I was wondering if it is possible to do something like this in Haskell
data Word = Det String | Adj String | Noun String | Verb String | Adverb String
data NounPhrase = Noun | Det Noun
If I'm going about this wrong, what I am trying to say is - a "Word" is either a "Det", "Adj", "Noun", etc. and a "NounPhrase" is a "Noun" OR a "Det" followed by a "Noun".
When I try to do this I get the error: "Undefined type constructor "Noun""
How can I go about this so it performs as stated above.
When you define an algebraic data type like
data MyType = Con1 String | Con2 Int
then Con1 and Con2 are data constructors which themselves are
functions Con1 :: String -> MyType and Con2 :: Int -> MyType.
Your example is having 2 problems:
Det and Noun) for Word and for NounPhrase.
So you need to choose different names for the constructors of
NounPhrase.Det Noun does not make sense, since Noun is a data
constructor, whereas the argument of Det needs to be a type, e.g., String.See Constructors in Haskell to help clear things up.
You are confusing types and value constructors. When you say
data Word = Det String | Adj String | Noun String | Verb String | Adverb String
You define the type Word as being one of a number of forms, for example
Det String
says that Det is a constructor that takes a String and gives you back a value of type Word. Same goes with Noun which is defined to be a constructor for Words and not a type.
There are various ways you might encode what you want in Haskell. By far the simplest is to use
data Word = Det String | Adj String | Noun String | Verb String | Adverb String
data NounPhrase = JustNoun String | Compound String String
and this is the "learning Haskell" way of doing it. It is "stringly typed", but is probably sufficient for your purposes.
