F# equivalent of `is` keyword in C#?

Question

My first F# day. If I have this:

let cat = Animal()

Now how do I check at later stage if cat is Animal?

In C#

bool b = cat is Animal;

In F#?

@ildjarn You should add that as an answer instead of a comment, so it can be accepted. — Jack P., May 21 '13 at 18:55
@Jack : On a mobile so I can't give a thorough answer. Feel free. :-] — ildjarn, May 21 '13 at 18:58
@ildjarn thanks, I knew it had something to do with `:?`, but did not get the syntax right! — nawfal, May 21 '13 at 19:03
You can pattern match on type too. Check other SO question [here][1]. [1]: http://stackoverflow.com/questions/16679503/how-to-perform-multiple-styles-of-pattern-matching — Przemysław Lewandowski, May 22 '13 at 09:00

score 28 · Accepted Answer · answered May 21 '13 at 19:29

@ildjarn deserves the credit here for answering first, but I'm submitting the answer here so it can be accepted.

The F# equivalent of the C# is keyword is :?. For example:

let cat = Animal()
if cat :? Animal then
    printfn "cat is an animal."
else
    printfn "cat is not an animal."

score 9 · Answer 2 · answered May 21 '13 at 19:15

9

For demonstration only (don't define an is function):

let is<'T> (x: obj) = x :? 'T

type Animal() = class end
type Cat() = inherit Animal()

let cat = Cat()
cat |> is<Animal> //true

answered May 21 '13 at 19:15

Daniel

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Daniel, can you tell me what is `|>` ? Why should one include it before a function call? Does it have a name? – nawfal May 21 '13 at 19:54
3

It's called the forward pipe operator. It applies the operand on the left as the last argument to the function on the right. ([see MSDN](http://msdn.microsoft.com/en-us/library/ee340273.aspx)) – Daniel May 21 '13 at 20:11
Daniel, why wouldn't you define is as a function, looks pretty useful to me? – sacha barber Dec 17 '14 at 15:55
2

@sacha: Given that there's a built-in type test operator (`:?`), `is<'T>` doesn't provide much value. Compare `cat is` to `cat :? Animal`. – Daniel Dec 17 '14 at 16:16
@Daniel : Yeah I guess, I think Is reads better personally, but I guess once you are familiar with all the weird FSharp operators, its fine. – sacha barber Dec 18 '14 at 08:48
2

You're right. It requires translation, i.e., reading `:?` as "is"—much the same as reading `>` as "greater than." – Daniel Dec 18 '14 at 16:55

score 4 · Answer 3 · answered May 29 '17 at 18:52

I know I'm late. If you try to test the type of a collection in fsi with :? it will give an error, if the item types do not match. E.g.

let squares = seq { for x in 1 .. 15 -> x * x }  
squares :? list<int> ;;   // will give false  
squares :? list<string> ;; // error FS0193: Type constraint mismatch

Wrapping in a function like Daniels is<'T> works.

F# equivalent of `is` keyword in C#?

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