you need,
let x = (1, 1.2, "hello")
let y = ((box x) :?> System.Tuple<int, float, string>);;
y.Item3 //returns "hello"
However, I would never use that, instead opting for the pattern match extraction. (also, I've read places that the F# compiler may not always choose to represent its tuples as .NET 4.0 tuples, so there may be a possibility that the cast would fail).
Reading your comments in some of the other answers, I'm unsure why the pattern matching solution doesn't work for you. Perhaps you want to access the tuple item at a certain place within an expression? If so, the previous would certainly work:
let str = "hello" + ((box x.[1]) :?> System.Tuple<int,string>).Item2 //though might as well use snd and fst for length 2 F# tuples
but you can achieve the same ends using the pattern matching extraction technique too (again, assuming this is even what you're after):
let str = "hello" + (let (_,name) = x.[1] in name)

you can access individual tuple from the list of tuple using List.nth.
let first, second = List.nth x 0
first and second represents the individual element of the tuple.

Related

unexpected return type from list comprehension

I am teaching myself a bit of F# by doing a bit of simple matrix mathematics. I decided to write a set of simple functions for combining two matrices as I thought that this would be a good way of learning list comprehensions. However when I compile it my unit tests produce a type mismatch exception.
//return a column from the matrix as a list
let getColumn(matrix: list<list<double>>, column:int) =
[for row in matrix do yield row.Item(column)]
//return a row from the matrix as a list
let getRow(matrix: list<list<double>>, column:int) =
matrix.Item(column)
//find the minimum width of the matrices in order to avoid index out of range exceptions
let minWidth(matrix1: list<list<double>>,matrix2: list<list<double>>) =
let width1 = [for row in matrix1 do yield row.Length] |> List.min
let width2 = [for row in matrix2 do yield row.Length] |> List.min
if width1 > width2 then width2 else width1
//find the minimum height of the matrices in order to avoid index out of range exceptions
let minHeight(matrix1: list<list<double>>,matrix2: list<list<double>>) =
let height1 = matrix1.Length
let height2 = matrix2.Length
if height1 > height2 then height2 else height1
//combine the two matrices
let concat(matrix1: list<list<double>>,matrix2: list<list<double>>) =
let width = minWidth(matrix1, matrix2)
let height = minHeight(matrix1, matrix2)
[for y in 0 .. height do yield [for x in 0 .. width do yield (List.fold2 (fun acc a b -> acc + (a*b)), getRow(matrix1, y), getColumn(matrix2, x))]]
I was expecting the function to return a list of lists of type
double list list
However what it actually returns looks more like some kind of lambda expression
((int -> int list -> int list -> int) * double list * double list) list list
Can somebody tell me what is being returned, and how to force it to be evaluated into the list of lists that I originally expected?
There's a short answer and a long answer to your question.
The short answer
The short version is that F# functions (like List.fold2) take multiple parameters not with commas the way you think they do, but with spaces in between. I.e., you should NOT call List.fold2 like this:
List.fold2 (function, list1, list2)
but rather like this:
List.fold2 function list1 list2
Now, if you just remove the commas in your List.fold2 call, you'll see that the compiler complains about your getRow(matrix1, y) call, and tells you to put parentheses around them. (And the outer pair of parentheses around List.fold2 isn't actually needed). So this:
(List.fold2 (fun acc a b -> acc + (a*b)), getRow(matrix1, y), getColumn(matrix2, x))
Needs to turn into this:
List.fold2 (fun acc a b -> acc + (a*b)) (getRow(matrix1, y)) (getColumn(matrix2, x))
The long answer
The way F# functions take multiple parameters is actually very different from most other languages such as C#. In fact, all F# functions take exactly one parameter! "But wait," you're probably thinking right now, "you just now showed me the syntax for F# functions taking multiple parameters!" Yes, I did. What's going on under the hood is a combination of currying and partial application. I'd write a long explanation, but Scott Wlaschin has already written one, that's much better than I could have written, so I'll just point you to the https://fsharpforfunandprofit.com/series/thinking-functionally.html series to help you understand what's going on here. (The sections on currying and partial application are the ones you want, but I'd recommend reading the series in order because the later parts build on concepts introduced in earlier parts).
And yes, this "long" answer appears shorter than the "short" answer, but if you go read that series (and then the rest of Scott Wlaschin's excellent site), you'll find that it's much longer than the short answer. :-)
If you have more questions, I'll be happy to try to explain.

Why data parameter comes last

Why have the data parameter in F# to come last, like the following code snippet shows:
let startsWith lookFor (s:string) = s.StartsWith(lookFor)
let str1 =
"hello"
|> startsWith "h"
I think part of your answer is in your question. The |> (forward pipe) operator lets you specify the last parameter to a function before you call it. If the parameters were in the opposite order, then that wouldn't work. The best examples of the power of this are with chaining of functions that operate on lists. Each function takes a list as its last parameter and returns a list that can be passed to the next function.
From http://www.tryfsharp.org/Learn/getting-started#chaining-functions:
[0..100]
|> List.filter (fun x -> x % 2 = 0)
|> List.map (fun x -> x * 2)
|> List.sum
The |> operator allows you to reorder your code by specifying the last
argument of a function before you call it. This example is
functionally equivalent to the previous code, but it reads much more
cleanly. First, it creates a list of numbers. Then, it pipes that list
of numbers to filter out the odds. Next, it pipes that result to
List.map to double it. Finally, it pipes the doubled numbers to
List.sum to add them together. The Forward Pipe Operator reorganizes
the function chain so that your code reads the way you think about the
problem instead of forcing you to think inside out.
As mentioned in the comments there is also the concept of currying, but I don't think that is as easy to grasp as chaining functions.

How to invoke the function in Seq.whatever without "printf"?

I'm new to f# and I tried to write a program supposed to go through all files in a given dir and for each file of type ".txt" to add an id number + "DONE" to the file.
my program:
//const:
[<Literal>]
let notImportantString= "blahBlah"
let mutable COUNT = 1.0
//funcs:
//addNumber --> add the sequence number COUNT to each file.
let addNumber (file : string) =
let mutable str = File.ReadAllText(file)
printfn "%s" str//just for check
let num = COUNT.ToString()
let str4 = str + " " + num + "\n\n\n DONE"
COUNT <- COUNT + 1.0
let str2 = File.WriteAllText(file,str4)
file
//matchFunc --> check if is ".txt"
let matchFunc (file : string) =
file.Contains(".txt")
//allFiles --> go through all files of a given dir
let allFiles dir =
seq
{ for file in Directory.GetFiles(dir) do
yield file
}
////////////////////////////
let dir = "D:\FSharpTesting"
let a = allFiles dir
|> Seq.filter(matchFunc)
|> Seq.map(addNumber)
printfn "%A" a
My question:
Tf I do not write the last line (printfn "%A" a) the files will not change.(if I DO write this line it works and change the files)
when I use debugger I see that it doesn't really computes the value of 'a' when it arrives to the line if "let a =......" it continues to the printfn line and than when it "sees" the 'a' there it goes back and computes the answer of 'a'.
why is it and how can i "start" the function without printing??
also- Can some one tells me why do I have to add file as a return type of the function "addNumber"?
(I added this because that how it works but I don't really understand why....)
last question-
if I write the COUNT variable right after the line of the [] definition
it gives an error and says that a constant cannot be "mutable" but if a add (and this is why I did so) another line before (like the string) it "forgets" the mistakes and works.
why that? and if you really cannot have a mutable const how can I do a static variable?
if I do not write the last line (printfn "%A" a) the files will not change.
F# sequences are lazy. So to force evaluation, you can execute some operation not returning a sequence. For example, you can call Seq.iter (have side effects, return ()), Seq.length (return an int which is the length of the sequence) or Seq.toList (return a list, an eager data structure), etc.
Can some one tells me why do I have to add file : string as a return type of the function "addNumber"?
Method and property access don't play nice with F# type inference. The type checker works from left to right, from top to bottom. When you say file.Contains, it doesn't know which type this should be with Contains member. Therefore, your type annotation is a good hint to F# type checker.
if I write the COUNT variable right after the line of the [<Literal>] definition
it gives an error and says that a constant cannot be "mutable"
Quoting from MSDN:
Values that are intended to be constants can be marked with the Literal attribute. This attribute has the effect of causing a value to be compiled as a constant.
A mutable value can change its value at some point in your program; the compiler complains for a good reason. You can simply delete [<Literal>] attribute.
To elaborate on Alex's answer -- F# sequences are lazily evaluated. This means that each element in the sequence is generated "on demand".
The benefit of this is that you don't waste computation time and memory on elements you don't ever need. Lazy evaluation does take a little getting used to though -- specifically because you can't assume order of execution (or that execution will even happen at all).
Your problem has a simple fix: just use Seq.iter to force execution/evaluation of the sequence, and pass the 'ignore' function to it since we don't care about the values returned by the sequence.
let a = allFiles dir
|> Seq.filter(matchFunc)
|> Seq.map(addNumber)
|> Seq.iter ignore // Forces the sequence to execute
Seq.map is intended to map one value to another, not generally to mutate a value. seq<_> represents a lazily generated sequence so, as Alex pointed out, nothing will happen until the sequence is enumerated. This is probably a better fit for codereview, but here's how I would write this:
Directory.EnumerateFiles(dir, "*.txt")
|> Seq.iteri (fun i path ->
let text = File.ReadAllText(path)
printfn "%s" text
let text = sprintf "%s %d\n\n\n DONE" text (i + 1)
File.WriteAllText(path, text))
Seq.map requires a return type, as do all expressions in F#. If a function performs an action, as opposed to computing a value, it can return unit: (). Regarding COUNT, a value cannot be mutable and [<Literal>] (const in C#). Those are precise opposites. For a static variable, use a module-scoped let mutable binding:
module Counter =
let mutable count = 1
open Counter
count <- count + 1
But you can avoid global mutable data by making count a function with a counter variable as a part of its private implementation. You can do this with a closure:
let count =
let i = ref 0
fun () ->
incr i
!i
let one = count()
let two = count()
f# is evaluated from top to bottom, but you are creating only lazy values until you do printfn. So, printfn is actually the first thing that gets executed which in turn executes the rest of your code. I think you can do the same thing if you tack on a println after Seq.map(addNumber) and do toList on it which will force evaluation as well.
This is a general behaviour of lazy sequence. you have the same in, say C# using IEnumerable, for which seq is an alias.
In pseudo code :
var lazyseq = "abcdef".Select(a => print a); //does not do anything
var b = lazyseq.ToArray(); //will evaluate the sequence
ToArray triggers the evaluation of a sequence :
This illustrate the fact that a sequence is just a description, and does not tell you when it will be enumerated : this is in control of the consumer of the sequence.
To go a bit further on the subject, you might want to look at this page from F# wikibook:
let isNebraskaCity_bad city =
let cities =
printfn "Creating cities Set"
["Bellevue"; "Omaha"; "Lincoln"; "Papillion"]
|> Set.ofList
cities.Contains(city)
let isNebraskaCity_good =
let cities =
printfn "Creating cities Set"
["Bellevue"; "Omaha"; "Lincoln"; "Papillion"]
|> Set.ofList
fun city -> cities.Contains(city)
Most notably, Sequence are not cached (although you can make them so). You see then that the dintinguo between the description and the runtime behaviour can have important consequence as the sequence itself is recomputed which can incur a very high cost and introduce quadratic number of operations if each value is itself linear to get !

f# iterating over two arrays, using function from a c# library

I have a list of words and a list of associated part of speech tags. I want to iterate over both, simultaneously (matched index) using each indexed tuple as input to a .NET function. Is this the best way (it works, but doesn't feel natural to me):
let taggingModel = SeqLabeler.loadModel(lthPath +
"models\penn_00_18_split_dict.model");
let lemmatizer = new Lemmatizer(lthPath + "v_n_a.txt")
let input = "the rain in spain falls on the plain"
let words = Preprocessor.tokenizeSentence( input )
let tags = SeqLabeler.tagSentence( taggingModel, words )
let lemmas = Array.map2 (fun x y -> lemmatizer.lookup(x,y)) words tags
Your code looks quite good to me - most of it deals with some loading and initialization, so there isn't much you could do to simplify that part. Alternatively to Array.map2, you could use Seq.zip combined with Seq.map - the zip function combines two sequences into a single one that contains pairs of elements with matching indices:
let lemmas = Seq.zip words tags
|> Seq.map (fun (x, y) -> lemmatizer.lookup (x, y))
Since lookup function takes a tuple that you got as an argument, you could write:
// standard syntax using the pipelining operator
let lemmas = Seq.zip words tags |> Seq.map lemmatizer.lookup
// .. an alternative syntax doing exactly the same thing
let lemmas = (words, tags) ||> Seq.zip |> Seq.map lemmatizer.lookup
The ||> operator used in the second version takes a tuple containing two values and passes them to the function on the right side as two arguments, meaning that (a, b) ||> f means f a b. The |> operator takes only a single value on the left, so (a, b) |> f would mean f (a, b) (which would work if the function f expected tuple instead of two, space separated, parameters).
If you need lemmas to be an array at the end, you'll need to add Array.ofSeq to the end of the processing pipeline (all Seq functions work with sequences, which correspond to IEnumerable<T>)
One more alternative is to use sequence expressions (you can use [| .. |] to construct an array directly if that's what you need):
let lemmas = [| for wt in Seq.zip words tags do // wt is tuple (string * string)
yield lemmatizer.lookup wt |]
Whether to use sequence expressions or not - that's just a personal preference. The first option seems to be more succinct in this case, but sequence expressions may be more readable for people less familiar with things like partial function application (in the shorter version using Seq.map)

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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