Bottom Type in F#

It is ALMOST possible to express the bottom type in F#.

The bottom type is the type that has no values. It is useful as the return type of a function when every execution either does not terminate or throws an exception. Some languages include the bottom type. The suggestion for F# to add the bottom type was declined. Interestingly, the suggestion for C# to add the bottom type lives on.

Nevertheless, it is possible to define the bottom type in F#. Here is how I would do it.

1linktype Bottom = private Bottom of Bottom

My original thought was to omit of Bottom and then have this line be alone in its module (so that a public function does not provide access to this private case constructor). Then I read the suggestion to include of Bottom in this post on Reddit from three years ago. By including of Bottom, this discriminated union is a recursive type but lacks a base case. As such, it is clear from this one line that it is impossible to obtain an instance of Bottom.

In addition to having no values, there is another property of the bottom type. In subtyping systems, it is a subtype of all types. That is the motivation for the name "bottom". It sits at the bottom of the type hierarchy. My bottom type does not have this property. I think the suggestion for F# to add the bottom type was specifically requesting this feature.

Added 2020-09-23

I just realized that this does not work as I had intended because Unchecked.defaultof<> exists.

F# Interactive
1link> type Bottom = Bottom of Bottom;;

2linktype Bottom = | Bottom of Bottom

3link

4link> Unchecked.defaultof<Bottom>;;

5linkval it : Bottom = <null>

This definition of Bottom is a reference type, and reference types can be null. Now the recursive nature is a detriment. I was aiming for a type with zero values, but I actually created a type with infinitely many values since <null>, Bottom <null>, Bottom (Bottom <null>), and so on are a rather direct translation of the Church numerals.

If we change Bottom to a struct, then the compiler complains about the recursive definition.

1link[<Struct>]

2linktype Bottom~~~~~~ = Bottom of Bottom

Error FS0954 This type definition involves an immediate cyclic reference through a struct field or inheritance relation

After removing of Bottom to make the type non-recursive, both the reference and struct variants of the definition now have exactly one value. This is now just a more verbose way to define the unit type, the type with just one value, which F# already has.

My six-year-old daughter has been saying lately that "there is no perfect; there is only good enough". I am going to take her advice here and be content with this approximation of the theoretically perfect bottom type in the pragmatic language that is F#.


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