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Custom types
Ela provides a support for user defined types. All user types in Ela are in fact algebraic types, however, it is possible to declare a type with just a single constructor. Also one can specify visibility level attributes on a type and on constructor level, which allows to create abstract types where all constructors are hidden and all operations with the type are done through regular functions.
Ela supports both regular algebraic types (declared using type keyword) and open algebraic types (declared using
opentype keyword). When a new type is introduced using opentype declaration, it is possible to extend this type
with new constructors. More details on that are provided in a separate section.
In order to declare a new type one can use a type declaration. It has the following formal definition:
"type" Name "=" [ "|" ] cons { "|" cons }
cons = ( Name { arg } | arg op arg | op arg | arg op ) [ "#" attr { attr } ]
arg = "!" name | name | "(" Type name ")" | "(" Type "!" name ")"
Type names should always start with a capital letter. Constructor names should either start with a capital letter or should be combined from operator symbols.
Let's look at some examples of a type definition:
type Maybe = None | Some a
type Complex = a :+ b
As it was mentioned above, it is possible to declare a type with just a single product:
type Any = Any a
Constructors in type declarations are separated by a semicolon character. A semicolon character is also allowed before the first constructor and may be used there in case when it might improve readability:
type Color =
| Red
| Green
| Blue
A type can have an unlimited number of constructors. Each constructor can have an unlimited number of parameters
(however, parameters are not required, as you can see from a definition of Maybe type above). Parameter names shouldn't
start with a capital letter, but, besides this, there are no limitations. You can use any names you like. For example,
the following declarations are equivalent:
type Complex1 = a :+ a
type Complex2 = a :+ b
However, parameter names are not wasted during compilation, but instead are preserved as metadata and can be queried at
run-time (using functions, provided with standard generic module).
It is possible to specify attributes - both on the type and the constructor level. For example, the following declaration:
type Foo # private
type Foo = Foo | Bar
creates a type Foo with two constructors, but a type itself and its both constructors are not visible outside of a module.
However, the following declaration:
type Foo =
| Foo # private
| Bar # private
creates a public type Foo with two private constructors.
It is allowed to use all standard attributes (see below for details). Attributes on the type level are always inherited by constructors, e.g. it is not possible to declare a non-private constructor in a private type, however, a private constructor in a public type is a completely legal scenario.
Type constructors are in fact regular global bindings. For example, having a type declaration like so:
type Maybe = None | Some a
we can use both None and Some as any other regular name bindings declared at top level. None here is a constant, and
Some is a one-argument function, that is no different from any other functions in Ela, is curried and is a first-class
value.
In order to create instances of Maybe type, you can write code like so:
x = None
y = Some 42
As it was mentioned above, constructors with arguments are just functions and can be used in all cases where functions can be used:
open core list
map Some [1..10]
Constructors can also be qualified by a module alias (and even declared with qualified attribute) just like any other
regular bindings:
open core
x = core.Some 42
Ela also allows to specify type constaints for constructor arguments. A type constraint is always enclosed in parentheses and is presented as a juxtaposition of a type name (optionally qualified with a module alias) and a parameter name:
type Circle = Circle (Double a)
When a type constraint is specified invocation of a constructor would fail if a parameter is of different type.
A single constructor can mix parameters with and without type constraints, e.g.:
type Person = Person (String name) (Double age) info
Type checking in constructors is done in a conservative manner - it never evaluates arguments if these arguments are thunks; therefore, the following code would work without errors:
c = Circle (& "string")
If this is not a desired behavior, you can mark a constructor parameter as strict, using a so called bang pattern:
type Circle = Circle (Double !a)
Now Circle constructor will always evaluate its argument.
It is also possible to use bang patterns without type constraints, e.g.:
type Complex = !a :+ !b
Types, declared using type definition, are closed, which means that this type cannot be extended with new constructors.
This is no always a desired behavior. That is why Ela provides a way to create open types. Open types are declared using
opentype construct which syntax is completely equivalent to the type constructor, except of a different keyword:
opentype Product = Cellphone | Laptop | Display
Types, declared using opentype has only one difference from types, declared using type construct - one can extend
a definition of an opentyp by new constructors even from a different module. This done using data construct like so:
data Product = TV | Smartphone
Again, syntax for data declaration is almost the same as for type declaration. However, in data declaration you can
optionally prefix a name of a type with a module alias like so:
data alias.Product = TV | Smartphone
Also data declaration is not used to introduce new types, but rather to extend existing types, therefore, it is an error
to reference undeclared types inside data declaration.
Types are constructed using constructors. In order to deconstruct a type instance, one can use pattern matching:
s = Some 42
(Some xx) = s
xx
Type names should be unique within the same module. However, two different modules can define types with the same name, and this is completely legal. In fact types in Ela are not identified by their names, but by their unique IDs which are generated during link phase. As a result, one can easily declare a type that has the same name as one of Ela built-in types:
type Int = Int a
Constructors for types are just regular Ela bindings, therefore, shadowing rules for them are exactly the same as for other
bindings. Types can be prefixed with modules aliases as well (it might be useful when type checking or inside data
declarations).
A type can provide instances for existing classes. However, because of reflective abilities of Ela, it is not always required
to manually implement these instances for each custom type. For example, module generic constains complete default
instances for many of the standard classes. An instance can be generated by a compiler based on the default instance if
you leave a body of an instance empty like so:
instance Show Foo
However, this is not always convinient, and that is the reason why Ela provides additional syntax to specify automatically
generated instances for your type - a deriving clause:
type Foo = Foo
deriving Eq Show
You can specify an unlimited number of classes in the deriving clause. You can use deriving clause in type, opentype
and data declarations.
It is possible to check a type of a value using is expression like so:
bar 42 is Foo
As long as type names are not unique, it is allowed to use module aliases in such patterns, e.g.:
open foobarModule@fb
bar 42 is fb.Foo