[Template-haskell] reifyDecl: syntax and semantics

[Derek Elkins]

On 02 May 2003 16:51:46 +0200
Fran=E7ois-R=E9gis Sinot <francois-regis.sinot@laposte.net> wrote:

[...]=20
> My second point is that the syntax is inconsistent with the intended=20
> semantics: for example, the documentation says that it should be
> possible to reify a function definition and a prototype.
> But if I ask reifyDecl length, what do I expect ? The def or the proto
> ? Same thing with reification of instance declarations.
> As a matter of fact, the present implementation does not seem to
> handle the cases of prototypes and instances, and I don't think it is
> an urgent matter. However, I think the syntax should not already
> forbid that.
[...]

I can't comment on the other parts, I would imagine it's an=20
implementation restriction that could be lifted, but with quite a bit of
effort.  However, which documntation says reifyDecl reifies the type?  I
don't remember reading that anywhere (though I may have just forgotten).
 I've always thought of reifyDecl and it has always worked that way for
me as providing the definition and only the definition.  There is a
seperate construct (reifyType) for type information (I think last time I
checked it wasn't implemented or maybe I'm thinking of something else).

My solution for abstracting away which datatype to process, is simply to
have the generation function be f :: Decl -> Decl, actually in my case
it was genAlgebra :: String -> [Decl] -> Q [Dec].  Then, one would use
it as:
$(genAlgebra "Lang" [reifyDecl Type,reifyDecl Expr])
not the prettiest solution, but not much worse than what it would be
anyways.  However, I can think of other problems where this limitation
may be an issue, your problem may be in that category.