Re: How to correctly use (second-class) modules?

The third solution is to use a OO approach: define a record of
pretty-printer operations.

For the SML/NJ pretty-printing library, I chose the functor approach,
where the pretty-printing engine is parameterized over a device
module.  Different backends (e.g., text, html, ...) are implemented
by different devices.  I could have used a record type for the device
interface, which might be a better choice.

	- John

On Feb 7, 2007, at 7:24 AM, Geoffrey Alan Washburn wrote:

>
> My software uses a pretty-printer abstraction, and when I opted to
> provide two different pretty-printing mechanisms the naïve decision I
> made was to just write another module satisfying the signature.
> However, now that I want to be able to choose a pretty-printer via a
> command-line switch I seem to be left with one of two options
>
> (1) Functorize my program and then instantiate two completely separate
> versions and dispatch using a wrapper script.
>
> (2) Write a third implementation of the signature that is a very  
> tedious
> series of conditionals.
>
> Both of these solutions are hard to take seriously.  Is there some  
> other
> idiom people use in practice to overcome the restrictions of
> second-class modules?  I did see the dynamic loading solution in  
> the CM
> manual, but that would not appear to be able to handle modules that
> contain abstract types.  Similarly, attempting to recast the  
> abstraction
> as a record of functions fails also fails because of the need for
> abstract types.  Is this really that uncommon of a design decision?
>
>
>
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