Enums

What you’ll build

A blog’s article workflow: each article has a status, from draft to archived, and each editor has a set of permissions. An Enum names each possible value, so the program can’t hold a status that doesn’t exist.

Declaring an Enum

An Enum lists its members, separated by commas or on lines of their own. Each member is used through the Enum’s name, like Status.Review, and prints as its name. Behind each member is an integer, from 0 up, unless a member is given its own value.

Import std.core

Enum Status
    Draft
    Review
    Published = 10
    Archived
End Enum

Local status = Status.Review
Print(status, status == Status.Review)
Print(Status.Draft As Int, Status.Archived As Int)
Print(10 As Status)

Review True
0 11
Published

Archived follows Published, so it’s 11. As turns a member into its integer and back. An integer that isn’t one of the Enum’s values is an error when the compiler can see it, as in 3 As Status, and a panic when the program runs.

Select over every member

A Select on an Enum must handle every member, or have a Default. Leaving a member out is an error that names it, so adding a member later points at each Select that has to handle it. Since this Select covers every member, the function returns a value on every path.

Import std.core

Enum Status
    Draft, Review, Published, Archived
End Enum

Function Label(status: Status): String
    Select status
        Case Status.Draft
            Return "being written"
        Case Status.Review
            Return "waiting for an editor"
        Case Status.Published
            Return "live"
        Case Status.Archived
            Return "taken down"
    End Select
End Function

Local statuses = [Status.Draft, Status.Published, Status.Archived]
For status In statuses
    Print("{status:<10} {Label(status)}")
End For

Draft      being written
Published  live
Archived   taken down

Without the Case for Status.Archived, the program doesn’t build. fb names the member left out, and says too that Label then has a path that returns nothing:

Import std.core

Enum Status
    Draft, Review, Published, Archived
End Enum

Function Label(status: Status): String
    Select status
        Case Status.Draft
            Return "being written"
        Case Status.Review
            Return "waiting for an editor"
        Case Status.Published
            Return "live"
    End Select
End Function

Print(Label(Status.Draft))

   Compiling articles.fb
articles.fb:7:10: error: 'Label' doesn't return a value on every path
  |
7 | Function Label(status: Status): String
  |          ^^^^^
articles.fb:8:12: error: Select doesn't cover Status.Archived; add a Case for each, or a Default
  |
8 |     Select status
  |            ^^^^^^
error: couldn't build articles.fb because of 2 errors

Flags

A @Flags Enum’s members are bits, 1, 2, 4, and so on, so a value can hold several at once. | combines them, and Has asks whether a value has all the flags given. A combination prints as its members’ names. Since a @Flags value can be any combination, a Select on one always needs a Default.

Import std.core

@Flags Enum Access
    Read, Write, Publish, Delete
End Enum

Local author = Access.Read | Access.Write
Local editor = author | Access.Publish
Print(author, editor)
Print(author.Has(Access.Write), author.Has(Access.Publish))
Print(editor.Has(Access.Read | Access.Publish))
Print(editor & ~Access.Write)

Read | Write Read | Write | Publish
True False
True
Read | Publish

~ flips every bit, so ~Access.Write is every flag but Write, and editor & ~Access.Write keeps the editor’s flags without Write.

An Enum in a type

An Enum is a value like a number, so it fits in a field. A field of one starts at the Enum’s zero: its first member, or no flags at all for a @Flags Enum.

Import std.core

Enum Status
    Draft, Review, Published
End Enum

Class Article
    ReadOnly Field Title: String
    Field status: Status

    New(title: String)
        Title = title
    End New

    Method Advance()
        Select status
            Case Status.Draft
                status = Status.Review
            Case Status.Review
                status = Status.Published
            Case Status.Published
                Print("{Title} is already published")
        End Select
    End Method
End Class

Local article = Article("Hello")
For step = 1 To 3
    article.Advance()
    Print(article.status)
End For

Review
Published
Hello is already published
Published

What you learned

• An Enum names a fixed set of values, each an integer underneath.
• Members print as their names, and As converts to and from integers.
• A Select on an Enum must cover every member or have a Default; on a @Flags Enum, it always has a Default.
• @Flags members are bits that combine with |, and Has tests them.
• An Enum’s zero is its first member, or no flags for a @Flags Enum.