[Haskell-cafe] putting the result of a function in `infix` declaration

[TP]

Hi everyone,

Consider the following code:

--------------
import Prelude hiding ((+))

data Expr = Plus Expr Expr | Minus Expr Expr | Expr String deriving Show

f :: Expr -> Int
f (Plus _ _) = 1
f (Minus _ _) = 2

-- infix (f (Plus undefined undefined)) +  -- (*)
infix 6 +
(+) :: Expr -> Expr -> Expr
e1 + e2 = Plus e1 e2

main = do

let a = Expr "a"
let b = Expr "b"
print $ a + b
--------------

I would like to declare the infix precedence of (+) with a function 
(commented line (*)) instead of directly as above.

Is there any means to do that?
Do we need some kind of preprocessing machinery? How to do that in Haskell?

In a more general way, let us suppose I have a Haskell library able to 
perform some calculations: how to use it in a pre-processing step, before 
compilation of the executable? It could be a function to compute 
symbolically roots of a polynomial of second degree to be used at runtime. 
We would put some placeholders in the code where the result of the pre-
processing calculation would enter.

Thanks in advance,

TP