Haskell Language Creating Custom Data Types Creating a simple data type
Example
The easiest way to create a custom data type in Haskell is to use the data keyword:
data Foo = Bar | Biz
The name of the type is specified between data and =, and is called a type constructor. After = we specify all value constructors of our data type, delimited by the | sign. There is a rule in Haskell that all type and value constructors must begin with a capital letter. The above declaration can be read as follows:
Define a type called
Foo, which has two possible values:BarandBiz.
Creating variables of our custom type
let x = Bar
The above statement creates a variable named x of type Foo. Let's verify this by checking its type.
:t x
prints
x :: Foo
