Xtykutl

My code currently looks like this. It is supposed to show the possible first symbols in the regular expression definition given to us beforehand. I am supposed to print these out as a list. For example, if the answer is supposed to be [1,2], it will come out [1,2] but when the answer is supposed to be ['1','2'] it will come out "12" or when it is supposed to be ['a', 'b'] it will come out "ab". What am I doing wrong?

data RE a         -- regular expressions over an alphabet defined by 'a'

    = Empty          -- empty regular expression

    | Sym a          -- match the given symbol

    | RE a :+: RE a  -- concatenation of two regular expressions

    | RE a :|: RE a  -- choice between two regular expressions

    | Rep (RE a)     -- zero or more repetitions of a regular expression

    | Rep1 (RE a)    -- one or more repetitions of a regular expression

    deriving (Show)



firstMatches :: RE a -> [a]

firstMatches Empty = 

firstMatches (Sym a)= a:list

firstMatches(Rep(a))=firstMatches a

firstMatches(Rep1(a))=firstMatches a

firstMatches (Empty :+: b)= firstMatches b

firstMatches (a :+: _) = firstMatches a

firstMatches (a :|: b)= firstMatches a ++ firstMatches b

You're not doing anything wrong.

String is a type synonym for [Char], so if you try to print a [Char] it will print as a String. This is somewhat of a special case, and it can be a little weird.

Show is the typeclass used to print things as a string. The definition of Show is something like this:

class Show a where

    showsPrec :: Int -> a -> ShowS

    show      :: a -> String

    showList  :: [a] -> ShowS

The showList function is optional. The documentation states:

The method showList is provided to allow the programmer to give a specialised way of showing lists of values. For example, this is used by the predefined Show instance of the Char type, where values of type String should be shown in double quotes, rather than between square brackets.

So if you define a new type and instantiate Show, you can optionally define a special way to show a list of your type, separate from the way it's normally shown and separate from the way lists are normally shown. Char takes advantage of this, in that a [Char] (or equivalently, a String), is shown with double-quotes instead of as a list of Char values.

I can't think of a way to get it to use the default show for a [Char]. I don't think there is one. A workaround might be to create a newtype wrapping Char with its own Show that uses the default showList implementation, but that doesn't seem appropriate here.

If this is homework, I'd expect the grader to know about this already, and I seriously doubt you'd get marked down for it, especially since the problem doesn't appear to be about show at all.