[Haskell-cafe] Re: [Haskell] FP-style vs. OO-style AST structure

Robert Dockins robdockins at fastmail.fm
Wed May 10 12:02:29 EDT 2006

[moved to haskell-cafe]

On Wednesday 10 May 2006 11:09 am, Doug Kirk wrote:
> Hi,
> I'm a Haskell newbie, but not new to programming, and I have a
> question regarding style (I think).
> I'm writing a parser for OMG's OCL, and have two ways of defining the
> AST model of a constraint. Each constraint in OCL has the following 4
> characteristics:
> 1. name :: Maybe String
> 2. context :: UmlElement
> 3. expr :: OclExpression
> 4. type :: OclConstraintType
> Now, having come from an O-O background, this looks right; however, in
> an FPL, it may not be. OclConstraintType is essentially an enumeration
> of the values:
>      Invariant
>    | Precondition
>    | Postcondition
>    | InitialValue
>    | Derivation
>    | Body
> The question is this: is it better to create a single type as above
> with a 'type' attribute, or would it be better to use the types as
> separate constructors of a Constraint, each constructor taking the
> same attributes?

Opinions may differ on this, bit I'm going to go out on a limb and say that 
using separate constructors with the same field names is the more idiomatic 
approach in Haskell.

Having a product type or a big record with an enumerated tag field is usually 
just a way to simulate sum types.  Why do that if the language supports them 

> I'm looking to avoid any pitfalls that could occur with either
> decision, and at this point I don't know the benefits of doing it one
> way vs. the other.

So long as every constraint has exactly the same fields, there isn't much 
practical difference.  If later you discover that different kinds of 
constraints need different fields, you'll be better off with the separate 

> BTW, I am using UUST Parser Combinator AG for the parser definition,
> in case that makes a difference.
> Thanks!
> --doug

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