Editing Tutorials/Programming Haskell/Argument handling (section)

====A type for flags====

The first thing to do is define a data type representing the valid flags.
First, let's import all the libraries I'll use:

<haskell>
    import Control.Monad
    import Data.Char
    import Data.List
    import System.Console.GetOpt
    import System.Environment
    import System.Exit
    import System.IO
    import Text.Printf
</haskell>

Now in a new file, Cat.hs, we'll write:

<haskell>
    data Flag
        = Blanks                -- -b
        | Dollar                -- -e
        | Squeeze               -- -s
        | Tabs                  -- -t
        | Unbuffered            -- -u
        | Invisible             -- -v
        | Number                -- -n
        | Help                  -- --help
        deriving (Eq,Ord,Enum,Show,Bounded)
</haskell>

The 'data' keyword defines a new data type, 'Flag', which can have one of several 
values. Such a type is often called a <i>sum</i> (or union) type. So 'Flag' is
a new user-defined type, just like other types, such as Bool or Int.
The identifiers on the right hand side of the | are the types <i>constructors</i>. That is, 
values which have type 'Flag'. We ask the compiler to also derive some
instances of various common classes for us (so we don't have to write the code ourselves).

With just this we can already start playing around with the flag data type in GHCi:

    > :reload
    > :m + Data.List
    > let s = [Number, Squeeze, Unbuffered, Squeeze]

    *Main Data.List> let s = [Number, Squeeze, Unbuffered, Squeeze]

    *Main Data.List> sort s
    [Squeeze,Squeeze,Unbuffered,Number]

    *Main Data.List> nub s
    [Number,Squeeze,Unbuffered]

    *Main Data.List> map fromEnum s
    [6,2,4,2]

    *Main Data.List> [Blanks .. ]
    [Blanks,Dollar,Squeeze,Tabs,Unbuffered,Invisible,Number,Help]

As you may already know, user defined data types are really first class citizens in Haskell, and behave just like
the 'inbuilt' types.