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 | [EMAIL PROTECTED]
"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> ".")*<?xml:namespace prefix = o ns =
> "urn:schemas-microsoft-com:office:office" />
>
> <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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