[Haskell-cafe] Newbie Question on type constructors

[Jon Fairbairn]

On 2004-11-01 at 23:01+0100 Benjamin Franksen wrote:
> On Monday 01 November 2004 21:51, Jon Fairbairn wrote:
> > Put the data declaration in a module, export the type, but
> >
> > not the constructor you want to hide:
> > > module Shape (Shape(Square), circle) where
> 
> Since we were talking about 'what can one do with a constructor as compared 
> with a function', it should be mentioned that this also has a disadvantage: 
> you loose pattern matching outside the module. I.e. no longer can say
> 
> 	...
> 	case x of
> 		Circle radius -> ...
> 		Square edge -> ...
> 
> BTW, would it be possible (in principle) to allow constructor export 
> 'read-only', so matching against a constructor is allowed but not 
> constructing a value?

I don't see why not. To add something to the debate about
what Circle /is/, I'd like to mention that the declaration

   data Shape = Circle Double | Square Double

is a convenience that declares three new names (Shape,
Circle and Square), but five entities.

There's Shape: a type, Circle, Square:: Double -> Shape:
constructor functions, and Circle, Square:: Shape -> (Double
-> t) -> t -> t: destructor functions.

The first three are available for general use by the
programmer, but the last two are hidden in the works of
pattern matching. I think this is what confuses things.

In all contexts bar pattern matching and export lists,
Circle and Square refer to the constructor functions (and
they really are just functions in these contexts). In
patterns they indirectly refer to the destructor functions,
and in export lists to both.

I'd like to see the separation made more accessible, and if
this were done, one could export whatever combination of the
entities one desired. I hesitate to raise the question of
what syntax we might choose to represent this for fear of
invoking Wadler's law.

  Jón

-- 
Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk