How extract the int from a FsCheck.Gen.choose - f#

I'm new on F#, and can't see how extract the int value from:
let autoInc = FsCheck.Gen.choose(1,999)
The compiler say the type is Gen<int>, but can't get the int from it!. I need to convert it to decimal, and both types are not compatible.

From a consumer's point of view, you can use the Gen.sample combinator which, given a generator (e.g. Gen.choose), gives you back some example values.
The signature of Gen.sample is:
val sample : size:int -> n:int -> gn:Gen<'a> -> 'a list
(* `size` is the size of generated test data
`n` is the number of samples to be returned
`gn` is the generator (e.g. `Gen.choose` in this case) *)
You can ignore size because Gen.choose ignores it, as its distribution is uniform, and do something like:
let result = Gen.choose(1,999) |> Gen.sample 0 1 |> Seq.exactlyOne |> decimal
(* 0 is the `size` (gets ignored by Gen.choose)
1 is the number of samples to be returned *)
The result should be a value in the closed interval [1, 999], e.g. 897.

Hi to add to what Nikos already told you, this is how you can get an decimal between 1 and 999:
#r "FsCheck.dll"
open FsCheck
let decimalBetween1and999 : Gen<decimal> =
Arb.generate |> Gen.suchThat (fun d -> d >= 1.0m && d <= 999.0m)
let sample () =
decimalBetween1and999
|> Gen.sample 0 1
|> List.head
you can now just use sample () to get a random decimal back.
In case you just want integers between 1 and 999 but have those converted to decimal you can just do:
let decimalIntBetween1and999 : Gen<decimal> =
Gen.choose (1,999)
|> Gen.map decimal
let sampleInt () =
decimalIntBetween1and999
|> Gen.sample 0 1
|> List.head
what you probably really want to do instead
Is use this to write you some nice types and check properties like this (here using Xunit as a test-framework and the FsCheck.Xunit package:
open FsCheck
open FsCheck.Xunit
type DecTo999 = DecTo999 of decimal
type Generators =
static member DecTo999 =
{ new Arbitrary<DecTo999>() with
override __.Generator =
Arb.generate
|> Gen.suchThat (fun d -> d >= 1.0m && d <= 999.0m)
|> Gen.map DecTo999
}
[<Arbitrary(typeof<Generators>)>]
module Tests =
type Marker = class end
[<Property>]
let ``example property`` (DecTo999 d) =
d > 1.0m

Gen<'a> is a type that essentially abstracts a function int -> 'a (the actual type is a bit more complex, but let's ignore for now). This function is pure, i.e. when given the same int, you'll get the same instance of 'a back every time. The idea is that FsCheck generates a bunch of random ints, feeds them to the Gen function, out come random instances of the type 'a you're interested in, and feeds those to a test.
So you can't really get out the int. You have in your hands a function that given an int, generates another int.
Gen.sample as described in another answer essentially just feeds a sequence of random ints to the function and applies it to each, returning the results.
The fact that this function is pure is important because it guarantees reproducibility: if FsCheck finds a value for which a test fails, you can record the original int that was fed into the Gen function - rerunning the test with that seed is guaranteed to generate the same values, i.e. reproduce the bug.

Related

FsCheck test change the range of values used for testing

My code is automatically testing for values from -99 to 99 while using FsCheck.
Check.Quick test
where my test function takes integer values.
I would like to test using values from 1 to 4999.
You can use Gen.elements combined with Prop.forAll:
let n = Gen.elements [-99..99] |> Arb.fromGen
let prop = Prop.forAll n (fun number ->
// Test goes here - e.g.:
Assert.InRange(number, -99, 99))
prop.QuickCheck()
Gen.elements takes a sequence of valid values and creates a uniform generator from that sequence. Prop.forAll defines a property with that custom generator.
You can combine it with FsCheck's Glue Library for xUnit.net, which is my preferred method:
[<Property>]
let ``Number is between -99 and 99`` () =
let n = Gen.elements [-99..99] |> Arb.fromGen
Prop.forAll n (fun number ->
// Test goes here - e.g.:
Assert.InRange(number, -99, 99))
By default, FsCheck generates integers between 1 and 100. You can change this by supplying a Config object to the Check.
let config = {
Config.Quick with
EndSize = 4999
}
Check.One(config,test)
EndSize indicates the size to use for the last test, when all the tests are passing. The size increases linearly between StartSize, which you can also set should you wish to generate test data in a range starting from some value other than 1, and EndSize. See the implementation of type Config in https://github.com/fscheck/FsCheck/blob/master/src/FsCheck/Runner.fs

Converting a list of strings into floats/ints in F#

Is there a quick and simple way to convert an entire list of strings into floats or integers
and add them together similar to this in F#?
foreach(string s in list)
{
sum += int.Parse(s);
}
If you want to aim for minimal number of characters, then you can simplify the solution posted by Ganesh to something like this:
let sum = list |> Seq.sumBy int
This does pretty much the same thing - the int function is a generic conversion that converts anything to an integer (and it works on strings too). The sumBy function is a combination of map and sum that first projects all elements to a numeric value and then sums the results.
Something like this should have the same effect:
let sum = list |> Seq.map System.Int32.Parse |> Seq.sum
F# doesn't seem to support referring to the method on int so I had to use System.Int32 instead.
In F# the type seq is an alias for the .NET IEnumerable, so this code works on arrays, lists etc.
Note the use of Parse in "point-free" style - a function without its argument can be used directly as an argument to another function that expects that type. In this case Seq.map has this type:
('a -> 'b) -> seq<'a> -> seq<'b>
And since System.Int32.Parse has type string -> int, Seq.map System.Int32.Parse has type seq<string> -> seq<int>.
Technically, there are at least 3 different approaches:
1) The Seq.sum or sumBy approach described in the other answers is the canonical way of getting the sum in F#:
let sum = Seq.sumBy int list
2) For instructional purposes, it may be interesting to see how closely one can simulate C# behavior in F#; for instance, using a reference cell type:
let inline (+=) x y = x := !x + y
let sum = ref 0
for s in list do sum += int s
3) Same idea as 2), but using a byref pointer type:
let inline (+=) (x:_ byref) y = x <- x + y
let mutable sum = 0
for s in list do &sum += int s

For loops in 2D arrays giving incorrect types in F#

I suspect that I am missing something very obvious here but this doesn't work:
let t = Array2D.create 1 1 1.0
for x in t do printfn "%f" x;;
It fails with
error FS0001: The type 'obj' is not compatible with any of the types float,float32,decimal, arising from the use of a printf-style format string
Interestingly using printf "%A" or "%O" prints the expected values which suggests to me that the problem is with the type inference
The corresponding code for a 1D array works fine
let t = Array.create 1 1.0
for x in t do printfn "%f" x;;
For reference this is on version 2.0 (both interactive and compiler) running on the latest mono
In .NET, a 1D array implicitly implements IList, which means it also implements (by inheritance) IEnumerable<T>. So, when you run:
let t = Array.create 1 1.0
for x in t do printfn "%f" x;;
the F# compiler emits code which gets an implementation of IEnumerable<T> (seq<T> in F#) from t, then iterates over it. Since it's able to get an IEnumerable<T> from the array, x will have type T.
On the other hand, multi-dimensional arrays (2d, 3d, etc.) only implement IEnumerable (not IEnumerable<T>) so the F# compiler infers the type of x as System.Object (or obj, in F#).
There are two solutions for what you want:
Cast each individual value within the loop, before printing it:
for x in t do printfn "%f" (x :?> float);;
Or, use Seq.cast to create and iterate over a strongly-typed enumerator:
for x in (Seq.cast<float> t) do printfn "%f" x;;
As Jack pointed out, this is a problem. One easy solution is:
let t = Array2D.create 2 2 1.0
t |> Array2D.iter (printfn "%f");;
And if you really like the for .. in .. do syntax:
type Array2DForLoopBuilder() =
member __.Zero() = ()
member __.For(a, f) = Array2D.iter f a
member __.Run e = e
let a2dfor = Array2DForLoopBuilder()
let t = Array2D.init 2 2 (fun a b -> float a + float b)
a2dfor { for x in t do printfn "%f" x }

F# Units of measure, problems with genericity

(I'm still banging on with units of measure in F#)
I'm having a problem making 'generic' functions which take 'typed' floats.
The following mockup class is intended to keep tabs on a cumulative error in position, based on a factor 'c'. The compiler doesn't like me saying 0.<'a> in the body of the type ("Unexpected type parameter in unit-of-measure literal").
///Corrects cumulative error in position based on s and c
type Corrector(s_init:float<'a>) =
let deltaS ds c = sin (ds / c) //incremental error function
//mutable values
let mutable nominal_s = s_init
let mutable error_s = 0.<'a> //<-- COMPILER NO LIKE
///Set new start pos and reset error to zero
member sc.Reset(s) =
nominal_s <- s
error_s <- 0.<'a> //<-- COMPILER NO LIKE
///Pass in new pos and c to corrector, returns corrected s and current error
member sc.Next(s:float<'a>, c:float<'a>) =
let ds = s - nominal_s //distance since last request
nominal_s <- s //update nominal s
error_s <- error_s + (deltaS ds c) //calculate cumulative error
(nominal_s + error_s, error_s) //pass back tuple
Another related question, I believe, still to do with 'generic' functions.
In the following code, what I am trying to do is make a function which will take a #seq of any type of floats and apply it to a function which only accepts 'vanilla' floats. The third line gives a 'Value Restriction' error, and I can't see any way out. (Removing the # solves the problem, but I'd like to avoid having to write the same thing for lists, seqs, arrays etc.)
[<Measure>] type km //define a unit of measure
let someFloatFn x = x + 1.2 //this is a function which takes 'vanilla' floats
let MapSeqToNonUnitFunction (x:#seq<float<'a>>) = Seq.map (float >> someFloatFn) x
let testList = [ 1 .. 4 ] |> List.map float |> List.map ((*) 1.0<km>)
MapSeqToNonUnitFunction testList
You can change the first 'compiler no like' to
let mutable error_s : float<'a> = 0.0<_>
and the compiler seems to like that.
As for the second question, I am not seeing the same error as you, and this
[<Measure>] type km
//define a unit of measure
let someFloatFn x = x + 1.2 //this is a function which takes 'vanilla' floats
let MapSeqToNonUnitFunction (x:seq<float<_>>) = Seq.map (float >> someFloatFn) x
let testList = [ 1 .. 4 ] |> List.map float |> List.map ((*) 1.0<km>)
let testList2 = testList :> seq<_>
let result = MapSeqToNonUnitFunction testList2
printfn "%A" result
compiles for me (though the upcast to seq<_> is a little annoying, I am not sure if there is an easy way to get rid of it or not).
Aside, I think convention is to name units parameters 'u, 'v, ... rather than 'a, 'b, ...
Units of measure cannot be used as type parameters. This is because the are erased by the compiler during compilation. This question is quite similar:
F# Units of measure - 'lifting' values to float<something>

F-Sharp (F#) untyped infinity

I wonder why F-Sharp doesn't support infinity.
This would work in Ruby (but not in f#):
let numbers n = [1 .. 1/0] |> Seq.take(n)
-> System.DivideByZeroException: Attempted to divide by zero.
I can write the same functionality in much complex way:
let numbers n = 1 |> Seq.unfold (fun i -> Some (i, i + 1)) |> Seq.take(n)
-> works
However I think that first one would be much more clear.
I can't find any easy way to use dynamically typed infinity in F#.
There is infinity keyword but it is float:
let a = Math.bigint +infinity;;
System.OverflowException: BigInteger cannot represent infinity.
at System.Numerics.BigInteger..ctor(Double value)
at .$FSI_0045.main#()
stopped due to error
Edit: also this seems to work in iteration:
let numbers n = Seq.initInfinite (fun i -> i+1) |> Seq.take(n)
First of all, F# lists are not lazy, (I'm not sure Ruby lists are lazy), so even with a general notion of infinity your first example can never work.
Second, there is no infinity value in Int32. Only MaxValue. There is a positive and negative infinity in Double though.
Putting it together, this works:
let numbers n = seq { 1. .. 1./0. } |> Seq.take(n)
I feel however Seq.initInfinite is your best option. The code above looks strange to me. (Or at least use Double.PositiveInfinity instead of 1./0.)
At first sight, a nice option to have in the language would be an infinite range operator like in haskell: seq { 1.. } The problem is that it would only work for seq, so I guess the extra work to support postfix operators is not worth it for this feature alone.
Bottom line: in my opinion, use Seq.initInfinite.
I think the following is the best solution to infinite ranges in F#; by marking the function inline we do better than "dynamically typed infinity" we get structurally typed infinite ranges (works with int32, int64, bigint, ... any type that which has a static member + which takes two arguments of its own type and returns a value of it's own type):
let inline infiniteRange start skip =
seq {
let n = ref start
while true do
yield n.contents
n.contents <- n.contents + skip
}
//val inline infiniteRange :
// ^a -> ^b -> seq< ^a>
// when ( ^a or ^b) : (static member ( + ) : ^a * ^b -> ^a)

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