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97 lines
2.9 KiB
Markdown
97 lines
2.9 KiB
Markdown
# Sum Type
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> This section isn't specific to xplr. However, since xplr configuration makes
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> heavy use of this particular data type, even though it isn't available in
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> most of the mainstream programming languages (yet), making it a wild or
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> unfamiliar concept for many, it's worth doing a quick introduction here.
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>
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> If you're already familiar with [Sum Type / Tagged Union][1] (e.g. Rust's
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> enum), you can skip ahead.
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While reading this doc, you'll come across some data types like [Layout][2],
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[Color][4], [Message][3] etc. that says something like "x is a sum type that
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can be any of the following", and then you'll see a list of strings and/or lua
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tables just below.
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Yes, they are actually sum types, i.e. they can be any of the given set of
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tagged variants listed there.
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Notice the word "be". Unlike classes or structs (aka product types), they can't
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"have" values, they can only "be" the value, or rather, be one of the possible
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set of values.
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Also notice the word "tagged". Unlike the single variant `null`, or the dual
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variant `boolean` types, the variants of sum types are tagged (i.e. named), and
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may further have, or be, value or values of any data type.
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A simple example of a sum type is an enum. Many programming languages have
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them, but only a few modern programming languages allow nesting other types
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into a sum type.
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```rust
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enum Color {
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Red,
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Green,
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}
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```
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Here, `Color` can be one of two possible set of values: `Red` and `Green`, just
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like `boolean`, but unlike `boolean`, being tagged allows `Color` to have more
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than two variants if required, by changing the definition.
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e.g.
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```rust
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enum Color {
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Red,
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Green,
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Blue,
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}
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```
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We'd document it here as:
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> Result is a sum type that can be one of the following:
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>
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> - "Red"
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> - "Green"
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> - "Blue"
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But some languages (like Rust, Haskell, Elm etc.) go even further, allowing us
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to associate each branch of the enum with further nested types like:
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```rust
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enum Layout {
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Table,
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HelpMenu,
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Horizontal {
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config: LayoutConfig, // A product type (similar to class/struct)
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splits: Vec<Layout> // A list of "Layout"s (i.e. list of sum types)
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},
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}
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```
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Here, as we can see, unlike the first example, some of `Layout`'s possible
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variants can have further nested types associated with them. Note that
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`Horizontal` here can have a sum type (e.g. enum), or a product type (e.g.
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class/struct), or both (any number of them actually) nested in it. But the
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nested values will only exist when `Layout` is `Horizontal`.
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We'd document it here as:
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> Layout is a sum type that can be one of the following:
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>
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> - "Table"
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> - "HelpMenu"
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> - { Horizontal = { config = Layout Config, splits = { Layout, ... } }
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And then we'd go on documenting whatever `Layout Config` is.
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So, there you go. This is exactly what sum types are - glorified enums that can
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have nested types in each branch.
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[1]: https://en.wikipedia.org/wiki/Tagged_union
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[2]: layout.md
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[3]: message.md
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[4]: style.md#color
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