On Sat, Dec 15, 2012 at 9:24 AM, satvik chauhan
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.
Hi Satvik Perhaps you could put the variables whose evaluations are shared in a 'let':
f p1 p2 = $(mkG [| g_1 |]) mkG body = liftM2 LetE [d| g_1 = $(dyn "p1") |] body
The above example won't work exactly because the two occurences of g_1 are different names. But you could replace the [d| |] with something that has a [Dec] with variables defined in such a way that they can be captured. On a somewhat unrelated note, GHC is less able to infer types for expression splices than for top level bindings. The issue had something to do with needing to typecheck all the $( x :: ExpQ ) as a group, even if the values could be defined in separate modules. It might not be an issue in your case, but one possible way around it is have the definition of 'f p1 p2 = ... ' done by template haskell. But then there is the issue that top level splices are run in order, so -- this doesn't work: {-# LANGUAGE TemplateHaskell #-} [d| y = x |] [d| x = 1 |] -- This does work: {-# LANGUAGE TemplateHaskell #-} [d| x = 1 |] [d| y = x |]