Another interesting code using Template Haskell

While writing the post about performance, I got interested in Template Haskell, specially in the $(lift) construction.  Investigating a little bit about it, I got to the following code:

{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH.Syntax
import X

main :: IO ()
main = print $(lift $ x)

With X.hs:

module X where

import System.IO.Unsafe

x :: Int
x = read $ unsafePerformIO $ putStr "Number: " >> getLine

The result was a surprise to me. It asked for the number in compilation time.

$ ghc --make main
[2 of 2] Compiling Main             ( main.hs, main.o )
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package array-0.3.0.0 ... linking ... done.
Loading package containers-0.3.0.0 ... linking ... done.
Loading package pretty-1.0.1.1 ... linking ... done.
Loading package template-haskell ... linking ... done.
Number: 5
Linking main ...
marcot@zezinho:~/codigo/haskell$ ./main
5