[Template-haskell] (no subject)

[Alastair Reid]

> > For example, I'd like to be able to write (this doesn't parse):
> >
> >   $(test [t[ Float ]] "1.0")
> >   test :: Q Typ -> String -> Q [Dec]
> >   test qty x = do
> >     ty <- qty
> >     print (read x :: $ty)    -- The ':: $ty' part is the important bit
> >     return []

Sean Seefried replied:
> I think the following should do the job
> [...]
> test :: TypeQ -> ExpQ
> test rtype =
>   lamE [varP "x"] (sigE (appE  (varE "read") (varE "x")) rtype)
> [...]
> main = putStrLn (show ($(test rtype) "123.45"))

Thanks.

This seems to be subtly different from what I want though.
What I want is for the string to be parsed at compile time based on a type 
that is provided in the code being compiled.
Your code seems to generate code which will parse the string at runtime.

The example of parsing a string is a vastly simplified variant of my actual 
goal.  The reason I want to do things at compile time is that I want to use 
typeclasses to control how code generation is done.  The idea is that the 
user would define a bunch of typeclasses containing code generators and then 
when they invoke a template, the appropriate code generator would be selected 
according to the type they specify.

After much pondering of what template haskell would have to do for this to 
work (dictionary lookups, etc.), I suspect that template haskell can't do 
this.  Instead of defining instances of typeclasses, I will have templates 
that add entries to a Typ-indexed lookup table and instead of type splicing I 
will do lookups in the table.  It's not as elegant as I'd hoped (because it 
will largely duplicate the typeclass mechanism) but it will work.

--
Alastair