Of course we can also define our own data types in haskell. One very common type is the \emph{enumeration}. For example, we could define a data type for the Week:
\begin{haskellcode}
data WeekDay = Monday
             | Tuesday
             | Thursday
             | Wednesday
             | Friday
             | Saturday
             | Sunday
\end{haskellcode}
This declares the new data type \code{WeekDay} with 7 \emph{constructors}. That means \code{Monday}, \code{Tuesday} etc. are all values of the type \code{WeekDay}.
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We could now define a whole week, by creating a list:
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\begin{haskellcode}
week :: [WeekDay]
week = [Monday, Tuesday, Thursday, Wednesday
       , Friday, Saturday, Sunday]
\end{haskellcode}