[Haskell-cafe] Template Haskell newbie questions

Hugo Pacheco hpacheco at gmail.com
Sat Sep 22 13:56:08 EDT 2007

Hi all,
I'm try to write some function in TH that I don't even know if it is

Consider the example from the tutorials

sel 1 2 = [| \(x,_) -> x |]

sel 2 2 = [| \(_,x) -> x |]

If I want to write some function that will dynamically create a selection
function according to its arguments and do something with it.

fsel :: Int -> Int -> x
fsel x y = $(sel x y) ...

Imagine a stupid case for

fsel :: Int -> Int -> Int
fsel x y = $(sel x y) (x,y)

The compiler complains that

GHC stage restriction: `x'
      is used in a top-level splice, and must be imported, not defined
    In the first argument of `sel', namely `x'
    In the expression: $[splice](sel x y) (x, y)
    In the definition of `fsel': fsel x y = $[splice](sel x y) (x, y)

what, indeed, makes sense.

However, is there a possible way to generate Haskell functions from TH
without having to instanciate its arguments?

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