[Template-haskell] RE: Template Haskell...

[Wolfgang Thaller]

> For some time now I've been saying that Tim and I are working on the
> design of some enhancements to Template Haskell.  It's taken longer 
> than
> I'd hoped, and we aren't done yet, but you can find a draft at
> 	http://research.microsoft.com/~simonpj/tmp/notes.ps

Great stuff!

I've recently done some TH work again, so let me add something to the 
wish-list:

Another thing that I'm really missing with the current Template Haskell 
is a "decent" way to pass state between different meta-functions. Let 
me provide a few examples of the things I want to do:

I have one meta-function that generates declarations; and I have 
another meta function that needs to know what definitions generated by 
the first meta function there are (accross module boundaries).
In my case, the first meta function would declare an Objective-C class 
and the second one would register all classes with the Objective-C 
runtime.
Manuel Chakravarty provided me with a nice workaround involving the use 
of (unsafePerformIO $ newIORef ...) global variables from the Q monad. 
But to make that scheme work transparently, I would still need things 
like

reifyImportedModules :: [String] -- or Q [String]
reifyCurrentModuleName :: String -- or Q String

And of course, using unsafePerformIO is generally not a nice thing to 
do, so if somebody could think of a more general solution, that would 
be great.

Another "ugly" idiom that I've been using a lot:

I have a function declareClass :: String -> String -> Q [Dec].
$(declareClass "Bar" "Foo")
declares Haskell data types for an Objective-C class "Bar", which is 
derrived from another Objecive-C class "Foo".
But it also defines
super_Bar = "Foo"
because another meta-function will require that information. The 
identifier "super_Bar" will never be explicitly used by the user of my 
template library.
So I have lots of identifiers where the associated meta-information is 
stored in separate variables whose name is somehow based on the 
original name. It works, but all that text manipulation feels more like 
shell scripts than Haskell...


Cheers,

Wolfgang