[Hs-Generics] Sample code

[Johan Jeuring]

About the test suite:

> geq - generic equality
>   - Must this be "extensible/specializable" to count? (If so,  
> cannot do with LIGD, SYB1/2.

I wouldn't think so. We can compare implementations on some standard  
types, and
let extensibility/specializability be a separate evaluation criterium.

> gshow
>   - Must this be "extensible/specializable" to count? (If so,  
> cannot do with LIGD, SYB1/2).
>   - Must this match "deriving Show" to count? (If so, should be  
> updated for LIGD.)

I think that would be a desirable requirement. Yes, I think the LIGD  
show example
should be updated with a function that places parentheses as deriving  
Show does.

> bits
>   - Must all versions produce the *same* binary format. This seems  
> like a potentially
>     unfair comparison as small details (constructor order in view)  
> can make significant
>     differences.
>     I propose we let each version choose its own binary form,  
> perhaps encouraging each
>     to produce the most compact format possible, while still  
> requiring fromBin . toBin = id.
>     (Currently, SYB doesn't use XBitz because I just copied the  
> example from Ralf's page,
>     but I'd like to change that.)

That makes sense. I can imagine requiring exactly the same binary  
format would lead to
some very nasty implementations for some views.

> foldTree
>   - An example from Ralf's SYB webpage. Accumulate all ints in a  
> tree. Then accumulate
>     only those ints wrapped with the "Leaf" constructor.

foldTree is maybe better called crushTree (as Ralf L calls it, after  
a paper by Lambert
Meertens many years ago), to avoid confusion with the `algebraic'  
fold. I agree it would
be nice to add generic crush to the test suite.

And maybe even generic fold, but I doubt if there is any appraoch  
with which generic
fold can be nicely defined.

> paradise
>   - Another of Ralf's examples. Should the LIGD version count?

This is the increase salaries example, isn't it? I prefer to call it  
something like increase
salaries, or raise salaries, as Oleg calls it (I suppose I don't care  
about my salary anymore
in paradise, that is, if I ever get to go there, of course).

And should the LIGD version count? Good question. Without `open  
datatypes' I would
think that this example is not implementable in LIGD, since it  
requires adapting the
library, so we maybe should leave it out.

> For all of these examples, there is the question of what tests to  
> run for each.
> geq, gshow, bits and paradise currently work for the  
> "CompanyDatatypes" (I extended LIGD to cover floats).  foldTree  
> works for a specific Tree. Perhaps future examples can stress other  
> datatypes.
>
> Other extensions that we need are some test harness to run all of  
> these tests and create a summary
> of which frameworks have which tests.

I think we have to create test data types in each category of data  
types you mentioned in
a previous message. And possibly multiple data types in the `standard  
data types' category.

Organizing this shootout is quite a task, I hope we'll find a  
volunteer to organize it. (I'm afraid
my time is too divided to do it.)

-- Johan