algebraic data type
algebraic data type includes
product types and
sum types (disjoint unions, variant types), the former one is the normal type like BookInfo, the other one can have multiple variants and multiple constructors. enum is a special sum type whose constructors take no arguments.
Till now, I know when define a type, we shall provides type name and value constructor name. Because haskell supports disjoint unions which can have multiple variants which need multiple constructors.
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