Question

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.

Answer 1

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.