[Haskell-cafe] Template Haskell Splicing

[satvik chauhan]

On Sat, Dec 15, 2012 at 1:30 PM, Michael Sloan <mgsloan at gmail.com> wrote:

> I don't think that there is a particular reason for not supporting
> quasi-quotes in where clauses..  It should be added!
>
> The reason for /splices/ to not be supported in here statements is that
> they are run during type checking.  That way calls to "reify" can access
> type information for things before your splice.  It also allows checking
> any AST quotes used inside your splice.  Since type-checking comes after
> renaming, splices can't be used in patterns (because it would affect the
> lexical scope).
>
> Quasi-quotes, on the other hand, are run in the renamer, and ought to be
> able to be used in where clauses.  Yet for some reason they can't - I get
> "parse error (possibly incorrect indentation or mismatched brackets)" when
> I try to put one under a where.
>
> Good catch!
> -Michael
>

Yeah, that is the problem. I have a function inside which I need to
generate some declarations using TH. I can not generate these at the top
level as these generations depend on the function's parameters which are
local to the function.

Something like

f p1 p2= ...
  where
-- this has to be generated by TH
     g_1 = p1
     g_2 = p2
     g_3 = p1 `xor` p2

something like the above. In the above I have shown only 2 parameters but
in my case it is much more. I am able to get the above splice as toplevel
declaration but I am still unsuccessful in getting it inside the where.
I can always make `g` a function and take parameters of `f` as arguments in
the top level splice but that will defeat the purpose of optimization
here(which I am trying to do), as that would result in a function call
every time I use `g` instead of above variables.
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