Hi, You're pretty close, actually. In general, remember the following: 'type' introduces a type synonym, which means that you're just renaming an existing type, not creating a new one. this means that you don't need constructors. 'data' means that you're defining a new type; in order to do this, you need to specify constructors. for instance:

type Position = Int

here, we've just given Int a new name, Position.

data Bool = True | False

here we've defined a new type (Bool) with two constructors, True and False. or, if the constructors take arguments:

data PairOfInts = PairOfInts Int Int

here, the type is called PairOfInts and the constructor is called PairOfInts; the constructor takes two ints as arguments. HTH, - Hal -- Hal Daume III | hdaume@isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume On Mon, 3 Nov 2003, Patty Fong wrote:

Hello, I'm fairly new to haskell and functional programming and i'm still trying to get my head around certain concepts.

I'm wondering if anyone can help me convert this abstract syntax into Haskel data and type declarations:

<prolog> ::= (<assertion> ".")*

<assertion> :: = <structure> | <rule>

<rule> ::= <structure> ":-" <structure>("," <structure>)*

<structure> ::= <name> [�(� <term> (�,� <term>)* �)�]

<term> ::= <number> | <variable> | <structure>

<variable> ::= <name>

<name> is simply a String and <number> an Int

This was my attempt feeble attempt but i ran into numerous errors... :

type Prolog = Assertion

data Assertion = Structure | Rule

type Rule = Structure Structure (Structure)

type Structure = Name [ Term (Term)]

data Term = Number | Variable | Structure

type Variable = String

type Name = String

type Number = Int

Any help would be appreciated

TIA

Patrick.

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