I have a discriminated union like this:
type A = |B | C of int*A
I have to pattern match like this (the parenthesis appear to be needed):
match x with
| B -> printfn "B"
| C (i,a) -> printfn "%A, %A" i a
Is there a way to instead match like this with something like an active pattern:
match x with
| B -> printfn "B"
| C i a -> printfn "%A, %A" i a
And if not how come F# is designed such that this matching with curried arguments doesn't work and it instead forces you to use a tuple?
Edit: This was inspired by the F# list in which you can use h::t without any tupling or anything like that. And the source code is like:
type List<'T> =
| ([]) : 'T list
| (::) : Head: 'T * Tail: 'T list -> 'T list
I think examining the definitions of a curried function and an active pattern will make this clear for you.
Curried function:
A function which takes multiple parameters but which allows you to pass them in one at a time in order to return a function which does the same thing but takes one fewer parameters. Example:
let add a b = a + b
//val add : a:int -> b:int -> int
let add5 = add 5
//val add5 : (int -> int)
Active Pattern:
A way of applying pattern matching where the matching can be done using parsing or other complex logic. Takes one parameter and returns the result of the parsing. So input -> single return parameter.
//Example taken from https://fsharpforfunandprofit.com/posts/convenience-active-patterns/
let (|Int|_|) str =
match System.Int32.TryParse(str) with
| (true,int) -> Some(int)
| _ -> None
val ( |Int|_| ) : str:string -> int option
Since the whole point of currying a function is to be able to partially apply the function, the concept simply makes no sense when applied to the result of an active pattern.
Put another way, the result of an active pattern can't be "curried" because you can only curry functions and the result of an active pattern is data which is not a function. In your example, 'C (i,a)' is defining the return type of the Active Pattern case, not a function call.
You cannot have whitespace as delimiter between bound patterns, because neither union cases nor active patterns support this. Syntax as per the F# spec:
6.9.8 Evaluating Union Case
Case(e1,…,en)
7.2.3 Active Patterns
(|CaseName|) arg1 ... argn inp
(|CaseName|_|) arg1 ... argn inp
So it's necessarily one tupled argument for a union case; and n+1 arguments for the banana function, of which n arguments are parameters. Only the last argument binds to the pattern. Consider:
type X = B | C
let (|C|) a b = C (a, b)
let i = 42
match C with
| B -> printfn "B"
| C i a -> printfn "%A, %A" i a // prints 42, (42, C)
The case C in your discriminated union has a value of a tuple type (int * A).
The (i,a) part of your pattern matching isn't a parameter, it's matching the i to the int part and the a to the A part.
You could equally match with C x and x would hold a tuple of (int * A).
Related
I am trying to access the fst element of a tuple. Normally I use fst (tuple), but this situation is a little different.
let getCard (pl : player) : (card * player) option =
let plDeck = pl
match plDeck with
| c1::re -> Some (c1,(re))
| [] -> None
This is my f# code. the player type is a list of ints, and the output is tuple with the first int of the player list, and the player list minus the first int.
It's an assignment from my computer science class, so it is required that I use the option type.
I am trying to access the fst element of the tuple in another function by writing
let gc = fst (getCard [1,2,3])
but it seems like I can't do it this way, since I am getting the warning:
This expression was expected to have type
''a * 'b' but here has type
'(card * player) option'
How do I work around this?
The compiler is telling you that you're trying to access an option of tuple card * player while the function fst expects a tuple of card * player.
You could pattern match on your getCard function and extract the card.
let result =
match getCard [1..5] with
| Some card -> fst(card)
| None -> -1
You could also use pattern matching to extract the first part of your tuple.
let result =
match getCard [1..5] with
| Some (card, _) -> card
| None -> -1
As suggested by #Guran you shouldn't return magic numbers
let result =
match getCard [1..5] with
| Some (card, _) -> Some card
| None -> None
I have the following Discriminated Union (DU) declaration:
type Book =
| Dictionary of string[]
| Novel of int[]
| Comics of bool[]
An example:
let x = Dictionary [|"a"; "b"|]
How can I extract the length of the array inside without doing pattern matching and without caring about the data type of the array (in this case: string, int, bool). Note: I have no control over the DU declaration; as a result, I can't write new member method within Book, like getArrayLength()
Of course, we can do it in some way as followed:
match x with
| Dictionary (x: _[]) -> x |> Array.length
| Novel (x: _[]) -> x |> Array.length
| Comics (x: _[]) -> x |> Array.length
But typing x |> Array.length a lot is incovenient. This is a simple example, but we can think of a general problem:
type Animal =
| Dog of DogClass
| Cat of CatClass
| Cow of CowClass
...
... and DogClass, CatClass, etc. may share something. We want to get that shared thing. E.g. those classes inherit from AnimalClass, within which there is countLegs() method. Suppsed there are many animals, pattern matching for all of them while the code block after -> is almost the same. I love the principle DRY (Don't Repeat Yourself).
Is there any convenient way to tackle such problem?
==
EDITED 21.10.2019
I was also looking for some syntax like:
let numEles =
match x with
| _ (arr: _[]) -> x |> Array.Length
| _ -> failwith "No identifiers with fields as Array."
let numLegs =
match anAnimall with
| _ (animal: ?> Animal) -> animal.countLegs()
| _ -> failwith "Can't count legs because of not being an animal."
I think this still follows the spirit of matching, but seem like this approach is not supported.
Realistically, there's no getting around pattern matching here. DUs were, in a way, built for it. Since you don't control the type, you can always add a type extension:
type Book with
member this.Length =
match this with
| Dictionary d -> d.Length
| Novel n -> n.Length
| Comics c -> c.Length
let x = Dictionary [|"a"; "b"|]
printfn "%d" x.Length // Prints 2
Though it's also equally valid to define a Book module with a length function on it if you prefer that:
module Book =
let length b =
match b with
| Dictionary d -> d.Length
| Novel n -> n.Length
| Comics c -> c.Length
let x = Dictionary [|"a"; "b"|]
printfn "%d" (x |> Book.length) // prints 2
But you'll need to write a pattern match expression on the Book type at least once. The fact that every case is made up of data that all has the same property doesn't really help the fact that you need to still identify every case individually.
I need a function that could take an arbitrary number of arguments, each could be either of type 'T or seq<'T>. Inside the function I need to process it as a single seq<'T> with all inputs combined in the same order as they sere supplied.
The obvious way was to have something like:
module Test =
let flatten ([<ParamArray>] args) =
let flat = seq {
for a in args do
match box a with
| :? int as x -> yield x
| :? seq<int> as sq ->
for s in sq do
yield s
| _ -> failwith "wrong input type"
}
flat // this should be seq<int>
but I cannot make it work in FSI even with the simplest case
let fl = Test.flatten 1;;
----------------------^
...: error FS0001: The type 'int' is not compatible with the type 'seq<'a>'
What is wrong here and how to get it work as needed? Probably this could be done in some completely different way?
From msdn :
In F#, parameter arrays can only be defined in methods. They cannot be
used in standalone functions or functions that are defined in
modules.
So instead of a module, declare a type with a static method.
open System
type Test() =
static member flatten ([<ParamArray>] args: obj[]) =
let flat = seq {
for a in args do
match box a with
| :? int as x -> yield x
| :? seq<int> as sq ->
for s in sq do
yield s
| _ -> failwith "wrong input type"
}
flat
If you have other let bindings you can still declare a module with the same name.
Also note that in the second guard of the match you can avoid the for loop by doing:
| :? seq<int> as sq -> yield! sq
And box is not required.
I'd like to check that a value is of a particular case of a discriminated union, without having to also check any included data. My motivation is to only test one thing with each unit test.
An example is as follows (the last two lines give compilation errors):
module MyState
open NUnit.Framework
open FsUnit
type MyState =
| StateOne of int
| StateTwo of int
let increment state =
match state with
| StateOne n when n = 10 -> StateTwo 0
| StateOne n -> StateOne (n + 1)
| StateTwo n -> StateTwo (n + 1)
[<Test>]
let ``incrementing StateOne 10 produces a StateTwo`` ()=
let state = StateOne 10
(increment state) |> should equal (StateTwo 0) // works fine
(increment state) |> should equal (StateTwo _) // I would like to write this...
(increment state) |> should be instanceOfType<StateTwo> // ...or this
Can this be done in FsUnit?
I'm aware of this answer but would prefer not to have to write matching functions for each case (in my real code there are far more than two).
If you don't mind using reflections, the isUnionCase function from this answer could be handy:
increment state
|> isUnionCase <# StateTwo #>
|> should equal true
Note that it's a bit verbose because you need a function call before comparing values.
A similar but lighter approach could be comparison of tags:
// Copy from https://stackoverflow.com/a/3365084
let getTag (a:'a) =
let (uc,_) = Microsoft.FSharp.Reflection.FSharpValue.GetUnionFields(a, typeof<'a>)
uc.Name
increment state
|> getTag
|> should equal "StateTwo"
Beware that this is not type-safe and you can easily misspell a union case name.
What I would do is to create a similar DUs for comparison purpose:
type MyStateCase =
| StateOneCase
| StateTwoCase
let categorize = function
| StateOne _ -> StateOneCase
| StateTwo _ -> StateTwoCase
In this way, you define categorize once and use it multiple times.
increment state
|> categorize
|> should equal StateTwoCase
It appears FSUnit doesn't (or can't, I'm not sure) directly support this use case.
The next best thing I've found is to declare a TestResult type like the following and use a match to reduce the result to this type.
type TestResult =
| Pass
| Fail of obj
Here is the reducing match
let testResult =
match result with
| OptionA(_) -> Pass
| other -> Fail(other)
Now you can just use should equal to ensure the correct result.
testResult |> should equal Pass
The benefits of this solution are strong typing but more importantly in the failure case you can see what the invalid result was.
It doesn't look very elegant, but you can extract type from a value of state:
let instanceOfState (state: 'a) =
instanceOfType<'a>
And then use it in the test:
(increment state) |> should be (instanceOfState <| StateTwo 88)
EDIT
Yes, unfortunately the type is always MyState. Looks like pattern matching or ugly reflection are inevitable.
What if FsUnit already supports an assertion against a specific union case, albeit one restricted to values of the type Microsoft.FSharp.Core.Choice<_,...,_>?
Let's leverage this with a multi-case active pattern, which uses Reflection to check against the union case name.
open System.Reflection
open Microsoft.FSharp.Reflection
let (|Pass|Fail|) name (x : obj) =
let t = x.GetType()
if FSharpType.IsUnion t &&
t.InvokeMember("Is" + name,
BindingFlags.GetProperty, null, x, null )
|> unbox then Pass
else Fail x
Should be working now:
increment state
|> (|Pass|Fail|) "StateTwo"
|> should be (choice 1)
I keep learning F# pattern matching with my simple function which should return square root if argument is number, argument otherwise. I've modified it a bit and it looks like as follows.
let my_sqrt (o: obj) =
match o with
| :? float as d -> (sqrt d).ToString()
| _ as x -> x.ToString()
It is working fine for my purpose, but what if I don't want to cast return value to string? How can I return "some object" and then use it in printfn "%A" (my_sqrt [| 1; 2; 3 |]) construction?
Even though your example is just a demonstration of what you're trying to do, it is worth pointing out that this is probably not a good design. In F#, you would not normally use functions that operate on objects using casts - a better way to represent this would be a discriminated union:
type MyInput =
| Numeric of float
| Other of obj
let my_sqrt = function
| Numeric d -> Numeric (sqrt d)
| Other o -> Other o
This function works on a type that is either Numeric or Other, but thanks to the DU, you do not need any casting. I think something along these lines would be a better approach to your actual problem too.
I think you want
let my_sqrt (o: obj) =
match o with
| :? float as d -> (sqrt d) :> obj
| _ as x -> x
just upcast to object
I think your function is ok. When you want to compute each square root, you have to map your function over array like this:
Array.map my_sqrt [| 1.0; 2.0; 3.0 |] |> printfn "%A"
As mentioned in TP answer, the general idea should be, wherever possible, to surface information to your type system.
It is then easier for you to read and reason your program, as you have named things.
That means F# can actually work for you and tell you when you made something wrong
That makes it always worth the investment.
I agree with Tomas that using a Discriminated Union would be better. There is no Either monad built into F# but you could use the Choice union to standardize the interface:
let my_sqrt (o : obj) =
match o with
| :? float as d -> Choice1Of2 (sqrt d)
| o -> Choice2Of2 o;;