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This is the definition for a new data type:
data Total a = Total a :+: Total a
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What does :+: mean in this type definition?
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The Data Keyword and Constructors In general, we define a new data type by using the data keyword, followed by the name of the type we're defining. The type has to begin with a capital letter to distinguish it from normal expression names. To start defining our type, we must provide a constructor.
The most basic way of defining a function in Haskell is to ``declare'' what it does. For example, we can write: double :: Int -> Int double n = 2*n. Here, the first line specifies the type of the function and the second line tells us how the output of double depends on its input.
Type Classes are a language mechanism in Haskell designed to support general overloading in a principled way. They address each of the concerns raised above. They provide concise types to describe overloaded functions, so there is no expo- nential blow-up in the number of versions of an overloaded function.
Nothing is a value, and its concrete type may be any possible instantiation of Maybe a . Otherwise said, values of type Maybe a continue to have concrete types Maybe Int , Maybe String , Maybe Whatever , and in particular, Nothing is able to be typed by each of them, depending on the context.
It is a data constructor written in infix form. You can have an operator as constructor if it begins with : .
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