CODEWITHSUNDEEP
f#
Luiso
Luiso

Reputation: 4113

What is the use of the F# :> operator

I have seen some code using the :> operator to accomplish something similar to type-casting in C# but even though I've searched a lot online I've seen no documentation about it.

What is that operator used for?

How does it work?

Where can I find some documentation about it?

Upvotes: 4

Views: 140

Answers (2)

Tomas Petricek
Tomas Petricek

Reputation: 243041

As @TheInnerLight explains, the :> operator represents an upcast. If you are coming from C#, this takes a bit of time to get used to, because in C# both (safe) upcast and (unsafe) downcast are written as (SomeType)value.

In F#, you do not need :> very often, because the compiler will insert automatic upcasts in the most common places - just like the C# compiler. For example, say we have foo that takes obj:

let foo (a:obj) = 0

The F# compiler accepts the following just fine, even though the argument is Random rather than obj (as the function foo expects):

foo (System.Random())

You could write this more explicitly, but you do not have to because the compiler inserts upcast:

foo (System.Random() :> obj)

One case where you need explicit upcasts is when returning different values from different branches of if or match constructs. For example:

if true then obj()
else System.Random()

This does not type check and you get an error:

error FS0001: This expression was expected to have type System.Object but here has type System.Random

You can fix this with an explicit upcast:

if true then obj()
else System.Random() :> obj

Upvotes: 8

TheInnerLight
TheInnerLight

Reputation: 12184

:> is the upcast operator. It's used to cast upward in an hierarchy so it's a type of casting that can be verified at compile time.

Its counterpart :?> is the downcast operator but the success of this can only be resolved at runtime.

See this page for more details: https://msdn.microsoft.com/visualfsharpdocs/conceptual/casting-and-conversions-[fsharp]

Upvotes: 9

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