Defining a datatype in Haskell

Question

Greetings, I am new to Haskell and I've gotten stuck in defining a datatype for an assignment.

I need to create the "Strategy" type, it's basically a string with 1-6 characters each representing a numeric value, and I must represent values greater than 9 as a letter (up to 35 total different values), I tried defining an auxiliary type representing each possible value and using that to create my type, but my code isn't working and I have ran out of ideas. This is the definition I have been trying:

data Value = '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' | 'A' | 'B' |
 'C' | 'D' | 'E' | 'F' | 'G' | 'I' |'J' | 'K' | 'L' | 'M' |  'N' | 'O' | 'P' |
 'Q' |'R' | 'S' | 'T' | 'U' | 'V' | 'W' | 'Y' | 'X' | 'Z'

data Strategy = Value | Value:Value | Value:Value:Value |
Value:Value:Value:Value | Value:Value:Value:Value:Value |             
Value:Value:Value:Value:Value:Value

The Value type isn't accepting the numbers, and the Strategy type "sort of" works up to the second constructor after which it goes bust. Thanks for your help!

Answer 1

Just like your previous (deleted) question, this looks a lot like homework. It also helps if you pick a real name and not use this throw-away account.

This page tells you how to use haskell's data type declarations. What are you doing different/wrong?

1. You use characters instead of constructors for Value . The '1' '2' , etc are all characters, you need a constructor along with zero or more fields like ValueCon1 Char and ValueCon2 String

2. You are using list constructors and what I assume is a field of Value instead of defining any new constructors for Strategy . Perhaps you want data Strategy = Strat [Value] , which will accept a list of any size. While more painful you can define many many strategy constructors with a finite number of separate Value fields.

BTW - where is your course taught? It seems late in the term to be covering these basics.

Answer 2

type Strategy = [Char]

^^ how I'd actually do it.

Or, maybe

newtype Strategy = Strategy [Char]

This is less principled, as the obligation is on you to not fill it with nonsense. Using the newtype solution, you can use "smart constructors" and say, eg

mkStrategy :: [Char] -> Strategy
mkStrategy x
   | {- list is valid -} = Strategy x
   | otherwise = error "you gave me a bad strategy!"

in real code, you don't want to throw error, but return an option type, but that's another story...