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# Enums
Tomo supports tagged enumerations, also known as "sum types." Users
can define their own using the `enum` keyword:
```tomo
enum VariousThings(AnInteger(i:Int), TwoWords(word1, word2:Text), Nothing)
...
a := VariousThings.AnInteger(5)
b := VariousThings.TwoWords("one", "two")
c := VariousThings.Nothing
```
## Pattern Matching
The values inside an enum can be accessed with pattern matching
```tomo
when x is AnInteger(i)
say("It was $i")
is TwoWords(x, y)
say("It was $x and $y")
is Nothing
say("It was nothing")
```
Pattern matching blocks are always checked for exhaustiveness, but you can add
an `else` block to handle all unmatched patterns.
## Tag Checking
Tags can also be quickly checked using the `.TagName` field:
```tomo
>> a.AnInteger
= yes
>> a.TwoWords
= no
```
## Reducing Boilerplate
There are three main areas where we can easily reduce the amount of boilerplate
around enums. We don't need to type `VariousThings.` in front of enum values
when we already know what type of enum we're dealing with. This means that we
don't need the name of the type for pattern matching (because we can infer the
type of the expression being matched). We also don't need the name of the type
when calling a function with an enum argument, nor when returning an enum value
from a function with an explicit return type:
```tomo
enum ArgumentType(AnInt(x:Int), SomeText(text:Text))
enum ReturnType(Nothing, AnInt(x:Int))
func increment(arg:ArgumentType -> ReturnType)
when arg is AnInt(x)
return AnInt(x + 1)
is SomeText
return Nothing
...
>> increment(AnInt(5))
= AnInt(6)
>> increment(SomeText("HI"))
= Nothiing
```
This lets us have overlapping tag names for different types, but smartly infer
which enum's value is being created when we know what we're expecting to get.
This also works for variable assignment to a variable whose type is already
known.
## Namespacing
Enums can also define their own methods and variables inside their namespace:
```tomo
enum VariousThings(AnInteger(i:Int), TwoWords(word1, word2:Text), Nothing)
meaningful_thing := AnInteger(42)
func doop(v:VariousThings)
say("$v")
```
Functions defined in an enum's namespace can be invoked as methods with `:` if
the first argument is the enum's type or a pointer to one (`vt.doop()`).
## Anonymous Enums
In some cases, you may want to use anonymous inline-defined enums. This lets
you define a lightweight type without a name for cases where that's more
convenient. For example, a function that has a simple variant for an argument:
```tomo
func pad_text(text:Text, width:Int, align:enum(Left,Right,Center) = Left -> Text)
...
...
padded := pad_text(text, 10, Right)
```
This could be defined explicitly as `enum TextAlignment(Left,Right,Center)` with
`pad_text` defining `align:TextAlignment`, but this adds a new symbol to the
top-level scope and forces the user to think about which name is being used. In
some applications, that overhead is not necessary or desirable.
Anonymous enums can be used in any place where a type is specified:
- Declarations: `my_variable : enum(A, B, C) = A`
- Function arguments: `func foo(arg:enum(A, B, C))`
- Function return values: `func foo(x:Int -> enum(Valid(result:Int), Invalid(reason:Text)))`
- Struct members: `struct Foo(x:enum(A,B,C))`, `enum Baz(Thing(type:enum(A,B,C)))`
In general, anonymous enums should be used sparingly in cases where there are
only a small number of options and the enum code is short. If you expect users
to refer to the enum type, it ought to be defined with a proper name. In the
`pad_text` example, the anonymous enum would cause problems if you wanted to
make a wrapper around it, because you would not be able to refer to the
`pad_text` `align` argument's type:
```tomo
func pad_text_wrapper(text:Text, width:Int, align:???)
...pad_text(text, width, align)...
```
**Note:** Each enum type is distinct, regardless of whether the enum shares the
same values with another enum, so you can't define another enum with the same
values and use that in places where a different anonymous enum is expected.
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