What is a "sealed" type in F# and why doesn't the type test pattern operator (:?) work on it out of the `box`?

The F# language guide (see Signatures article) has a very sparse definition of what a "sealed" type is:

Attribute	Description
[<Sealed>]	For a type that has no abstract members, or that should not be extended.
[<Interface>]	For a type that is an interface.

I searched the entire PDF version of the language guide using the term seal, and this was the most informative (out of all 8 results).

Context: :? fails on "simple types" with error FS0016

For example, I was playing with type abbreviations when both matches failed

type Lofa = | A | B
type Miez = Lofa

let (a: Miez) = A

match a with | :? Miez -> a
match a with | :? Lofa -> a

with

error FS0016: The type 'Miez' does not  have any
proper subtypes and cannot be used as the source
of a type test or runtime coercion.

error FS0016: The type 'Lofa' does not  have any
proper subtypes and cannot be used as the source
of a type test or runtime coercion.

The SO thread F# Why can't I use the :? operator in F# interactive? provides an answer (i.e., to use box, hence the mnemonic pun in the title),

match (box a) with | :? Miez -> a
match (box a) with | :? Lofa -> a

but it doesn't explain why it works or what "sealed" types are, and also raises questions:

• Does this mean that "sealed" types are "simple" F# types (i.e., not defined as "class types")?

  Found the issue #1320: Allow types to be Sealed by default, but I think that it only proposes "class" types to be "sealed" by default (as the "simple" types already are). Then again, this may be a misinterpretation.

• I thought I understood what the box operator does (even mentioned the same scenario there), but just realized that I don't know what "box a returns value a wrapped in a .NET System.Object instance" means.

  Does "wrapping" mean that a boxed "simple" F# type is redefined the same way as the original input, but as a "class type", inheriting everything from System.Object?

Please read before considering to close this question

^{This was intended for the trigger happy SO audience, but doesn't hurt to include it here:)}

Yes, there are two questions in the title, but before closing this question, please consider my reasoning: I tried to separate and link the questions bi-directionally, but

• an abstract answer to the first one (similar to the guide's) without examples would probably just as unhelpful without context, and

• asking the 2nd question would inevitable duplicate efforts to define what a "sealed" type is.

Thank you!

posted 4 months ago

CC0

4mo ago

toraritte‭

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